[cctalk] Re: vt220 firmware source?

[Rob Jarratt via cctalk]

> -Original Message-
> From: Paul Flo Williams via cctalk 
> Sent: 05 February 2024 16:24
> To: cctalk@classiccmp.org
> Cc: Paul Flo Williams 
> Subject: [cctalk] Re: vt220 firmware source?
> 
> On Sun, 04 Feb 2024 16:26:57 -0700
> Richard via cctalk  wrote:
> 
> > In article  you
> > write:
> > >I'm about to dive in to commenting the disassembly listings, but
> > >figured I'd ping here to see if anybody might have done this already
> > >in case I wouldn't have to start from scratch?
> >
> > Give IDA Pro or Ghidra a crack at it to help you make sense of the raw
> > disassembly.
> >
> > I made pretty good progress on VT100 ROMs before Paul Williams put up
> > a completely reverse engineered commented listing.
> 
> Well, in case it helps, I'll explicitly disclaim any intention of 
> disassembling the
> VT220 :-)
> 
> After the VT100, I did make a start on the VT102/131 combination but it
> seemed too samey to hold my interest.

Hello Paul,

Your annotated VT100 listing was invaluable in helping me to fix my own VT100 
(still not working though, but on the video output side). Would be interested 
in how you did it, I get the impression from your message that you used an 
emulator to observe the behaviour?

Thank you so much!

Rob

> 
> More than 20 years ago, I started on the VT320, as my personal favourite, but
> I didn't understand how to tackle emulation at the time, the 8051 emulator I
> wrote had flaws, and I spent a long time on getting the emulated video timing
> correct enough to even pass self test! If only the VT320 had had a technical
> reference as comprehensive as that published for the VT100. I will return to 
> it
> at some point, probably using some else's 8051 core, as Peter Sichel (last 
> head
> of DEC's terminals group) bet me that I wouldn't be able to do it.
> 
> At the moment, I'm tackling another full commentary, but it may well have an
> even smaller audience than that for the VT100; I'm 83% through gutting the
> arcade game Star Force. I've even fixed two æsthetic bugs in the game, 40
> years too late for anyone to care. At least this is Z80 assembler, and 
> tinkering
> with that has always made me *very* happy.
> 
> Paul.

[cctalk] Re: RIP: Software design pioneer and Pascal creator Niklaus Wirth

[Rob Jarratt via cctalk]

> -Original Message-
> From: Paul Koning 
> Sent: 05 January 2024 00:34
> To: r...@jarratt.me.uk; cctalk@classiccmp.org
> Cc: Robert Jarratt 
> Subject: Re: [cctalk] RIP: Software design pioneer and Pascal creator
Niklaus
> Wirth
> 
> 
> 
> > On Jan 4, 2024, at 5:38 PM, Rob Jarratt via cctalk

> wrote:
> >
> > Sad news but a great article Liam, thanks for the interesting history.
> >
> > The second language I taught myself was Algol 68 (!), on a DECSYSTEM20.
I
> learned Pascal at University, when I did my M.Sc I used VAX Pascal on a
VAX
> 11/780 and went on to use it as my first professional language. I loved
VAX
> Pascal, I could do just about anything I wanted with it. Of course, it
wasn't
> very "pure" Pascal.
> >
> > Incidentally, I have asked a couple of times before, but if anyone knows
of
> any media with Algol68C for PDP10 I would love to get hold of it.
> 
> That would be neat, indeed.  Where did that one come from?  I remember a
> well regarded Algol68 subset compiler from the Royal Radar Establishment
in
> the UK.  Don't remember what machine(s) it targeted.

Algol68C came from the University of Cambridge. It was written on IBM
hardware as a portable compiler and it was ported to PDP10 at the University
of Essex.

> 
> There's an Algol68g (GPL-licensed open source implementation).  I haven't
> tried it yet.
> 
> I think the CDC 6000 Algol 68 is still around somewhere.  That one was
> created in Holland.
> 
>   paul

[cctalk] Re: RIP: Software design pioneer and Pascal creator Niklaus Wirth

[Rob Jarratt via cctalk]

Sad news but a great article Liam, thanks for the interesting history.

The second language I taught myself was Algol 68 (!), on a DECSYSTEM20. I 
learned Pascal at University, when I did my M.Sc I used VAX Pascal on a VAX 
11/780 and went on to use it as my first professional language. I loved VAX 
Pascal, I could do just about anything I wanted with it. Of course, it wasn't 
very "pure" Pascal.

Incidentally, I have asked a couple of times before, but if anyone knows of any 
media with Algol68C for PDP10 I would love to get hold of it.

Regards

Rob

> -Original Message-
> From: Liam Proven via cctalk 
> Sent: 04 January 2024 21:32
> To: General Discussion: On-Topic and Off-Topic Posts 
> Cc: Liam Proven 
> Subject: [cctalk] RIP: Software design pioneer and Pascal creator Niklaus 
> Wirth
> 
> Evangelist of lean software and devisor of 9 programming languages and an
> OS was 89
> 
> https://www.theregister.com/2024/01/04/niklaus_wirth_obituary/
> 
> The great man has left us. I wrote an obituary.
> 
> --
> Liam Proven ~ Profile: https://about.me/liamproven
> Email: lpro...@cix.co.uk ~ gMail/gTalk/FB: lpro...@gmail.com
> Twitter/LinkedIn: lproven ~ Skype: liamproven
> IoM: (+44) 7624 277612: UK: (+44) 7939-087884 Czech [+
> WhatsApp/Telegram/Signal]: (+420) 702-829-053

[cctalk] VT100: Replacement for B411 Transistor

[Rob Jarratt via cctalk]

I think I have got my VT100 basically working now except for the actual
video display. I think there is a problem with the transistor that drives
the flyback transformer. This is Q414 on page 58 of the Feb82 schematic on
BitSavers
(https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.
pdf). I have removed it from the circuit and tested it with the diode tester
of my multimeter. It does not test as two diodes, indeed across
Collector-Base the multimeter beeps for a short circuit, and so I am fairly
sure it is bad,

 

The part is the one with the heatsink as shown in this picture:
https://rjarratt.files.wordpress.com/2023/12/img_20231221_112305.jpg. I have
found some information on it here:
https://www.web-bcs.com/transistor/tc/b0/B411.php but the Feb82 schematic
shows it is a BU407D. My video board is not exactly the same as the one in
the schematic despite having the same DEC part number, but the circuit that
drives the flyback is the same on the secondary side of T403, and includes
the optional diode CR406 in the form of another B411 transistor that is not
connected at the emitter.

 

I am trying to identify a replacement and could do with some help
identifying one. I can't find a full datasheet for the B411, all I can find
is this https://www.web-bcs.com/transistor/tc/b0/B411.php, which seems to
match the part I need to replace, and I have also found one for a BU407 (not
BU407D) here https://www.mouser.co.uk/datasheet/2/308/1/BU407_D-2310257.pdf.

 

I have tried to find something that meets or exceeds the voltage, current
and switching time specs. I have found a couple of possible replacements and
would welcome opinions on their suitability:

 

https://www.farnell.com/datasheets/2861437.pdf

and

https://www.farnell.com/datasheets/3033460.pdf (assuming that the switching
times are a typo and they are in microseconds rather than seconds)

 

Do these seem like suitable replacements?

 

Thanks

 

Rob

[cctalk] Re: VT100 Monitor Board

[Rob Jarratt via cctalk]

> -Original Message-
> From: Christian Corti via cctalk 
> Sent: 30 November 2023 08:43
> To: Rob Jarratt via cctalk 
> Cc: Christian Corti 
> Subject: [cctalk] Re: VT100 Monitor Board
> 
> On Wed, 29 Nov 2023, 'General Discussion: On-Topic and Off-Topic Posts'
> wrote:
> > I am working on the schematic and when it is done I will post it for
> > information. However, I was wondering what you suggest for testing the
> 
> What am I missing? Why don't you use the official schematics of the VT100
> that also include the schematic of the monitor board?
> It is on bitsavers PDF page 43.
> /pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf

My board does not match the one in the schematic.

> 
> Christian

[cctalk] Re: VT100 Monitor Board

[Rob Jarratt via cctalk]

> -Original Message-
> From: Tony Duell 
> Sent: 26 November 2023 06:35
> To: r...@jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: [cctalk] VT100 Monitor Board
> 
> On Sat, Nov 25, 2023 at 11:12 PM Rob Jarratt 
> wrote:
> 
> > > Have you checked the diodes and capacitors associated with the
> > > flyback transformer? If something is shorted there. it's the
> > > equivaent of running a power supply into a short circuit. The input 
> > > current
> will go up.
> >
> > I have checked all the diodes (in circuit) and they appear to be OK. One
> capacitor may be suspect but without the full schematic I am not sure. I may
> have to lift it to check.
> 
> Without the schematic you are going to have problems.

Hello Tony,

I am working on the schematic and when it is done I will post it for 
information. However, I was wondering what you suggest for testing the board 
once I have done this. Simply replacing the blown fuse and plugging it in 
doesn't sound like a good idea. Equally, not sure if using the light bulb 
method for limiting current into the PSU is going to help is it?

Thanks

Rob

> 
> With a schematic you can see
> (a) Can you isolate certain areas -- for example the vertical deflection syste
> 
> > > Trace out a schematic of the board so I know what I am dealing with.
> >
> > Yes, I may have to do that. Historically I have done a poor job of this
> because of my insufficient understanding of electronics.
> 
> It gets easier with experience. Looking at every small monochrome monitor
> schematic you can find will give you an idea of the sort of things to look 
> for.
> 
> > > I do wonder what the problem is with testing it with the flyback
> > > connected, though...
> >
> > Only that I know it can produce lethal voltages and I would rather avoid 
> > that
> if I can.
> 
> The high voltage outputs can't supply that much current and are unlikely to
> be lethal. Mains, and even worse the rectified mains in an SMPSU, is a lot
> more liely to kill you.
> 
> That said, work with one hand in your pocket (current flow arm-to-arm is the
> most dangerous) and take care.
> 
> I don't see how you can debug a monitor without having the flyback in place.
> It's part of the highest power circuit on the board. And it provides voltages 
> for
> many other areas.
> 
> -tony

[cctalk] Re: VT100 Monitor Board

[Rob Jarratt via cctalk]

> -Original Message-
> From: Tony Duell 
> Sent: 25 November 2023 14:17
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Rob Jarratt 
> Subject: Re: [cctalk] VT100 Monitor Board
> 
> On Sat, Nov 25, 2023 at 12:07 PM Rob Jarratt via cctalk
>  wrote:
> >
> > Hello,
> >
> >
> >
> > As some may recall I have been working on getting a VT100 going again.
> > I have made good progress and I think the main board is probably OK
> > now (see here if you are interested:
> > https://robs-old-computers.com/2023/11/19/vt100-keyboard-constant-clic
> > king-f ault/). Possibly I still need to replace the NVRAM, but I am
> > leaving that until I fix the problem I want to describe next.
> >
> >
> >
> > The problem is that there is no image on the screen. This is because
> > the monitor board is not doing anything, there is no glow from the
> > neck of the tube etc. I have found that this is because the fuse on
> > the 12V input to the monitor board is open circuit.
> 
> 
> OK. My first suspicion would be problem round the horizontal output
> stage/flyback transformer. But that is just a guess
> >
> >
> >
> > Of course the worry is, why? There could be a fault on the board. I
> > have tested the transistors in circuit with a multimeter and they
> > appear to be OK. I used a bench PSU to give the board 12V and it drew
> > no current (with all connectors disconnected). I tried again with the
> > round connector attached to the end of the tube and it drew about
> > 100mA and there was a faint glow from the neck of the tube.
> 
> The CRT has a heater fillament rated at about 11V or so.  Most, if not all, 
> such
> monitors run it from the12V input via a suitable resistor.
> So you know the CRT filament is good but you don't know much else so far.
> 
> 
> >
> >
> >
> > I am hesitant just to replace the fuse and try it. I am hoping for
> > some suggestions on how to test this safely (in particular without
> > involving the flyback transformer) to find if there is a fault.
> 
> I don't think you can do much without the flyback connector plugged in. You
> need the flyback transformer to do any sane tests on the horizontal side, and
> you need the iron-cored inductor, normally wired on the same connector, to
> get the vertical output stage to work. Also you may need the deflection yoke
> connected for some tests, the inductance of the horizontal deflection
> windings can make quite a difference (factor of 2) to the voltages produce by
> the flyback.
> 
> The flyback transformer started as a 'something for nothing' idea. To deflect
> the electron beam horizontally, you store considerable energy in the
> horizontal yok windings. Rather than waste that at the end of the line, you 
> use
> it to power other bits of the monitor, like the EHT for the CRT final anode.
> 
> Have you checked the diodes and capacitors associated with the flyback
> transformer? If something is shorted there. it's the equivaent of running a
> power supply into a short circuit. The input current will go up.

I have checked all the diodes (in circuit) and they appear to be OK. One 
capacitor may be suspect but without the full schematic I am not sure. I may 
have to lift it to check.

> 
> Does this unit have a horizontal oscllator? Quite a lot do not, they simply 
> use
> the horizontal pulses from the logic,suitably amplified, to drive the 
> horizontal
> output transistor. This could never have worked for television as interference
> pulses could send the thing crazy with voltages going all over the place, but
> it's not uncommon in small monitors. The IBM5151 was like that. If there is
> no horizontal oscillator then you need to provide a drive signal of the 
> correct
> frequency and duty cycle. Rather than use the logic (which might be faulty
> and thus mis-driving the monitor), I've been known to cobble something up
> using a 555 timer chip,
> 
> Here's roughtly what I would do :
> 
> Trace out a schematic of the board so I know what I am dealing with.

Yes, I may have to do that. Historically I have done a poor job of this because 
of my insufficient understanding of electronics.

> 
> Check all diodes and capactiors hung off the flyback transformer.

Agreed, will check carefully.

> 
> Ring-test the flyback transformer.

You sent me a circuit for this years ago and I built the device, so this is 
definitely something else I can do.

> 
> Make a test oscillator to drive it if necessary
> 
> Connect it up, run it from a current limited supply. If it tries to draw too 
> much
> current, then I've mssed something. Might tr

[cctalk] Re: VT100 Monitor Board

[Rob Jarratt via cctalk]

> -Original Message-
> From: Dave Dunfield via cctalk 
> Sent: 25 November 2023 14:06
> To: cctalk@classiccmp.org
> Cc: Dave Dunfield <1297.dunfi...@gmail.com>
> Subject: [cctalk] Re: VT100 Monitor Board
> 
> > Rob Jarratt 25 Nov 2023 8:47 a.m.
> > Of course the worry is, why? There could be a fault on the board.
> > I am hesitant just to replace the fuse and try it...
> 
> An older CRT terminal is probably a bit too much current draw for this, but
> you can prob use it in a setup to test parts...
> 
> A very handy gadget you can make very easily, which I use all the time when
> testing small devices in "unknown operational state" is a simple current
> limiter. (following discussion based on North America power, numbers may
> be different if you are in a different part of the world).
> 
> The "smallest" typical line circuit is 15A which is more than enough to cause
> damage to small devices experiencing excessive power draw through a fault
> (often indicated by a blown fuse).
> 
> The "limiter" relies on the fact that an incandescent light bulb will 
> draw/pass a
> fair bit of current when it is cold, and much less when it's warm. (this is
> because they are designed to "turn on" fast)

Thanks. I am familiar with the light bulb current limiter, although I must say 
I hadn't thought of using it in this case. I have used it when testing PSUs, 
but as this is downstream of the PSU I wonder if it would help? My bench PSU 
has current limiting abilities though, so I can provide current limited 
voltages within the range of the PSU. It is actually a twin PSU each with a 
0-30V range, so I probably could drive the board that way.

Thanks

Rob

> 
> In my case, I have three light sockets wired in parallel, all in series with 
> the hot
> side of a receptacle. This lets me change from a single 25w bulb (very little
> current possible) up to 3 100w bulbs (a good part of amp before it seriously
> limits). For example, 100w bulbs draw .833ish (100/120) when operating fully
> lit - x3 = 2.5A max current - this would only happen if the device under test
> was "shorted", presenting 0 series resistance and would therefore effectively
> have 0 volts across it.
> 
> In practice, you could prob. draw 1/2 amp (160ish ma per bulb) without
> warming them "too much" to seriously drop a lot of voltage. Much more than
> that and the bulbs will light up rather than hearing "popping" sounds from
> the device under test :-)
> 
> 
> -- Btw, I've given most of my CRT terminals away - For VT100's I use my
> "PC100"
> program - It provides very good VT100 emulation using an old DOS (or
> DosBox) PC - it remains "text" mode, so it turns "smooth scroll" into "slow
> scroll"
> and
> large fonts into "double spaced" fonts - but in all other respects nothing 
> I've
> used it on has been able to tell it's not an actual VT100!
> (I'm sure there are better/graphical VT100 emulations "out there")
> 
> Dave
> 
> --
> --
> Search "Dave's Old Computers" see "my personal" at bottom!

[cctalk] VT100 Monitor Board

[Rob Jarratt via cctalk]

Hello,

 

As some may recall I have been working on getting a VT100 going again. I
have made good progress and I think the main board is probably OK now (see
here if you are interested:
https://robs-old-computers.com/2023/11/19/vt100-keyboard-constant-clicking-f
ault/). Possibly I still need to replace the NVRAM, but I am leaving that
until I fix the problem I want to describe next.

 

The problem is that there is no image on the screen. This is because the
monitor board is not doing anything, there is no glow from the neck of the
tube etc. I have found that this is because the fuse on the 12V input to the
monitor board is open circuit.

 

Of course the worry is, why? There could be a fault on the board. I have
tested the transistors in circuit with a multimeter and they appear to be
OK. I used a bench PSU to give the board 12V and it drew no current (with
all connectors disconnected). I tried again with the round connector
attached to the end of the tube and it drew about 100mA and there was a
faint glow from the neck of the tube.

 

I am hesitant just to replace the fuse and try it. I am hoping for some
suggestions on how to test this safely (in particular without involving the
flyback transformer) to find if there is a fault.

 

For information, the monitor is an Elston and I pre-emptively replaced all
the electrolytics on the monitor board apart from the non-polar one. Some
details of what I did are here
https://robs-old-computers.com/2023/10/01/vt100-ram-fault/. Although I have
since realised that I didn't replace two of them because they looked like
diodes. I don't think the board I have is the one in the available
printsets.

 

Thanks

 

Rob

 

[cctalk] Re: VT100: Failing 2114 Chip Replaced With One With The Same Fault

[Rob Jarratt via cctalk]

Thanks Adrian,

 

I am not sure if the eprom programmer I have will do what you suggest, I don’t 
have an Arduino but I do have a Raspberry Pi that I could probably employ in 
testing the RAM. I may look into that.

 

I will look again at the writes. I have seen that the test pattern (0xAA) is 
written consistently to many other locations, including some to the seemingly 
faulty chip, so it is hard to think how the wrong data may be getting written, 
but I think I can check this fairly easily just to be sure.

 

Regards

 

Rob

 

From: Adrian Godwin  
Sent: 08 October 2023 12:38
To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 

Subject: Re: [cctalk] Re: VT100: Failing 2114 Chip Replaced With One With The 
Same Fault

 

Another possible approach is to trigger the logic analyser on a write access to 
that ram address, preferably with the probes on the ram itself. Look at the 
resulting captures .. does  it seem consistent with the code and other accesses 
?

 

 

On Sun, Oct 8, 2023 at 12:35 PM Adrian Godwin mailto:artgod...@gmail.com> > wrote:

Do you have one of those eprom programmers which also do device checks ? They 
might do a check of the supposed faulty ram out of circuit. If you don't have 
one you could probably write one for any convenient device you have to hand 
such as an arduino. Exercising the ram with port writes will be painfully slow 
compared with a normal ram test but with only 2K to test it shouldn't take too 
long.

 

 

On Sun, Oct 8, 2023 at 9:35 AM Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote:



> -Original Message-
> From: wrco...@wrcooke.net <mailto:wrco...@wrcooke.net>   <mailto:wrco...@wrcooke.net> >
> Sent: 08 October 2023 04:15
> To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; Rob Jarratt via cctalk 
> mailto:cctalk@classiccmp.org> >
> Subject: Re: [cctalk] VT100: Failing 2114 Chip Replaced With One With The
> Same Fault
> 
> 
> 
> > On 10/07/2023 5:35 PM CDT Rob Jarratt via cctalk  > <mailto:cctalk@classiccmp.org> >
> wrote:
> >
> >
> > I find this really hard to explain. It can't be the chip selection
> > logic because then the addresses 0x2400-0x2407 would also fail and I
> > checked the CS signal with the logic analyser just to be sure. I also
> > checked the address lines directly on the RAM chip for any stuck bits
> > and they seemed fine too.
> >
> >
> >
> > What are the chances of two 2114 chips failing at exactly the same address?
> > Is there some failure mode I might not be considering?
> >
> > Rob
> 
> Perhaps it isn't the 2114 or its associated circuit at all.  Maybe some other
> device is being incorrectly selected by that address and driving (half) the 
> bus
> low?  Just a thought.

Many thanks for the suggestion. This hadn't crossed my mind, so I checked. All 
the things that I could identify on the schematic that connect to the bus 
(UART, interrupt vector, flag buffer and modem signals) seem not to be enabled. 
I have looked at what is sinking the data bus, there is a buffer which seems to 
be OK and the 8251 PIC. The PIC is harder to check but I can see it is not 
selected and the input pins don’t appear to be shorted.

Not really sure what else to consider.

> 
> Will
> 
> If you want to build a ship, don't drum up people to collect wood and don't
> assign them tasks and work, but rather teach them to long for the endless
> immensity of the sea.
> 
> Antoine de Saint-Exupery

[cctalk] Re: VT100: Failing 2114 Chip Replaced With One With The Same Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: wrco...@wrcooke.net 
> Sent: 08 October 2023 04:15
> To: r...@jarratt.me.uk; Rob Jarratt via cctalk 
> Subject: Re: [cctalk] VT100: Failing 2114 Chip Replaced With One With The
> Same Fault
> 
> 
> 
> > On 10/07/2023 5:35 PM CDT Rob Jarratt via cctalk 
> wrote:
> >
> >
> > I find this really hard to explain. It can't be the chip selection
> > logic because then the addresses 0x2400-0x2407 would also fail and I
> > checked the CS signal with the logic analyser just to be sure. I also
> > checked the address lines directly on the RAM chip for any stuck bits
> > and they seemed fine too.
> >
> >
> >
> > What are the chances of two 2114 chips failing at exactly the same address?
> > Is there some failure mode I might not be considering?
> >
> > Rob
> 
> Perhaps it isn't the 2114 or its associated circuit at all.  Maybe some other
> device is being incorrectly selected by that address and driving (half) the 
> bus
> low?  Just a thought.

Many thanks for the suggestion. This hadn't crossed my mind, so I checked. All 
the things that I could identify on the schematic that connect to the bus 
(UART, interrupt vector, flag buffer and modem signals) seem not to be enabled. 
I have looked at what is sinking the data bus, there is a buffer which seems to 
be OK and the 8251 PIC. The PIC is harder to check but I can see it is not 
selected and the input pins don’t appear to be shorted.

Not really sure what else to consider.

> 
> Will
> 
> If you want to build a ship, don't drum up people to collect wood and don't
> assign them tasks and work, but rather teach them to long for the endless
> immensity of the sea.
> 
> Antoine de Saint-Exupery

[cctalk] VT100: Failing 2114 Chip Replaced With One With The Same Fault

[Rob Jarratt via cctalk]

I posted recently that I had identified a faulty RAM chip in my VT100 and
having replaced it the terminal seemed to get further into the self test.
After further analysis with the logic analyser I have realised that it is
still failing the RAM test, despite an apparent change in behaviour. I think
the change in behaviour could be simply due to the slightly suspect keyboard
cable.

 

But here is the puzzle. When I first identified the faulty RAM chip if found
that address 0x2408 would read back as 0x0A instead of 0xAA. I reckon this
equates to E50 in the schematic, as it is the upper nibble of the second
bank of RAM. I replaced the chip with one I bought recently. It turns out
the self-test is still failing at the SAME address.

 

I find this really hard to explain. It can't be the chip selection logic
because then the addresses 0x2400-0x2407 would also fail and I checked the
CS signal with the logic analyser just to be sure. I also checked the
address lines directly on the RAM chip for any stuck bits and they seemed
fine too.

 

What are the chances of two 2114 chips failing at exactly the same address?
Is there some failure mode I might not be considering?

 

Thanks

 

Rob

[cctalk] Possible DC012 Control Chip Fault on a VT100

[Rob Jarratt via cctalk]

I have my faulty VT100 now passing the RAM test, but it is still not
producing any output to the screen. Looking at the DC012 chip it seems to be
permanently asserting the interrupt line and not generating any DMA HOLD
REQUEST signals. It seems that all the clock inputs are running (DOT CLK,
CHAR CLK, VERT RESET). Unless there are other clock inputs not shown on the
schematic, I think this suggests that the DC012 is faulty.

 

Does anyone know of any other reason why the DC012 might behave this way?
Anyone have a spare DC012 chip?

 

I have posted a bit more detail here:
https://robs-old-computers.com/2023/10/01/vt100-ram-fault/

 

Thanks

 

Rob

[cctalk] Tracking Down a RAM Issue in a VT100 (how does the 2114 chip work?)

[Rob Jarratt via cctalk]

I have a non-functioning VT100. I think it is failing in the POST during the
RAM check. I don't know for sure because I can't get it to light up the LEDs
on the keyboard, however I used my logic analyser (a HP1630G) to see what
values were written to the UART to send to the keyboard and I see it sends
the values FF then 1, 2, 3, 4 and finally 5. The last value corresponds to
the RAM test so I am fairly confident the RAM test is what is failing. 

 

I have disassembled the VT100 ROM and if I have understood it correctly it
zeroes out the RAM (high address to low) and then for each address (low
address to high) it tries first to read back the zero and then writes 0xAA
to the location and tries to read that back.

 

I am also confident the 8080 is working OK because I was able to capture an
address trace on the ROM that showed it executing the program as per the
disassembled ROM. 

 

My problem is getting the logic analyser reliably to tell me how each RAM
chip is being addressed and what data is being read or written. I am seeing
strange values for the addresses (sometimes) and I am not sure I have setup
the logic analyser correctly. I have read the datasheet for the 2114 chip
and I am not entirely clear that I have understood it correctly. Here is how
I have set it up:

 

Trigger on the -ve edge of Chip Select (pin 8)

Capture A9-A0 as the address

Capture WE as an indication of Read or Write

 

The timing diagrams show the write cycle where the WE signal and CE signal
seem to transition at the same time and the data may be only valid a bit
later then the CS -ve edge. But this may just be me not knowing how to read
the datasheet. Using the +ve edge of CS seems no better.

 

Is there something I am missing about how to analyse how the RAM chips are
being used?

 

Thanks

 

Rob

[cctalk] Re: VT100: Datasheet for Intel 8228

[Rob Jarratt via cctalk]

Thanks for the suggestions but I think I was already aware of all of those. I 
have double checked against your list and still don’t see the pulses with the 
DSO. There has to be something else I am doing wrong ☹

 

From: Tom Hunter  
Sent: 17 July 2023 06:24
To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 

Subject: Re: [cctalk] Re: VT100: Datasheet for Intel 8228

 

Common DSO trigger problems experienced by a novice are:

 

1) Trigger position is off the screen. The trigger position relative to the 
current visible capture is typically shown somewhere on the screen - often at 
the top of the screen. Reduce the horizontal resolution to something like 10 
ms/division and then scroll the trigger position to be in the exact centre of 
the screen and then increase the horizontal resolution to something suitable 
e.g 500 ns/division. Typically the DSO will keep the trigger position centred 
if you had it close enough to the centre position before increasing the 
horizontal resolution.

 

2) The horizontal resolution is set so that the pulse is invisible. For example 
1 ms/division for a 10 ns pulse.

 

3) Triggering on the wrong channel.

 

4) Trigger level above or below the signal.

 

5) Trigger mode set to "Auto" as opposed to "Normal" or "Single" causing a 
momentary display of the triggered signal which is immediately overwritten by 
the current signal.

 

Tom Hunter

 

On Mon, Jul 17, 2023 at 1:14 AM Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote:

I used a logic analyser to check the 8228 and it does appear to be working as 
expected as can be seen here 
https://rjarratt.files.wordpress.com/2023/07/8228-operation.jpg. The labels are 
not very clear, but the 3rd from top is the WR input and the 4th from top is 
the I/O W output. So it does look like it is working.

The odd thing is that I can't pick this up with my DSO. I have tried messing 
with the trigger, but I just can't pick up anything that shows I/O W going low. 
I am using -ve edge triggers. I have tried a pulse trigger with a width >20ns 
and I have tried nth edge trigger, neither of the latter two ever trigger. 
Obviously it is my method that is wrong, but I really don't know what it is 
about my method that is wrong. Are there any common mistakes that novices make?

Thanks

[cctalk] Re: VT100: Datasheet for Intel 8228

[Rob Jarratt via cctalk]

I used a logic analyser to check the 8228 and it does appear to be working as 
expected as can be seen here 
https://rjarratt.files.wordpress.com/2023/07/8228-operation.jpg. The labels are 
not very clear, but the 3rd from top is the WR input and the 4th from top is 
the I/O W output. So it does look like it is working.

The odd thing is that I can't pick this up with my DSO. I have tried messing 
with the trigger, but I just can't pick up anything that shows I/O W going low. 
I am using -ve edge triggers. I have tried a pulse trigger with a width >20ns 
and I have tried nth edge trigger, neither of the latter two ever trigger. 
Obviously it is my method that is wrong, but I really don't know what it is 
about my method that is wrong. Are there any common mistakes that novices make?

Thanks

> -Original Message-
> From: Rob Jarratt 
> Sent: 09 July 2023 20:51
> To: 'Jonathan Chapman' ; r...@jarratt.me.uk;
> 'General Discussion: On-Topic and Off-Topic Posts' 
> Subject: RE: [cctalk] Re: VT100: Datasheet for Intel 8228
> 
> Thanks for confirming that HLDA doesn't need to be asserted at this stage.
> 
> I looked at the address trace and matched it to the disassembled ROM. My
> ROM image compares to another ROM image that has been posted online at
> 9track.net, so I am fairly confident that the CPU and ROM are OK.
> 
> I don't think that the RAM test is passing though, it might not even be able 
> to
> start it, I have not traced this sufficiently to be sure, but it is in a loop 
> at one
> point, but not before sending data to the keyboard UART. That is what made
> me look at the 8228 because I traced back that far to where something did
> not seem be happening correctly.
> 
> In fact, I now realise that I should have checked MEM WR and MEM RD. I can
> see that MEM WR is never asserted, but MEM RD is asserted. I do see WR
> being asserted on the input side immediately after startup, but then it isn't
> asserted again.
> 
> It is possible that the firmware is in a loop before it gets to the RAM test,
> possibly because the 8228 driver is not working.
> 
> Regards
> 
> Rob
> 
> > -Original Message-
> > From: Jonathan Chapman 
> > Sent: Sunday, July 9, 2023 5:18 PM
> > To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > Posts 
> > Subject: Re: [cctalk] Re: VT100: Datasheet for Intel 8228
> >
> > HLDA should be deasserted, that puts the processor in a DMA mode where
> > it releases all the control signals.
> >
> > I/O W is asserted low when OUT is high and the low WR pulse comes
> along.
> > It's both status and timing, coming out of the 8228.
> >
> > You would probably do well seeing if MEMR and MEMW are asserting. If
> > the program is running off due to e.g. bad RAM, ROM bitrot, etc. you
> > may never see the I/O lines assert.
> >
> > Thanks,
> > Jonathan
> >
> > --- Original Message ---
> > On Sunday, July 9th, 2023 at 12:12, Rob Jarratt via cctalk
> >  wrote:
> >
> >
> > >
> > >
> > > Actually I have just found a more detailed description in the Intel
> > > 8080 Microcomputer Systems Users Manual 1975, but it still doesn't
> > > tell me exactly how it works. It isn't completely clear to me if it
> > > needs HLDA to be asserted for I/O W to be asserted.
> > >
> > > > -Original Message-
> > > > From: Rob Jarratt via cctalk cctalk@classiccmp.org
> > > > Sent: Sunday, July 9, 2023 4:39 PM
> > > > To: General Discussion: On-Topic and Off-Topic Posts
> > > > cctalk@classiccmp.org
> > > > Cc: Rob Jarratt robert.jarr...@ntlworld.com
> > > > Subject: [cctalk] VT100: Datasheet for Intel 8228
> > > >
> > > > Hello,
> > > >
> > > > I have a non-functioning VT100. I think I may have isolated the
> > > > problem to
> > >
> > > an
> > >
> > > > Intel 8228 chip (or 88228, the schematic says 8228, the part is
> > > > marked 88228C). Certainly, the part gets a bit hot and it doesn't
> > > > seem to be outputting anything on the I/O W pin (pin 27) despite
> > > > activity on STSTB
> > >
> > > (pin
> > >
> > > > 1), DBIN (pin 4) and WR (Pin 3). There is no activity on the HLDA
> > > > input though, but I am not sure if that is required because I
> > > > think the firmware
> > >
> > > is
> > >
> > > > just trying to send its status to the keyboard LEDs.
> > > >
> > > > I can find a brief datasheet for the 8228 but it doesn't tell me
> > > > the logic
> > >
> > > for
> > >
> > > > producing the I/O W signal, so I am not sure if it is behaving as
> > > > it
> > >
> > > should.
> > >
> > > > Does anyone have more comprehensive information on how the 8228 is
> > > > supposed to work?
> > > >
> > > > I have dumped the ROMs and been able to capture the ROM reads and
> > > > they match the disassembled code, so I think the 8080 CPU itself
> > > > is
> > working.
> > > >
> > > > Thanks
> > > >
> > > > Rob

[cctalk] Re: VT100: Datasheet for Intel 8228

[Rob Jarratt via cctalk]

Thanks for confirming that HLDA doesn't need to be asserted at this stage.

I looked at the address trace and matched it to the disassembled ROM. My ROM 
image compares to another ROM image that has been posted online at 9track.net, 
so I am fairly confident that the CPU and ROM are OK.

I don't think that the RAM test is passing though, it might not even be able to 
start it, I have not traced this sufficiently to be sure, but it is in a loop 
at one point, but not before sending data to the keyboard UART. That is what 
made me look at the 8228 because I traced back that far to where something did 
not seem be happening correctly.

In fact, I now realise that I should have checked MEM WR and MEM RD. I can see 
that MEM WR is never asserted, but MEM RD is asserted. I do see WR being 
asserted on the input side immediately after startup, but then it isn't 
asserted again.

It is possible that the firmware is in a loop before it gets to the RAM test, 
possibly because the 8228 driver is not working.

Regards

Rob

> -Original Message-
> From: Jonathan Chapman 
> Sent: Sunday, July 9, 2023 5:18 PM
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: [cctalk] Re: VT100: Datasheet for Intel 8228
> 
> HLDA should be deasserted, that puts the processor in a DMA mode where it
> releases all the control signals.
> 
> I/O W is asserted low when OUT is high and the low WR pulse comes along.
> It's both status and timing, coming out of the 8228.
> 
> You would probably do well seeing if MEMR and MEMW are asserting. If the
> program is running off due to e.g. bad RAM, ROM bitrot, etc. you may never
> see the I/O lines assert.
> 
> Thanks,
> Jonathan
> 
> --- Original Message ---
> On Sunday, July 9th, 2023 at 12:12, Rob Jarratt via cctalk
>  wrote:
> 
> 
> >
> >
> > Actually I have just found a more detailed description in the Intel
> > 8080 Microcomputer Systems Users Manual 1975, but it still doesn't
> > tell me exactly how it works. It isn't completely clear to me if it
> > needs HLDA to be asserted for I/O W to be asserted.
> >
> > > -Original Message-
> > > From: Rob Jarratt via cctalk cctalk@classiccmp.org
> > > Sent: Sunday, July 9, 2023 4:39 PM
> > > To: General Discussion: On-Topic and Off-Topic Posts
> > > cctalk@classiccmp.org
> > > Cc: Rob Jarratt robert.jarr...@ntlworld.com
> > > Subject: [cctalk] VT100: Datasheet for Intel 8228
> > >
> > > Hello,
> > >
> > > I have a non-functioning VT100. I think I may have isolated the
> > > problem to
> >
> > an
> >
> > > Intel 8228 chip (or 88228, the schematic says 8228, the part is
> > > marked 88228C). Certainly, the part gets a bit hot and it doesn't
> > > seem to be outputting anything on the I/O W pin (pin 27) despite
> > > activity on STSTB
> >
> > (pin
> >
> > > 1), DBIN (pin 4) and WR (Pin 3). There is no activity on the HLDA
> > > input though, but I am not sure if that is required because I think
> > > the firmware
> >
> > is
> >
> > > just trying to send its status to the keyboard LEDs.
> > >
> > > I can find a brief datasheet for the 8228 but it doesn't tell me the
> > > logic
> >
> > for
> >
> > > producing the I/O W signal, so I am not sure if it is behaving as it
> >
> > should.
> >
> > > Does anyone have more comprehensive information on how the 8228 is
> > > supposed to work?
> > >
> > > I have dumped the ROMs and been able to capture the ROM reads and
> > > they match the disassembled code, so I think the 8080 CPU itself is
> working.
> > >
> > > Thanks
> > >
> > > Rob

[cctalk] Re: VT100: Datasheet for Intel 8228

[Rob Jarratt via cctalk]

Actually I have just found a more detailed description in the Intel 8080
Microcomputer Systems Users Manual 1975, but it still doesn't tell me
exactly how it works. It isn't completely clear to me if it needs HLDA to be
asserted for I/O W to be asserted.

> -Original Message-
> From: Rob Jarratt via cctalk 
> Sent: Sunday, July 9, 2023 4:39 PM
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Rob Jarratt 
> Subject: [cctalk] VT100: Datasheet for Intel 8228
> 
> Hello,
> 
> 
> 
> I have a non-functioning VT100. I think I may have isolated the problem to
an
> Intel 8228 chip (or 88228, the schematic says 8228, the part is marked
> 88228C). Certainly, the part gets a bit hot and it doesn't seem to be
> outputting anything on the I/O W pin (pin 27) despite activity on STSTB
(pin
> 1), DBIN (pin 4) and WR (Pin 3). There is no activity on the HLDA input
> though, but I am not sure if that is required because I think the firmware
is
> just trying to send its status to the keyboard LEDs.
> 
> 
> 
> I can find a brief datasheet for the 8228 but it doesn't tell me the logic
for
> producing the I/O W signal, so I am not sure if it is behaving as it
should.
> Does anyone have more comprehensive information on how the 8228 is
> supposed to work?
> 
> 
> 
> I have dumped the ROMs and been able to capture the ROM reads and they
> match the disassembled code, so I think the 8080 CPU itself is working.
> 
> 
> 
> Thanks
> 
> 
> 
> Rob

[cctalk] VT100: Datasheet for Intel 8228

[Rob Jarratt via cctalk]

Hello,

 

I have a non-functioning VT100. I think I may have isolated the problem to
an Intel 8228 chip (or 88228, the schematic says 8228, the part is marked
88228C). Certainly, the part gets a bit hot and it doesn't seem to be
outputting anything on the I/O W pin (pin 27) despite activity on STSTB (pin
1), DBIN (pin 4) and WR (Pin 3). There is no activity on the HLDA input
though, but I am not sure if that is required because I think the firmware
is just trying to send its status to the keyboard LEDs.

 

I can find a brief datasheet for the 8228 but it doesn't tell me the logic
for producing the I/O W signal, so I am not sure if it is behaving as it
should. Does anyone have more comprehensive information on how the 8228 is
supposed to work?

 

I have dumped the ROMs and been able to capture the ROM reads and they match
the disassembled code, so I think the 8080 CPU itself is working.

 

Thanks

 

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Thank you to everyone who helped me with this. The PSU if finally fixed. I have 
posted about it on my blog: 
https://robs-old-computers.com/2023/06/29/h7842-finally-fixed/

Regards

Rob

> -Original Message-
> From: Rob Jarratt 
> Sent: 25 June 2023 18:07
> To: 'Tony Duell' ; r...@jarratt.me.uk
> Cc: 'General Discussion: On-Topic and Off-Topic Posts'
> ; 'Brent Hilpert' 
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> 
> 
> > -Original Message-
> > From: Tony Duell 
> > Sent: 25 June 2023 18:00
> > To: r...@jarratt.me.uk
> > Cc: General Discussion: On-Topic and Off-Topic Posts
> > ; Brent Hilpert 
> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> > On Sun, Jun 25, 2023 at 5:52 PM Rob Jarratt
> > 
> > wrote:
> > >
> > > Before I confuse matters too much I should point out that I was not
> > > using
> > the Rainbow power switch unit but one from a DECmate. I had
> > erroneously thought there was a fault with the Rainbow one. I think
> > the fan supply may be different on the DECmate switch, so ignore the bit
> about the fan turning.
> >
> > It appears that there are 2 ways of powering (different types of) fans
> > with this power supply. Whether one was used in the Rainbow and the
> > other in the Decmate I know not. But anyway :
> >
> > A 12V DC fan running off the DC output of the power supply. This is
> > the one shown in my schemtic for the supply. Note the jumper link in
> > the fan power plug that means the supply doesn't get mains if you forget to
> plug the fan in.
> 
> That is what I failed to notice and I thought that somehow there was a fault 
> in
> the switch unit.
> 
> >
> > A 115V AC fan connected to the unused pair of pins on the AC input
> > cable to the power supply (top right of the 'switch/fan assy' in my
> > schematc. This uses the primary winding of the startup transformer as
> > an autotransformer on 230V mains.
> >
> > Note that the latter fan will run if the chopper side of things is
> > dead, the former won't.
> >
> > -tony

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Tony Duell 
> Sent: 25 June 2023 18:00
> To: r...@jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts
> ; Brent Hilpert 
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> On Sun, Jun 25, 2023 at 5:52 PM Rob Jarratt 
> wrote:
> >
> > Before I confuse matters too much I should point out that I was not using
> the Rainbow power switch unit but one from a DECmate. I had erroneously
> thought there was a fault with the Rainbow one. I think the fan supply may
> be different on the DECmate switch, so ignore the bit about the fan turning.
> 
> It appears that there are 2 ways of powering (different types of) fans with 
> this
> power supply. Whether one was used in the Rainbow and the other in the
> Decmate I know not. But anyway :
> 
> A 12V DC fan running off the DC output of the power supply. This is the one
> shown in my schemtic for the supply. Note the jumper link in the fan power
> plug that means the supply doesn't get mains if you forget to plug the fan in.

That is what I failed to notice and I thought that somehow there was a fault in 
the switch unit. 

> 
> A 115V AC fan connected to the unused pair of pins on the AC input cable to
> the power supply (top right of the 'switch/fan assy' in my schematc. This uses
> the primary winding of the startup transformer as an autotransformer on
> 230V mains.
> 
> Note that the latter fan will run if the chopper side of things is dead, the
> former won't.
> 
> -tony

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Yes the chopper did blow up and I have since replaced it. So hopefully this is 
the last part that needs to be replaced!

Regards

Rob

> -Original Message-
> From: Tony Duell 
> Sent: 25 June 2023 17:52
> To: r...@jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts
> ; Brent Hilpert 
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> On Sun, Jun 25, 2023 at 5:33 PM Rob Jarratt 
> wrote:
> >
> > Thanks Tony. I removed the mains bridge rectifier and found it measured
> short across two of its terminals, so I am ordering a replacement. I tested 
> the
> PSU, without the mains bridge rectifier and without the diodes on the output
> of the startup transformer. I got 24VAC on the output of the , the lamp 
> limiter
> bulbs did not light and the fan in the power switch unit turned. So it looks 
> like
> the startup transformer is actually OK.
> >
> 
> 
> It's possible that the rectifier just failed. Or it might have been damaged
> because it had to pass too much current due to a problem in the main
> chopper circuit. I would certainly check the chopper transistor for shorts
> now.
> 
> -tony

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Before I confuse matters too much I should point out that I was not using the 
Rainbow power switch unit but one from a DECmate. I had erroneously thought 
there was a fault with the Rainbow one. I think the fan supply may be different 
on the DECmate switch, so ignore the bit about the fan turning.

Regards

Rob

> -Original Message-
> From: Rob Jarratt 
> Sent: 25 June 2023 17:34
> To: 'Tony Duell' ; 'r...@jarratt.me.uk'
> ; 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Cc: 'Brent Hilpert' 
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> Thanks Tony. I removed the mains bridge rectifier and found it measured
> short across two of its terminals, so I am ordering a replacement. I tested 
> the
> PSU, without the mains bridge rectifier and without the diodes on the output
> of the startup transformer. I got 24VAC on the output of the , the lamp 
> limiter
> bulbs did not light and the fan in the power switch unit turned. So it looks 
> like
> the startup transformer is actually OK.
> 
> Regards
> 
> Rob
> 
> .> -Original Message-
> > From: Tony Duell 
> > Sent: 25 June 2023 14:21
> > To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > Posts 
> > Cc: Brent Hilpert ; Rob Jarratt
> > 
> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> > On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk
> >  wrote:
> > >
> > > Hello,
> > >
> > > It has taken me ages to get back to this, but I think I have hit an
> > > insurmountable problem.
> > >
> > > I reassembled the PSU after the PWM started working again and tested
> > > it with a dummy load and using two 100W light bulbs in series on the
> > > input side to limit the current. However, it did not work. I traced
> > > this to the 7812 regulator that supplies Vstart (PSU Sheet 1) not
> > > getting enough voltage to run, about 3.7VDC. In the same conditions
> > > the working one gets about 10 or 11VDC. I also noticed that on the
> > > good PSU the 100W bulbs pulsed and were fairly dim, but on the bad
> > > PSU
> > they were brighter and glowed steadily.
> > >
> > > This made me suspect that the startup transformer may have been
> > > damaged. So I checked the output of the startup transformer. On the
> > > bad PSU this was about 2.4VAC, while on the working one it was about
> > > 12VAC. I lifted all 4 of the diodes connected to the startup
> > > transformer and
> > they all tested OK.
> > >
> > > Sadly, I think this means the transformer must have been damaged. I
> > > have the facility to do a ring test on the transformer, but don't
> > > want to remove it unless there is a need to. Is there any other
> > > failure mode that I am not considering? I didn't check the inputs to
> > > the transformer, so something could be short on the input side and
> > > pulling the input to the startup transformer down. It is worth
> > > reminding anyone reading this, that the 7812 on PSU Sheet 1 did fail
> > > with a short to
> > ground.
> >
> > I wouldn't be too hasty to condemn the transformer.
> >
> > This is a mains transformer, not part of a switch mode power supply
> > circuit. It takes in mains via the voltage selector switch, the centre
> > tapped secondary is full-wave rectified twice to produce the +12V and
> > -12V rails needed to get the rest of the supply going.
> >
> > Now, you mention the lamp limiter bulbs (series light bulbs, whatever
> > you call
> > them) are bright. That means they are dropping significant voltage. So
> > the input voltage to the startup  transformer will be less than mains.
> > Which will lead to a low output from its secondary winding.
> >
> > Try disconnectng the mains feed to the main chopper circuit. Perhaps
> > remove the mains bridge rectifier, or both of the inrush limiter
> > thermistors. Anything to prevent power getting to the main chopper
> > circuit. Power up again with the lamp limiter in series with the mains
> > (of course). My guess is that then the bulbs will be dark and the startup
> transformer will give the right voltages.
> > This would point to a problem with  the chopper circuit.
> >
> > -tony
> >
> >
> >
> >
> > >
> > > I have included a couple of minor responses to Brent's last email to
> > > me below.
> > >
> > > Regards
> > >
> > > Rob
> > >
> > > > -Original Message-
> > > > From: Brent Hilper

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Thanks Tony. I removed the mains bridge rectifier and found it measured short 
across two of its terminals, so I am ordering a replacement. I tested the PSU, 
without the mains bridge rectifier and without the diodes on the output of the 
startup transformer. I got 24VAC on the output of the , the lamp limiter bulbs 
did not light and the fan in the power switch unit turned. So it looks like the 
startup transformer is actually OK. 

Regards

Rob

.> -Original Message-
> From: Tony Duell 
> Sent: 25 June 2023 14:21
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Brent Hilpert ; Rob Jarratt
> 
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk
>  wrote:
> >
> > Hello,
> >
> > It has taken me ages to get back to this, but I think I have hit an
> > insurmountable problem.
> >
> > I reassembled the PSU after the PWM started working again and tested
> > it with a dummy load and using two 100W light bulbs in series on the
> > input side to limit the current. However, it did not work. I traced
> > this to the 7812 regulator that supplies Vstart (PSU Sheet 1) not
> > getting enough voltage to run, about 3.7VDC. In the same conditions
> > the working one gets about 10 or 11VDC. I also noticed that on the
> > good PSU the 100W bulbs pulsed and were fairly dim, but on the bad PSU
> they were brighter and glowed steadily.
> >
> > This made me suspect that the startup transformer may have been
> > damaged. So I checked the output of the startup transformer. On the
> > bad PSU this was about 2.4VAC, while on the working one it was about
> > 12VAC. I lifted all 4 of the diodes connected to the startup transformer and
> they all tested OK.
> >
> > Sadly, I think this means the transformer must have been damaged. I
> > have the facility to do a ring test on the transformer, but don't want
> > to remove it unless there is a need to. Is there any other failure
> > mode that I am not considering? I didn't check the inputs to the
> > transformer, so something could be short on the input side and pulling
> > the input to the startup transformer down. It is worth reminding
> > anyone reading this, that the 7812 on PSU Sheet 1 did fail with a short to
> ground.
> 
> I wouldn't be too hasty to condemn the transformer.
> 
> This is a mains transformer, not part of a switch mode power supply circuit. 
> It
> takes in mains via the voltage selector switch, the centre tapped secondary is
> full-wave rectified twice to produce the +12V and -12V rails needed to get the
> rest of the supply going.
> 
> Now, you mention the lamp limiter bulbs (series light bulbs, whatever you call
> them) are bright. That means they are dropping significant voltage. So the
> input voltage to the startup  transformer will be less than mains. Which will
> lead to a low output from its secondary winding.
> 
> Try disconnectng the mains feed to the main chopper circuit. Perhaps remove
> the mains bridge rectifier, or both of the inrush limiter thermistors. 
> Anything
> to prevent power getting to the main chopper circuit. Power up again with
> the lamp limiter in series with the mains (of course). My guess is that then
> the bulbs will be dark and the startup transformer will give the right 
> voltages.
> This would point to a problem with  the chopper circuit.
> 
> -tony
> 
> 
> 
> 
> >
> > I have included a couple of minor responses to Brent's last email to
> > me below.
> >
> > Regards
> >
> > Rob
> >
> > > -Original Message-
> > > From: Brent Hilpert 
> > > Sent: 26 May 2023 17:24
> > > To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > > Posts 
> > > Subject: Re: [cctalk] Rainbow H7842 PSU Fault
> > >
> > > On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
> > > >
> > > > This evening I went to check Vstart for any oscillation. However,
> > > > all of
> > a
> > > sudden, the current draw is down to 85mA and PWM has started
> > > working. I am at a loss to explain it. I wondered if there might be
> > > a dry joint, but
> > I have
> > > tried a few light taps and shakes and it continues to work. Perhaps
> > > your
> > idea
> > > of some debris causing a short might explain it, otherwise I just
> > > don't
> > know.
> > >
> > > Operation with only VStart+12 places the circuitry into an
> > > unspecified operating region - a region outside of the design
> > > inte

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Jon Elson via cctalk 
> Sent: 25 June 2023 16:24
> To: Rob Jarratt via cctalk 
> Cc: Jon Elson 
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> On 6/25/23 07:36, Rob Jarratt via cctalk wrote:
> > This made me suspect that the startup transformer may have been
> > damaged. So I checked the output of the startup transformer. On the
> > bad PSU this was about 2.4VAC, while on the working one it was about
> > 12VAC. I lifted all 4 of the diodes connected to the startup transformer and
> they all tested OK.
> >
> Well, apply current-limited 12 V DC to the output of the bridge rectifier and
> see what current it draws.
> 
> Could be a failed capacitor or regulator, or something downstream of the
> regulator that has failed.

If you mean the 7812 on PSU Sheet 1 then I believe it is fine as I have already 
tested it with a bench PSU supplying it with 15VDC and it works fine.

> 
> (Maybe you have already checked this.)
> 
> Jon

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Hello,

It has taken me ages to get back to this, but I think I have hit an
insurmountable problem.

I reassembled the PSU after the PWM started working again and tested it with
a dummy load and using two 100W light bulbs in series on the input side to
limit the current. However, it did not work. I traced this to the 7812
regulator that supplies Vstart (PSU Sheet 1) not getting enough voltage to
run, about 3.7VDC. In the same conditions the working one gets about 10 or
11VDC. I also noticed that on the good PSU the 100W bulbs pulsed and were
fairly dim, but on the bad PSU they were brighter and glowed steadily.

This made me suspect that the startup transformer may have been damaged. So
I checked the output of the startup transformer. On the bad PSU this was
about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of
the diodes connected to the startup transformer and they all tested OK.

Sadly, I think this means the transformer must have been damaged. I have the
facility to do a ring test on the transformer, but don't want to remove it
unless there is a need to. Is there any other failure mode that I am not
considering? I didn't check the inputs to the transformer, so something
could be short on the input side and pulling the input to the startup
transformer down. It is worth reminding anyone reading this, that the 7812
on PSU Sheet 1 did fail with a short to ground.

I have included a couple of minor responses to Brent's last email to me
below.

Regards

Rob

> -Original Message-
> From: Brent Hilpert 
> Sent: 26 May 2023 17:24
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: [cctalk] Rainbow H7842 PSU Fault
> 
> On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
> >
> > This evening I went to check Vstart for any oscillation. However, all of
a
> sudden, the current draw is down to 85mA and PWM has started working. I
> am at a loss to explain it. I wondered if there might be a dry joint, but
I have
> tried a few light taps and shakes and it continues to work. Perhaps your
idea
> of some debris causing a short might explain it, otherwise I just don't
know.
> 
> Operation with only VStart+12 places the circuitry into an unspecified
> operating region - a region outside of the design intentions. In part,
several
> semiconductor junctions and portions of circuitry are polarised opposite
to
> their normal/designed-for state. It is not surprising that you are seeing
> odd/unpredictable behaviour under this operating environment, nor is it
> surprising that it's different than the 'good' supply under the same
operating
> environment.
> 
> So why was it in shutdown earlier the other day but not now? :
> Who knows - it's operating in an unspecified region. Perhaps the room
> temperature is 2 degrees higher. That's a serious point, not phase-of-the-
> moon satire.

Well, the weather here has indeed got a bit warmer.

> 
> When you supplied the proper startup environment with both Vstart+12 and
> Vstart-12 both the bad and good unit behaved as expected for the design.
> 
> Why is the VStart+12 current draw higher when it was in shutdown versus
> when the PWM controller IC is pulsing? :
> Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held
> hard ON (conducting) (see datasheet).
> Holding this transistor ON subtracts it's off-state current (~ 17mA) but
adds
> it's on-state base current (~ 37mA) and it's on-state collector current (~
> 73mA), for a net up-to ~ 93mA increase (may be less dependant on duty
> cycle of PWM), to the Vstart+12 current.
> 

Thanks for explaining that, it makes more sense now. I need to learn to look
at other parts of the schematic beyond the immediate thing I am interested
in because I keep missing things like this.

> There remain two unexplained things here:
>   - Where was that unusual current-sense voltage that sends it into
> shutdown coming from?
>   (I provided one potential explanation earlier, but it remains
unknown
> at this time).
> 
>   - The 51-ohm current-sense resistor in the -12V supply vs the mode
> of operation of
>   the -12 supply remains unexplained/non-sensical. At the max current
> you mentioned (150mA),
>   the V drop across that R would be >7V (!), which makes no sense. If
I
> had it in hand, I'd be
>   double-checking the drawing of that current-sense circuit around the
> 51-ohm R as a start.

Agreed that this does not make a lot of sense. I have checked the actual
value of the resistor (in circuit) and it matches the schematic. I have also
checked the schematic against the actual circuit and it really does seem
like the schematic is correct.

> 
> But this is not to say that either of these has anything to do with the
fault you
> were/are dealing with, they'

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 20 May 2023 09:20
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> 
> 
> Ok, it looks like there is not a severe leak from the -12V line to ground 
> then.
> 
> I am puzzled by the extra current draw on Vstart by the bad PSU but I'm not
> sure that tracking this down would lead us to the real problem.
> 
> On the other hand, did you mention at one point that Vstart was varying?
> If this is the case, the reason for this would probably need to be found and
> fixed independent of whether it leads to finding the main problem as this is
> supposed to be a stable supply.
> 
> I don't think there is likely to be any serious leakage via E1b because the 
> link
> to the -12V line is via a 75K resistor which would limit any leakage current 
> to
> roughly 160uA.  Of course this applies if the resistor really is 75K and 
> doesn't
> have carbon deposits bridging the tracks and connections around it to
> somewhere else.
> 
> I would suggest looking carefully at the resistors around E3d to make sure
> they have the correct values, especially the 360K resistor and making sure
> there is no debris etc around these components that could be bridging any
> connections associated with them to somewhere else, also that no
> connections have been severed.  Problems here could be leading to E3d
> falsely triggering when there is no real overload.
> 
> It might be useful to check the voltages and resistor values in the -12V
> regulator and compare with same in the good power supply, especially the
> voltage across the zener diode.
> 
> > >
> > > Is this the same PSU whose chopper transistor exploded a while back?
> > > Could there be any carbon deposits remaining on the board or
> > > conductive remnants wedged under components etc causing leakage
> from
> > > the -12V line to ground?
> >
> > The component nearest to the exploded transistor is the 10uF capacitor
> > on the output of the 12V regulator. There are some carbon deposits on
> > it. I did a cursory check for resistance and ESR and it seemed OK.
> >
> 
> This capacitor is probably there to ensure the 7812 doesn't oscillate.  
> Looking
> at Vstart with an oscilloscope should confirm that this is not an issue.  If 
> it
> doesn't have excessive leakage current and has approximately the correct
> capacitance, it is probably ok.  However, if there is gunk trapped underneath
> it around the leads, this might account for the extra current draw on Vstart.
> 
> The explosion could have had other bad effects.  Maybe E3 got damaged by a
> surge in its power supply when the transistor blew up?  Maybe the -12V
> rectifier was affected?  It is probably not as robust as the rectifiers for 
> the
> other lines and the chopper transistor shorting would have likely caused a
> big current pulse in the chopper transformer primary, leading in turn to
> surges at it's secondaries.  Also the diode in parallel with the 51R sense
> resistor might be suspect.
> 
> I'm not sure how to test these components comprehensively without trying
> replacements for them.
> 
> If the 7812 was damaged at the time of the explosion, other components
> powered from Vstart could have experienced surges as well.  Maybe stuff on
> the input side of the 7812 too?
> 

This evening I went to check Vstart for any oscillation. However, all of a 
sudden, the current draw is down to 85mA and PWM has started working. I am at a 
loss to explain it. I wondered if there might be a dry joint, but I have tried 
a few light taps and shakes and it continues to work. Perhaps your idea of some 
debris causing a short might explain it, otherwise I just don't know.

I am thinking I may put it back together and test with a light bulb in series.

Regards

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 10 May 2023 12:04
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> 
> >
> > I will do all the suggested checks, but I won't be able to do this for
> > a number of days. However, I wanted to understand something in the
> meantime.
> > The conditions I am applying are (I think!) what would happen during
> > startup, and during startup the control board has to make the PWM run,
> > otherwise the main switching transistor won't operate and the
> > transformer won't operate to produce the -12V in the first place. So
> > surely in the startup condition I am applying (which is to supply
> > Vstart from a bench PSU) is valid? Where is the flaw in my reasoning here?
> >
> 
> The PSU also gives a kick start to the -12V line by applying some negative
> voltage probably around -15V to it via a 2k7 resistor (on sheet 1).  If you 
> are
> not providing this, perhaps this is why the -12V line is able to swing 
> slightly
> positive and upset conditions in the -12V current sense circuit enough to
> prevent the PWM from starting?
> 
> Even if you are providing this resistor limited -12V startup supply, could it 
> be
> insufficient due to a faulty component leaking current from the -12V line to
> ground meaning that the -12V line does not become sufficiently negative to
> fulfil the startup conditions?
> 
> Could the PWM be managing to start up and run for a cycle or two and then
> stop due to a fault in the PWM circuit somehow attempting to draw too much
> current from the -12V line?  I don't see any connections from the -12V line to
> the PWM so this is probably not the case.
> 

So, I did the following test. I used my bench PSU to apply +12V to the 
regulator output and -12V to the actual -12V output. Under these conditions the 
PWM operated correctly. I repeated the test on the good PSU and the result was 
the same.

The good PSU drew 13mA on the -12V line and 92mA from the regulator side. The 
bad PSU was 16mA and 86mA respectively. When I test only applying +12V to the 
regulator output, the good PSU draws about 90mA and the bad one 140mA.

It is worth repeating that the PWM operates correctly on the good PSU when I 
only apply +12V to the regulator, but not on the bad one. I guess there must be 
some kind of short somewhere, but really not sure where it might be. It has to 
be something that is pulling the -12V line slightly higher. The only place 
where this seems like it could happen is around E1b I think. Perhaps there is a 
path through the positive input to E1b to Vcc on the LM393 (Control Module 
Sheet 1). Does that seem logical?

> Is this the same PSU whose chopper transistor exploded a while back?  Could
> there be any carbon deposits remaining on the board or conductive
> remnants wedged under components etc causing leakage from the -12V line
> to ground?

The component nearest to the exploded transistor is the 10uF capacitor on the 
output of the 12V regulator. There are some carbon deposits on it. I did a 
cursory check for resistance and ESR and it seemed OK.

Regards

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 09 May 2023 17:58
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> >
> > I am going to read your answer more carefully later. But I wanted to
> > check one thing. I measured the base-emitter voltage as negative in
> > both cases, and yet the TIP121 appears to be conducting on the bad
> > PSU. Surely that means that the TIP121 is not working correctly?
> >
> 
> I'd be very reluctant to draw any conclusions from measurements made
> when the device is not biased correctly.  As Brent says, there could be 
> current
> flowing through the presumably forward biased base-collector junction.
> Also, this device is a not just a straighforward transistor.  It's data sheet 
> says it
> is a package containing two transistors and some other components,
> including a normally reverse biased diode between the collector and emitter
> terminals.  A positive voltage of sufficient magnitude on it's emitter could 
> be
> forward biasing this diode resulting in current flow through it and therefore
> also through the 20R resistors causing a voltage to be developed across
> them.
> 
> I think a better way to determine if the TIP121 is causing excess current draw
> on the -12V line is to make the voltage at it's emitter approximately correct
> and see if it then draws enough current through itself to cause the excess
> current trip to operate.  This current can then be measured by observing the
> voltage across the 20R resistors (or the smoke coming from them if they are
> not of sufficient power rating...).  It should also be possible to observe the
> conditions around the zener diode and the MPSA55 and see if these
> components are behaving reasonably.

I will do all the suggested checks, but I won't be able to do this for a number 
of days. However, I wanted to understand something in the meantime. The 
conditions I am applying are (I think!) what would happen during startup, and 
during startup the control board has to make the PWM run, otherwise the main 
switching transistor won't operate and the transformer won't operate to produce 
the -12V in the first place. So surely in the startup condition I am applying 
(which is to supply Vstart from a bench PSU) is valid? Where is the flaw in my 
reasoning here?

> 
> Regards,
> Peter.

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> 
> I don't think we can draw any conclusions from voltage measurements
> around the TIP121 without the -12V line being approximately correct or at
> the very least being negative.
> 
> It's collector voltage being 0V on the good PSU suggests there is no current
> flowing through the parallel 20R resistors and therefore no emitter-collector
> current through the TIP121 so it's not being switched on meaning that it's
> driver doesn't feel the need to pull the -12V line down nearer 0V because it's
> already low enough.  (Maybe having a load is enough to keep it at the correct
> level and the shunt regulator only starts doing stuff if the load is less than
> expected or removed altogether? Or maybe the -12V line is not present when
> the test was done?)
> 
> The collector voltage of the TIP121 on the bad PSU being slightly positive is
> probably due to the voltage on it's emitter (ie the -12V line) being positive
> instead of negative which makes it's bias conditions all wrong and any
> measurements around it mostly meaningless.



I am going to read your answer more carefully later. But I wanted to check one 
thing. I measured the base-emitter voltage as negative in both cases, and yet 
the TIP121 appears to be conducting on the bad PSU. Surely that means that the 
TIP121 is not working correctly?

> 
> How about applying -12V or slightly less from a bench supply current limited
> to about 150mA to the -12V line and seeing what happens?  If the PSU
> and/or -12V load tries to draw more than that there would seem to be a real
> overload and further investigation is needed to find out what is drawing the
> current.
> 
> Measure the voltage across the zener diode in the -12V regulator and
> compare with the good PSU.  Also compare the voltage across the parallel
> 20R resistors.
> A larger voltage here would indicate more current being drawn through the
> TIP121.
> 
> Under these conditions, there shouldn't be any voltage across the 51R
> current sensing resistor for the -12V line.  If there is, it suggests there 
> could
> be reverse leakage through the -12V rectifier diode.
> 
> 
> However, Brent's calculations show that the current trip value for the -12V
> line is as low as 1.3mA and I can't see any reason to disagree with his
> calculations or his conclusion that this seems very low (except that another
> tiny smidgen of current is available from the negative startup supply but this
> won't really have any bearing on things). If this is really the case, then 
> placing
> something like a 5k6 resistor across the -12V line on the good PSU should
> cause enough current to flow for the trip to operate.  Finding this level of
> leakage in the failed PSU is not going to be easy.
> 
> On the other hand, if this test doesn't trip it, then please look very 
> closely at
> the resistors and connections to the inputs of E3d and verify that they are as
> described on the circuit diagram.
> 
> It seems very strange indeed to have a trip value as low as 1.3mA combined
> with a shunt regulator whose method of regulation is to pull the voltage
> down by drawing current from the supply line.  Perhaps the shunt regulator
> might be able to pull enough current to cause the trip to operate if the -12V
> line was too high (in the negative sense) or if the shunt regulator was under
> the mistaken impression that the -12V line was too high?
> 
> (This is a bit unlikely but the 115V/230V switch is set correctly, isn't it?)
> 
> On the other other hand, if the manual says that the -12V line is supposed to
> be able to supply 150mA, then it doesn't make sense for the current trip to
> operate at 1.3mA and we must be going wrong somewhere.
> 
> Regards,
> Peter.
> 
> >
> > >
> > > Regards,
> > > Peter.
> > >
> > > >
> > > > Regards
> > > >
> > > >
> > > >
> > > > Rob
> > > >
> > > >
> > > >
> > > > From: Rob Jarratt 
> > > > Sent: 02 May 2023 08:19
> > > > To: 'Mattis Lind' ; r...@jarratt.me.uk;
> > > > 'General
> > > > Discussion: On-Topic and Off-Topic Posts' 
> > > > Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> > > >
> > > >
> > > >
> > > > That’s a good idea, I will try that
> > > >
> > > >
> > > >
> > > > From: Mattis Lind < 
> > > > mattisl...@gmail.com>
> > > > Sent: Tuesday, May 2, 2023 7:55 AM
> > > > To:   r...@jarratt.me.uk; General
> Discussion:
> > > > On-Topic and Off-Topic Posts < 
> > > > cctalk@classiccmp.org>
> > > > Cc: Rob Jarratt < 
> > > > robert.jarr...@ntlworld.com>
> > > > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > Not quite sure what you mean here. I had advice from a friend to
> > > > bench test the control module by providing 14V to the input of the
> > > > 7812. On the good PSU I can see the PWM operate, on the bad one
> > > > the PWM is
> > > shutdown.
> > > >
> > > >
> > > >
> > > 

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 07 May 2023 10:34
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> >
> > The comments about the tolerance of the 7812 were right, it doesn’t
> > appear to be an issue with the replacement 7812 regulator because when
> > I tried using the bench PSU to feed exactly 12V to the circuit from
> > the output of the 7812 the comparator still gave the wrong result. It
> > was still wrong if I applied only 11V
> >
> 
> What do you mean by "gave the wrong result"?  If the power supply to the
> comparator is "reasonable" and the comparator's output does not reflect
> what is happening at it's inputs, the comparator is faulty.
> 
> If the comparator is giving the "wrong result" because it's inputs are 
> telling it
> to, it is behaving correctly.

Sorry for the loose language. I mean that the comparator is working correctly 
but the output is not what should be expected, because its inputs are wrong.

> 
> With no mains supply connected and a positive startup voltage applied to
> Vstart and a negative startup voltage applied to the -12V line via a 2k7
> resistor, you could try shorting the inputs of the comparator together and see
> whether this changes the comparator's output.  I want to emphasise doing
> this without power going to the mains rectifier feeding the chopper so that if
> there is real overload, the magic smoke will not be released.
> 
> If the output if the comparator is then "correct" and this output being
> "wrong" was the source of the PSU not working, then the PWM should start
> up like it does in the good power supply.  This would confirm that there is a
> problem in the components providing and/or mmonitoring the -12V line.
> If the PWM does not start up, this suggests the problem is elsewhere.
> 

I can tell you that if I remove the diode on the output of the -12V comparator 
then the PWM operates normally.

> >
> > I then looked at the value of Vz on the good and bad PSUs, when
> > applying 12V to the 7812 output. That was 5.4V in both the good and
> > bad PSUs. Where I saw a difference was on the -12V output, it was
> > +0.4V on the good PSU and 0.56V on the bad one (the voltage varied so
> > this was an average). I checked the voltage drop across the current
> > sense resistor. It is 0.01V on the good PSU and 0.08V on the bad PSU,
> > which would explain the higher positive voltage on the -12V output and the
> comparator being turned on.
> >
> >
> >
> > I am wondering if there could be a problem on the -12V output circuit
> > (PSU Sheet 3). I am struggling to understand the purpose of the two
> > transistors and the Zener diode there, but maybe one of them should be
> > switched on and isn’t. I am also unsure now as to which diode is doing
> > the rectification (to -12V). Someone said it was the one attached to
> > pin 6 of the transformer, but is that right? Isn’t it the Zener diode
> > half way across the page?
> >
> 
> The TIP121 darlington transistor is a shunt regulator for the -12V line.
> The voltage at the base of the MPSA55 transistor varies with the -12V line
> because it is connected to the -12V line via the potential divider formed by
> the 1k24 and 1k10 resistors.  If the voltage on the -12V line increases in the
> negative direction, the voltage at the base of the MPSA55 increases in
> proportion to it via the potential divider.  It's emitter voltage is fixed by 
> the
> zener diode so the MPSA55 is turned on more and it pulls more current
> through the base of the TIP121 which results in the TIP121 conducting more
> and pulling the -12V line down closer to it's correct voltage.  The opposite
> happens if the -12V goes lower than it should be, the TIP121 is turned on less
> and this allows the -12V line to increase negatively to it's correct value.
> 
> None of this circuitry should be doing very much until the chopper
> transformer is producing the source for the -12V line.
> 
> I suppose if the TIP121 is sborted or the zener diode is shorted, it could be
> causing problems, however, from the test results etc we have been given so
> far, I am not completely convinced there is a problem with the -12V line.
> 
> If a negative startup voltage is applied to the -12V line as well as a 
> positive
> voltage to Vstart to better simultate startup conditions, it may reveal more
> about what is happening with the -12V line.
> It might then be possible to compare the voltages across the zener diodes in
> the working power supply and the non-working power supply for example.

Thanks for the explanation. I am getting a better understanding now. I have 
noticed a difference here between the good and the bad PSU. On the good PSU I 
found the collector on TIP121 at 0V. On the bad PSU it was at +0.016V. The 
base-emitter voltage drop is -0.02V on the good PSU and -0.09V on the bad one. 
This is making it feel like the TIP121 may be faulty, 

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Brent Hilpert 
> Sent: 07 May 2023 23:53
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: [cctalk] Rainbow H7842 PSU Fault
> 
> On 2023-May-07, at 1:54 AM, Rob Jarratt via cctalk wrote:
> > The comments about the tolerance of the 7812 were right, it doesn’t
> appear to be an issue with the replacement 7812 regulator because when I
> tried using the bench PSU to feed exactly 12V to the circuit from the output 
> of
> the 7812 the comparator still gave the wrong result. It was still wrong if I
> applied only 11V
> >
> > I then looked at the value of Vz on the good and bad PSUs, when applying
> 12V to the 7812 output. That was 5.4V in both the good and bad PSUs.
> Where I saw a difference was on the -12V output, it was +0.4V on the good
> PSU and 0.56V on the bad one (the voltage varied so this was an average). I
> checked the voltage drop across the current sense resistor. It is 0.01V on the
> good PSU and 0.08V on the bad PSU, which would explain the higher positive
> voltage on the -12V output and the comparator being turned on.
> 
> 
> From earlier measurements and the 45uA calc of current through the 51Ω
> sense resistor, the V across the 51Ω ISense-12 resistor should be only 0.002V.
> 
> So a question is where is this 0.08V coming from? An unfulfilled -12V supply
> for the E3 power pin might have been an explanation, as extra current might
> be drawn out of the E3d.+in input due to the +in being pulled below
> (negative to) the
> E3 -power pin. But you say that pin is connected to GND, so the source of the
> 0.08V should be sought, some more comprehensive measurements around
> the E3d inputs / ISenseR might help.


I thought I had answered where the Vcc and GND are connected on the comparator. 
Vcc is connected to Vstart and GND to GND. I will look again at all the inputs 
to check.


> 
> From calculations from design/schematic:
>   - E3d.+in should = 1/2 the voltage across the 51Ω ISense-12 resistor.

I measure 0.08V across the current sense resistor and indeed I found 0.04V on 
the E3d +IN

>   - E3d.-in should = 0.033 V.

And that is what I measured (actually 0.034V).

>   - The trip condition to send the E3d output high (shutdown) is: V(ISR-12) >
> 0.066V  (V across ISense-12 Resistor).

Indeed. The drop across the resistor is too high causing the comparator to turn 
off, when it should be on and sinking current to allow the PWN to operate. I 
think the comparator is working correctly, the problem seems to be the voltage 
drop across the resistor is genuinely too high, but I can't see why.

Under the same test conditions, I measure a 0.01V drop across the resistor in 
the good PSU.

> 
> Also: Is that ISense-12 resistor really a 51Ω resistor? Or is it 5.1Ω or 
> 0.51Ω?
> Again from calculation, 51Ω would imply a current limit of only 1.3mA, which
> is rather low.

It is 51R, I have measured it (in circuit) and the colour coding confirms it. 
The spec in the technical manual says the -12V is rated at 150mA max.

Thanks

Rob

PS Will reply to Peter's email later when I have had a chance to understand it 
and to try the suggestions.

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

The comments about the tolerance of the 7812 were right, it doesn’t appear to 
be an issue with the replacement 7812 regulator because when I tried using the 
bench PSU to feed exactly 12V to the circuit from the output of the 7812 the 
comparator still gave the wrong result. It was still wrong if I applied only 11V

 

I then looked at the value of Vz on the good and bad PSUs, when applying 12V to 
the 7812 output. That was 5.4V in both the good and bad PSUs. Where I saw a 
difference was on the -12V output, it was +0.4V on the good PSU and 0.56V on 
the bad one (the voltage varied so this was an average). I checked the voltage 
drop across the current sense resistor. It is 0.01V on the good PSU and 0.08V 
on the bad PSU, which would explain the higher positive voltage on the -12V 
output and the comparator being turned on.

 

I am wondering if there could be a problem on the -12V output circuit (PSU 
Sheet 3). I am struggling to understand the purpose of the two transistors and 
the Zener diode there, but maybe one of them should be switched on and isn’t. I 
am also unsure now as to which diode is doing the rectification (to -12V). 
Someone said it was the one attached to pin 6 of the transformer, but is that 
right? Isn’t it the Zener diode half way across the page?

 

Regards

 

Rob

 

From: Rob Jarratt  
Sent: 02 May 2023 08:19
To: 'Mattis Lind' ; r...@jarratt.me.uk; 'General 
Discussion: On-Topic and Off-Topic Posts' 
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault

 

That’s a good idea, I will try that

 

From: Mattis Lind <  mattisl...@gmail.com> 
Sent: Tuesday, May 2, 2023 7:55 AM
To:   r...@jarratt.me.uk; General Discussion: 
On-Topic and Off-Topic Posts <  
cctalk@classiccmp.org>
Cc: Rob Jarratt <  
robert.jarr...@ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault

 

 

 




Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.

 

And what happens if you feed in exactly 12 V on the output of the 7812? Would 
the PWM work then? By using a lab supply you could check if the circuit is 
sensitive to variation in the 12V supply.

 

 

/Mattis

 


Regards

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

That’s a good idea, I will try that

 

From: Mattis Lind  
Sent: Tuesday, May 2, 2023 7:55 AM
To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 

Cc: Rob Jarratt 
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault

 

 

 




Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.

 

And what happens if you feed in exactly 12 V on the output of the 7812? Would 
the PWM work then? By using a lab supply you could check if the circuit is 
sensitive to variation in the 12V supply.

 

 

/Mattis

 


Regards

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> -Original Message-
> From: Brent Hilpert via cctalk 
> Sent: Monday, May 1, 2023 11:23 PM
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Brent Hilpert 
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> On 2023-May-01, at 2:25 AM, Peter Coghlan via cctalk wrote:
> > It seems a bit odd that a power supply from someone like DEC of that
> > era would be designed to depend so critically on the absolute value of
> > a rail used for startup purposes.
> 
> Further to Peter's point above, the 1988 NatSemi databook - which is to
say,
> from the era of this power supply - specs the 7812 output min-max to be
> 11.4 to 12.6V (+/-5%). Your measured Vstart=12.4V is well within this.
> Looking at the schematic, nothing stands out where the distinction from 12
or
> 12.1 would matter.
> 
> You still haven't reported the IC power pin connections. If the neg-supply
> pins are supplied by -12 rather than GND, it could explain the odd voltage
> seen on the E3d +input.

Sorry, I thought I had done this. The two LM339 comparator power pins are
connected to Vstart and GND.

The reason for the odd output of E3d seems to be that the positive input is
actually higher than the negative one, but only by a small amount, and that
small amount would seem to correspond to the slightly higher value of
Vstart, because the negative input is the output of a voltage divider from
Vstart to ISense -12V+, which is GND.

> 
> There are 3 explicit components in the design which provide -12V at
startup.
> They didn't throw those components in there just to fill up board space
and
> look pretty. Why would you expect the control circuit to be testable for
valid
> startup state when you haven't provided the startup environment?

Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.

Regards

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

I need to order higher tolerance ones. However, I was wondering, would a 
sensible strategy be to go through the ones that I have and find one that 
outputs a voltage closer to the spec? Or would that just be storing up trouble 
for the future?

Regards

Rob

> -Original Message-
> From: Rob Jarratt 
> Sent: Sunday, April 30, 2023 9:35 PM
> To: 'Wayne S' 
> Cc: r...@jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> Looks like the one I used is an L7812CV, which has a looser tolerance. I am
> going to have to order some tighter tolerance parts.
> 
> > -Original Message-
> > From: Wayne S 
> > Sent: Sunday, April 30, 2023 7:42 PM
> > To: Wayne S 
> > Cc: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > Posts ; Rob Jarratt
> > 
> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> > The CT version is 4%, the ACT is 2%.
> >
> > Sent from my iPhone
> >
> > > On Apr 30, 2023, at 11:37, Wayne S  wrote:
> > >
> > > Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So
> > > the
> > voltages your measuring are within the 4% part. Maybe you do need the
> > 2% one.
> > >
> > > Sent from my iPhone
> > >
> > >> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk
> > >> 
> > wrote:
> > >>
> > >> I think I have found a possible cause, but not too sure what to do
> > >> about it if I have.
> > >>
> > >> I had to replace the 7812 regulator that supplies Vstart. The new
> > >> one has an output that seems to vary between 12.2 and 12.4V. The
> > >> 7812 on the good PSU outputs a steady 12.1V. I think that is enough
> > >> to cause the E3d comparator to turn the comparator off and allow
> > >> its output to
> > float high.
> > >>
> > >> I suspect I may have used one with a wide tolerance rather than a
> > >> narrow one, or just have a bad one. It should be a genuine part as
> > >> it came from CPC Farnell. Not quite sure why the output varies on
> > >> the bad
> > one though.
> > >>
> > >> Regards
> > >>
> > >> Rob
> > >>
> > >>
> > >>

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Looks like the one I used is an L7812CV, which has a looser tolerance. I am 
going to have to order some tighter tolerance parts.

> -Original Message-
> From: Wayne S 
> Sent: Sunday, April 30, 2023 7:42 PM
> To: Wayne S 
> Cc: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> ; Rob Jarratt 
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> 
> The CT version is 4%, the ACT is 2%.
> 
> Sent from my iPhone
> 
> > On Apr 30, 2023, at 11:37, Wayne S  wrote:
> >
> > Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So the
> voltages your measuring are within the 4% part. Maybe you do need the 2%
> one.
> >
> > Sent from my iPhone
> >
> >> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk 
> wrote:
> >>
> >> I think I have found a possible cause, but not too sure what to do
> >> about it if I have.
> >>
> >> I had to replace the 7812 regulator that supplies Vstart. The new one
> >> has an output that seems to vary between 12.2 and 12.4V. The 7812 on
> >> the good PSU outputs a steady 12.1V. I think that is enough to cause
> >> the E3d comparator to turn the comparator off and allow its output to
> float high.
> >>
> >> I suspect I may have used one with a wide tolerance rather than a
> >> narrow one, or just have a bad one. It should be a genuine part as it
> >> came from CPC Farnell. Not quite sure why the output varies on the bad
> one though.
> >>
> >> Regards
> >>
> >> Rob
> >>
> >>
> >>

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

I think I have found a possible cause, but not too sure what to do about it
if I have.

I had to replace the 7812 regulator that supplies Vstart. The new one has an
output that seems to vary between 12.2 and 12.4V. The 7812 on the good PSU
outputs a steady 12.1V. I think that is enough to cause the E3d comparator
to turn the comparator off and allow its output to float high.

I suspect I may have used one with a wide tolerance rather than a narrow
one, or just have a bad one. It should be a genuine part as it came from CPC
Farnell. Not quite sure why the output varies on the bad one though.

Regards

Rob

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

> It looks like the conditions in the control and overcurrent sensing parts
of the
> working and the non-working PSUs are different so some further
investigation
> may be required as to why this is the case.
> 

Yes, that is certainly the problem. Presumably more current is flowing
through the current sense resistor than should be. I thought that perhaps
the higher positive voltage might be enough to switch on the rectifier diode
while the lower positive voltage on the good PSU won't, but it seems to
switch on in both cases because I measured a voltage across the current
sense resistor in both cases. I measured 0.08V on the bad PSU and 0.01 on
the good PSU. So this would explain why an over current is being detected.

What I don't now get is why the -12V output is higher on the bad PSU because
if there is something shorted or a capacitor with high current leakage on
the -12V output, that would surely result in a lower voltage on the -12V
output, not a higher one?

> >
> > I hadn't noticed the connection to -12V on the non-inverting input of
E1b.
> > However, I don't think this can account for the 0.6V because in both
> > the working and non-working PSUs the non-inverting input to E1b is 4V.
> >
> 
> See further down.
> 
> >
> > However,
> > when I was checking this, I noticed that I must have made a mistake,
> > because the -12V output actually measures +0.4V on the working PSU
> > (not zero as I first thought), but the non-working PSU measures +0.6V.
> > Also, I noticed that the -12V output on the working PSU rises more
> > slowly to +0.4V than on the non-working PSU where it rises more quickly
to
> +0.6V.
> >
> > I still don't understand where this +ve voltage on the -12V output can
> > come from though? Whether on the working PSU or the non-working PSU.
> >
> 
> The 4V at the non-inverting input of E1b is linked to the -12V line via
the 75k
> resistor and from there via a smoothing choke and the chopper transformer
> secondary to the -12V line rectifier diode connected to pin 6 and on to
> ground via the parallel diode/resistor combination.  This should result in
a
> current of about 45 microamps flowing to ground through those components
> mentioned.
> This in turn should cause the rectifier to be forward biased and drop
> approximately 0.6V across it which is characteristic of silicon diodes.
> Does that make sense?

It took me a while to understand this, but yes I get that now.

> 
> (It's rather confusing in that the -12V line rectifier is on the ground
side of the
> chopper transformer secondary instead of on the supply side where it would
> more usually be encountered.  It's electrically all the same wherever it
is put
> as the components involved are in series anyway.)
> 
> It ought to be possible to measure the same 0.6V across the diode to
confirm
> this is where it is being dropped (and to measure the remaining 3.4V of
the
> 4V at E1b across the 75k resistor).  As to why it is only 0.4V on the
working
> power supply, I haven't thought that far ahead yet :-)

On the bad PSU I measure a 0.48V drop across the rectifier diode (the one
directly connected to the transformer, not the one in parallel with the
current sense resistor), on the good PSU I measure a 0.4V drop.


> 
> I think the +0.6V on the -12V line is explainable and to be expected under
the
> test conditions described.  It looks like there could be something wrong
in the
> control circuitry which is preventing the power supply from starting up.
> This might also account for the difference between the 0.4V and 0.6V.
> 
> Brent's suggestions for checking the condition around the comparators and
> how they are supplied with power are good ones.  I haven't made any
further
> suggestions because I don't have any right now :-)

I had forgotten about Brent's suggestion. Still need to check this.

> 
> Regards,
> Peter Coghlan.

[cctalk] Re: Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

Thanks for the various replies. Here are my responses and further
observations.

I was asked if I had a load on the PSU while bench testing it. The answer to
that is no.

I did check the two diodes around the -12V current sensing resistor (PSU
Sheet 3), but as was pointed out the current sensing resistor is very low
value (51 ohms). The two diodes measure the same, in circuit, on the working
and non-working PSUs. I suppose I may need to lift them to check them
properly.

I am not sure about the comment suggesting that the bench test conditions
could result an overcurrent being sensed on the -12V output. This is because
I use the same bench test conditions on both the working and non-working
PSUs, and on the working one an overcurrent is not sensed.

I hadn't noticed the connection to -12V on the non-inverting input of E1b.
However, I don't think this can account for the 0.6V because in both the
working and non-working PSUs the non-inverting input to E1b is 4V. However,
when I was checking this, I noticed that I must have made a mistake, because
the -12V output actually measures +0.4V on the working PSU (not zero as I
first thought), but the non-working PSU measures +0.6V. Also, I noticed that
the -12V output on the working PSU rises more slowly to +0.4V than on the
non-working PSU where it rises more quickly to +0.6V.

I still don't understand where this +ve voltage on the -12V output can come
from though? Whether on the working PSU or the non-working PSU.

Thanks

Rob

[cctalk] Rainbow H7842 PSU Fault

[Rob Jarratt via cctalk]

As some of you may be aware I am trying to find a fault in a Rainbow H7842
PSU. I am using Tony Duell's schematic from here
http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf.

 

I have been testing the Control Module by using a bench PSU to supply 15VDC
to the input of the 7812 regulator (p2, PSU Sheet 1).

 

My diagnosis shows that the control module is shutting down the PWM (p6,
Control Module Sheet 2) because it is detecting an overcurrent in the -12V
side (E3d on Control Module sheet 1, although I have determined that it is
actually E3c).

 

This seems to be because I measure a steady 0.6V on pin 6 of the transformer
(p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
chopper won't be running. I had previously removed the transformer and there
are no shorts between the pin 5-pin 6 winding and any of the other pins on
the transformer. I checked all the DC outputs of the PSU when powering the
7812 from the bench, both on a working PSU and the non-working one. They are
all at zero except the -12V output on the non-working PSU, which is +0.6V.
But the voltage can't come out of nowhere.

 

I am stumped and would appreciate any suggestions.

 

Thanks

 

Rob

[cctalk] Re: VMS V4.2 MUP (Mandatory Update) wanted!

[Rob Jarratt via cctalk]

I wouldn't mind having a copy for completeness too...

> -Original Message-
> From: Hans-Ulrich Hölscher via cctalk 
> Sent: 17 March 2023 13:25
> To: General Discussion: On-Topic and Off-Topic Posts 
> Cc: Plamen Mihaylov ; Hans-Ulrich
> Hölscher 
> Subject: [cctalk] Re: VMS V4.2 MUP (Mandatory Update) wanted!
> 
> Hi,
> that's great news!
> Could you please verify?
> Is it on diskette (probably RX50), tape (TK50) or an image for simh?
> Can I help you with other VMS software?
> 
> Thanks a lot!
> 
> Ulli
> 
> Am Fr., 17. März 2023 um 14:21 Uhr schrieb Plamen Mihaylov via cctalk <
> cctalk@classiccmp.org>:
> 
> > I probably have microvms 4.2 mup
> >
> > On Friday, March 17, 2023, Hans-Ulrich Hölscher via cctalk <
> > cctalk@classiccmp.org> wrote:
> >
> > > The mandatory update for VMS V4.2 is still missing in my collection.
> > >
> > > Because of that, VMS V4.2 is not fully usable, neither can layered
> > > software be installed nor can it be upgraded to VMS V4.3.
> > >
> > > Who can help???
> > >
> >

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

I have done a little more probing around. I have found that the 7812 regulator 
that drives Vstart on sheet 1 of Tony Duell’s schematic is shorted, so I will 
have to replace this too. I have not found anything else that looks obviously 
suspicious. I can’t test the output rectifiers for shorts without desoldering 
them, which I would rather avoid. I guess the next step is to replace the 
broken parts and use the light bulb current-limiter method to power on the PSU.

 

Regards

 

Rob

 

From: Rob Jarratt  
Sent: 24 November 2022 21:45
To: 'Mattis Lind' ; r...@jarratt.me.uk; 'General 
Discussion: On-Topic and Off-Topic Posts' 
Subject: RE: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

 

Regards

 

Rob

 

From: Mattis Lind mailto:mattisl...@gmail.com> > 
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk  ; General Discussion: 
On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 

Hello Rob!

 


Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.

 

 

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

 

I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

 

 

 


I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

 

Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

 

 




I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

 
lure/

 

/Mattis 


Thanks



Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

> -Original Message-
> From: Will Cooke via cctalk 
> Sent: 26 November 2022 19:12
> To: Rob Jarratt via cctalk 
> Cc: wrco...@wrcooke.net
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> 
> I'm not at all familiar with either this circuit or any of the mentioned 
> diodes.
> However, I would point out that a diode's forward voltage drop varies with
> current through it.  Usually, the datasheet will list the "max" forward drop, 
> at
> the rated current and typically at the lowest rated temp (the drop decreases
> as temp rises.)  So it is entirely possible that your tester is putting a 
> very small
> current through a high-current diode and getting that 0.5V.   It might be
> useful to feed it something close to its rated current and measure the drop
> for a more accurate estimate.

Thanks, I didn't know this, there is still so much I have to learn! I had 
noticed that the datasheets say "max" forward voltage but wasn't sure of the 
significance.

> 
> Will

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Just trying to decide what to replace the failed diode with, and looking at the 
UF400x series, as suggested by Mattis. It seems to me that as long as the PIV 
is 200V or higher it should be fine from that point of view, the switching 
speed is never higher than 70ns, while the original A114x (assuming it *is* an 
A114x) has a switching speed of 200ns (possibly even 200us from the datasheet).

However, I am wondering about the forward voltage drop. The datasheets suggest 
that the A114x parts have a 1.3V forward voltage drop. I have a spare H7842 
that was working (until I messed it up today, another story), so I tested the 
diode in that, its forward voltage appears to be 0.5V, using a little tester I 
have. The UF400x have ratings of either 1.0V or 1.7V.

How sensitive is the circuit going to be to the forward voltage on the diode? 
Given that the forward voltage of the suggested replacement is higher, would it 
slow down the speed with which the transistor is switched off too much and 
cause it to be overloaded and fail?

Thanks

Rob


> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 25 November 2022 09:44
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> It is often possible to infer the component ratings needed from the other
> components around them.  A component in the base circuit of a transistor is
> likely to experience lower currents and voltages than one in the collector
> circuit.
> 
> In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor
> in parallel with the diode.  Suppose it is a 1W resistor.  This means that the
> from P = I squared R, the average current the resistor is likely to pass is 
> less
> than 1A.  From Ohm's law, V/I = R, this means the average voltage across the
> resistor is likely to be no more than 2.7 Volts.
> 
> It is possible that the peak current / voltage involved could be higher than
> the average for short periods of time but we have plenty of margin for error
> here so we don't need to think about that too much.  A diode with a PIV of
> 200V should be fine here.
> 
> Regards,
> Peter Coghlan
> 
> >
> > Tony Duell has reverse engineered the following schematic.
> >
> >
> >
> > http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf
> >
> >
> >
> > I will go with the 1000V as you suggest anyway.
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Mattis Lind 
> > Sent: 25 November 2022 07:12
> > To: r...@jarratt.me.uk
> > Cc: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842
> > Power Supply
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > On 24 Nov 2022, at 22:45, Rob Jarratt   > wrote:
> >
> > 
> >
> > Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had
> > a suggestion that the PIV should be 200V. Not sure what rating I
> > should be going for here?
> >
> >
> >
> > Given that I didn’t have a schematic and this is on the primary side I
> > went for the recommendation of 1000V. 200V may a bit low on the
> > primary side depending on the application of the diode. On the primary
> > there can be sustained voltages up to 400V and peaks that go even
> > higher. Using a diode with higher PIV almost never affects the
> > operation as long as other parameters stay the same. In this case the
> > most important parameter is the trr. It has to be a fast recovery
> > diode. In this case the UF4007 is slightly slower than the UF4004. But
> > I doubt it has a big significance. Actually the
> > A114 is much slower. 200 ns.
> >
> >
> >
> > :Mattis
> >
> >
> >
> >
> >
> > Regards
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Mattis Lind mailto:mattisl...@gmail.com>
> > >
> > Sent: 22 November 2022 07:54
> > To: r...@jarratt.me.uk  ;
> > General Discussion: On-Topic and Off-Topic Posts
> > mailto:cctalk@classiccmp.org> >
> > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842
> > Power Supply
> >
> >
> >
> >
> >
> > Hello Rob!
> >
> >
> >
> >
> > Given that before the transistor blew up there had clearly been
> > another failure somewhere else, I tried to find the original failure.
> > There were no obviously damaged parts, so I just probed around near
> > the transistor for any parts that were open circuit or short circuit.
> > I found a diode connected to the base of the transistor that appeared
> > to be short circuit. So, I decided to lift one end to check it. As I
> > de-soldered one of the leads, the diode broke in two. So clearly the
> > diode was either damaged by the failure of the transistor, or it was the
> cause of the failure. This is the diode:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> >
> >
> >
> >
> >
> >
> >
> >
> > DEC used a lot of A114x diodes in their PSUs. They looked exactly like

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Tony Duell has reverse engineered the following schematic.

 

http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf

 

I will go with the 1000V as you suggest anyway.

 

Thanks

 

Rob

 

From: Mattis Lind  
Sent: 25 November 2022 07:12
To: r...@jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts 
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 





On 24 Nov 2022, at 22:45, Rob Jarratt mailto:robert.jarr...@ntlworld.com> > wrote:



Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

 

Given that I didn’t have a schematic and this is on the primary side I went for 
the recommendation of 1000V. 200V may a bit low on the primary side depending 
on the application of the diode. On the primary there can be sustained voltages 
up to 400V and peaks that go even higher. Using a diode with higher PIV almost 
never affects the operation as long as other parameters stay the same. In this 
case the most important parameter is the trr. It has to be a fast recovery 
diode. In this case the UF4007 is slightly slower than the UF4004. But I doubt 
it has a big significance. Actually the A114 is much slower. 200 ns. 

 

:Mattis



 

Regards

 

Rob

 

From: Mattis Lind mailto:mattisl...@gmail.com> > 
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk  ; General Discussion: 
On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 

Hello Rob!

 


Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.




 

 

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

 

I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

 

 

 


I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

 

Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

 

 




I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

 
lure/




 

/Mattis 


Thanks



Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

 

Regards

 

Rob

 

From: Mattis Lind  
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 

Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 

Hello Rob!

 


Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.



 

 

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

 

I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

 

 

 


I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

 

Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

 

 




I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

 
lure/



 

/Mattis 


Thanks



Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Thanks Antonio,

The location of the diode is arrowed on this picture: 
https://rjarratt.files.wordpress.com/2022/11/img_20221120_205802-arrowed.jpg

You can also see the heatsink where the transistor used to be.

Regards

Rob

> -Original Message-
> From: Antonio Carlini via cctalk 
> Sent: 20 November 2022 20:38
> To: Rob Jarratt via cctalk 
> Cc: Antonio Carlini 
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> On 20/11/2022 17:40, Rob Jarratt via cctalk wrote:
> > The H7842 PSU in my Rainbow failed yesterday. At first the machine
> > just powered down and there was a slight burning smell, I wasn't next
> > to the machine when this happened, so I didn't see or hear anything to
> > tell me where the problem might be. Not being sure if there was a
> > short in the machine or a problem in the PSU, I disconnected the fans,
> > FDD and HDD and, probably foolishly, I applied power again to see if the
> machine would work.
> > At this point there was a bang and a flash in the PSU.
> >
> >
> 
> >
> > I blogged this here (it repeats most of that I have said above):
> > https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-
> supp
> > ly-fai
> > lure/
> >
> 
> I have Rainbow PSUs H7842A, H78420 (which I suspect I may have misread!
> ...) and H7842D available. I can look tomorrow; if you can supply an overview
> picture and maybe circle the location of the offending parts that might help
> me identify them more quickly (always assuming that they are marked at all,
> of course).
> 
> 
> Antonio
> 
> 
> --
> Antonio Carlini
> anto...@acarlini.com

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Hello Peter,

Thanks for the analysis. There doesn't seem to be anything further back from
the diode, unless you mean further back behind the inductor?

I will check on the output side as you suggest.

The circuit breaker did pop out when it failed. The onboard fuse is intact.
The house RCD triggered and cut the power to the whole house when the
transistor exploded!

Do you, or anyone else, have an idea what the diode could be so that I can
find a replacement. Like I said, it seems to be marked D610, and there are
some other ones that look to be the same elsewhere in the PSU.

Thanks

Rob 

> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 20 November 2022 18:50
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> Hi Rob,
> 
> I'm only guessing here.  I think the sequence may have been that the main
> switching transistor failed first as it would be under more stress than a
diode
> in the base circuit.  If the transistor shorted E-B-C then the HT would
become
> connected to the circuitry at it's base which would be compelely unable to
> cope with voltages and currents involved.  This probably resulted in the
> failure of the diode.  I think it may be worth looking at the components
> further back the drive chain from the diode.
> The inductor could be ok unless it is a very frail little thing but small
signal
> semiconductor components and/or resistors further back may not have
> fared as well as it.
> 
> It might also be worthwhile checking for shorted rectifiers on the output
side
> in case this was the cause of the stress on the switching transistor.
> However, the power supply might have an overcurrent trip to reduce the
> possibility of this sort of damage.  If there is an overcurrent trip or
thermal
> trip, this may have been reset after the power supply was powered off for
a
> while and when it was powered on again, the already damaged transistor
> could have been teed up to fail more spectacularly?  Like I said, just
guessing
> here.
> 
> Were there no fuses failed or cutouts cut out?  Does it look like there
should
> have been? I would think a shorted switching transistor should have caused
> some safety device to operate.  Or is it the case of the old adage that
the
> faster acting transistor managed to sacrifice itself in time to protect
the quick-
> blow fuse from blowing?
> 
> Regards,
> Peter Coghlan.
> 
> >
> > The H7842 PSU in my Rainbow failed yesterday. At first the machine
> > just powered down and there was a slight burning smell, I wasn't next
> > to the machine when this happened, so I didn't see or hear anything to
> > tell me where the problem might be. Not being sure if there was a
> > short in the machine or a problem in the PSU, I disconnected the fans,
> > FDD and HDD and, probably foolishly, I applied power again to see if the
> machine would work.
> > At this point there was a bang and a flash in the PSU.
> >
> >
> >
> > On opening up the H7842 power supply I found that one of the
> > transistors had completely disintegrated. It looks to be the main
> > switching transistor, here is a picture of it:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg.
> > I have identified a source for this transistor, but if anyone can
> > suggest a modern replacement that would be useful too. However, that
> > is not my main problem.
> >
> >
> >
> > Given that before the transistor blew up there had clearly been
> > another failure somewhere else, I tried to find the original failure.
> > There were no obviously damaged parts, so I just probed around near
> > the transistor for any parts that were open circuit or short circuit.
> > I found a diode connected to the base of the transistor that appeared
> > to be short circuit. So, I decided to lift one end to check it. As I
> > de-soldered one of the leads, the diode broke in two. So clearly the
> > diode was either damaged by the failure of the transistor, or it was the
> cause of the failure. This is the diode:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> >
> >
> >
> > I can't quite make out the markings on the diode to know what to
> > replace it with. I think it says "D610". Would that be the right
> > designation? If so, can anyone suggest a suitable replacement please?
> >
> >
> >
> > The diode seems to connect an inductor to the base of the switching
> > transistor and the collector of the transistor is connected to a
> > transformer. Should I be looking for other failed parts? Not sure if
> > the diode failed first and then caused the transistor to fail? Or if
> > something else has failed which caused these parts to fail?
> >
> >
> >
> > I do know that there are no shorts in the Rainbow itself, because I
> > have a spare PSU that still works fine in the same machine.
> >
> >
> >
> > I blogged this here (it repeats most of that I have said above):
> 

[cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

The H7842 PSU in my Rainbow failed yesterday. At first the machine just
powered down and there was a slight burning smell, I wasn't next to the
machine when this happened, so I didn't see or hear anything to tell me
where the problem might be. Not being sure if there was a short in the
machine or a problem in the PSU, I disconnected the fans, FDD and HDD and,
probably foolishly, I applied power again to see if the machine would work.
At this point there was a bang and a flash in the PSU.

 

On opening up the H7842 power supply I found that one of the transistors had
completely disintegrated. It looks to be the main switching transistor, here
is a picture of it:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg.  I
have identified a source for this transistor, but if anyone can suggest a
modern replacement that would be useful too. However, that is not my main
problem.

 

Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.

 

I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?

 

The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.

 

I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
lure/

 

Thanks

 

Rob

[cctalk] Modern Replacement for H7140 in PDP 11/24

[Rob Jarratt via cctalk]

Given all the troubles I have had with the H7140 in my PDP 11/24 I am
considering whether to replace it with modern equivalents, installed inside
the H7140 enclosure. I am a bit puzzled by the specs listed in the PDP 11/24
Maintenance Card, it suggests the PSU outputs +12V and -12V from the memory
inverter/memory regulator, but the specs for the cards don't mention 12V so
I don't know if I need 12V from the PSU. My memory board is an M8722-BC
(MS11-MB). I can't find a manual or printset for this memory, so I am not
sure what voltages it will need, although I suspect it only needs +5V, +15V
and -15V. Is that right?

 

I know I will also have to replace the fans, because the ones in the machine
are AC and need 35V.

 

Thanks

 

Rob

[cctalk] Re: HP 150 software

[Rob Jarratt via cctalk]

I have a HP-150 but I am in the UK. I think it would be good to get the images 
made and archived before sending the floppies anywhere at all.

Regards

Rob

> -Original Message-
> From: Stan Sieler via cctalk 
> Sent: 07 September 2022 01:51
> To: ba...@googlegroups.com; General Discussion: On-Topic and Off-Topic
> Posts 
> Cc: Stan Sieler 
> Subject: [cctalk] HP 150 software
> 
> Hi,
> 
> I found a bunch of original HP 150 software on 3.5" floppies ...
> any HP 150 collectors here?Free, pickup, Cupertino.
> 
> Includes the following.  About 1/2 are original disks.
> 
> The most unusual are probably the compilers from Prospero, and the
> IMAGE-like database (Mirage?) from Datasoft International (the developer
> was likely Michel Kohon, from France, and a member of the HP 3000
> community).
> 
> Datacom:
>DSN/Link
>HP PCLink
>Kermit
>PC2622
>Reflection 1 Plus
> 
> Misc / Unknown:
>Ally/150
>Application Master Extended I/O Application
>Cardfile (full app)
>Cardfile demo
>Computer tutor 150
>Edit/150 from KSD systems Limited
>Infocom sampler
>Interex CSL/100 volume 56
>Interex CSL150  (contributed library)
>Mentor version 1.E.1 from KSD systems Limited
>System demo
>Thinkjet demo
>Visicalc
> 
> Games:
>Tick Tock, Radar, Othello, others
>Type attack, Temple of Apshai, Ricochet
>Winning Deal
>Zork
> 
> Programming...
>C (unknown...just says "C" on label)
>Lattice C
>MASM
>Modula 2
>MVP Forth (on misc games floppy)
>Pro Fortran from Prospero
>   Pro For 1  (possibly same as above)
>Pro Pascal from Prospero
>Borland Turbo Pascal 2.0
> 
>ISV Development  (from HP)
>ISV revision A.1.2 (Independent Software Vendor toolkit from HP?)
>Programmers Toolkit  (HP)
>Programmers tools: debug, sort, find, edlin, ece2bin, ...more... (HP)
> 
> Database: ???
>Mirager Version ii 2.A.1 Datasoft International
>Mirage Library
>Mirage I
> 
> //

[cctalk] Re: Advice on Repairing a HP1630G Logic Analyser

[Rob Jarratt via cctalk]

I have just worked out how to dismantle the keycaps, although still
wondering if cleaning them will actually help?

> -Original Message-
> From: Rob Jarratt via cctalk 
> Sent: 29 August 2022 11:26
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Rob Jarratt 
> Subject: [cctalk] Advice on Repairing a HP1630G Logic Analyser
> 
> I have a nice HP1630G logic analyser. It still works OK but has a number
of
> issues that I would like to fix.
> 
> 
> 
> First, I managed to break the power switch. It became stiff and wouldn't
> readily toggle the power. I think the shaft became bent. In pressing too
hard,
> I broke off the button on the end of the shaft. Looking at it, I think the
> button may have been glued onto the end of the shaft, possibly in a
previous
> repair. There are some sites that list the button as available so I am
trying to
> source an original, but I think it could be just the button and not the
shaft
> that goes with it. However, if that proves impractical or too expensive, I
am
> wondering if it is possible to source a new switch, with a long shaft to
reach
> the switch on the board, that would at least allow me to toggle power from
> the front panel? I am not sure what to search for though, any suggestions?
I
> have several possible workarounds: 1) just leave the switch on permanently
> and "toggle" the power just by plugging into the mains
> 2) try to glue the button back on, possibly with a metal pin for strength,
but I
> think there is a bend in the shaft that caused the action not to work well
and
> so this may not work well 3) Raid a spare from a HP1630D that I also have.
4)
> try to make a new shaft and switch, possibly 3D printed, or just use a
piece of
> wooden dowelling. Any comments on these approaches?
> 
> 
> 
> The other problem I have is that I get terrible switch bounce from the
front
> panel keyboard. I have removed the keyboard to see if I can get at the
> switches and see if cleaning them would help. However, the keys (made by
> Cherry) look like sealed units to me. Again, does anyone have any advice
on
> how to improve the performance of these switches? I could again raid my
> HP1630D for a spare (which has a keyboard that isn't as bad), but would
> rather not have to do so.
> 
> 
> 
> Finally, I get a bit of screen wobble on the display. I am going to check
all the
> capacitors on the display board, but any other suggestions?
> 
> 
> 
> Pictures of the broken switch and the keyboard are here:
> https://1drv.ms/u/s!AgETuZFainX8gZNMqek10fyE3n1rlw?e=xd7RWU
> 
> 
> 
> Regards
> 
> 
> 
> Rob

[cctalk] Advice on Repairing a HP1630G Logic Analyser

[Rob Jarratt via cctalk]

I have a nice HP1630G logic analyser. It still works OK but has a number of
issues that I would like to fix.

 

First, I managed to break the power switch. It became stiff and wouldn't
readily toggle the power. I think the shaft became bent. In pressing too
hard, I broke off the button on the end of the shaft. Looking at it, I think
the button may have been glued onto the end of the shaft, possibly in a
previous repair. There are some sites that list the button as available so I
am trying to source an original, but I think it could be just the button and
not the shaft that goes with it. However, if that proves impractical or too
expensive, I am wondering if it is possible to source a new switch, with a
long shaft to reach the switch on the board, that would at least allow me to
toggle power from the front panel? I am not sure what to search for though,
any suggestions? I have several possible workarounds: 1) just leave the
switch on permanently and "toggle" the power just by plugging into the mains
2) try to glue the button back on, possibly with a metal pin for strength,
but I think there is a bend in the shaft that caused the action not to work
well and so this may not work well 3) Raid a spare from a HP1630D that I
also have. 4) try to make a new shaft and switch, possibly 3D printed, or
just use a piece of wooden dowelling. Any comments on these approaches?

 

The other problem I have is that I get terrible switch bounce from the front
panel keyboard. I have removed the keyboard to see if I can get at the
switches and see if cleaning them would help. However, the keys (made by
Cherry) look like sealed units to me. Again, does anyone have any advice on
how to improve the performance of these switches? I could again raid my
HP1630D for a spare (which has a keyboard that isn't as bad), but would
rather not have to do so.

 

Finally, I get a bit of screen wobble on the display. I am going to check
all the capacitors on the display board, but any other suggestions?

 

Pictures of the broken switch and the keyboard are here:
https://1drv.ms/u/s!AgETuZFainX8gZNMqek10fyE3n1rlw?e=xd7RWU

 

Regards

 

Rob

[cctalk] PDP 11/24 Unibus Termination and Grant Cards

[Rob Jarratt via cctalk]

The 11/24 System Technical Manual says that the terminator (M9302 or M9312)
must be installed in the last slot (slot 9) of the backplane,  and that all
empty slots must have a grant card. What I am not clear on is what they mean
by an "empty" slot. Specifically, if I have the CPU, the M7134 memory map
and a memory board installed in slots 1-3 and then nothing else except the
terminator in slot 9, does this mean I have to install grant cards in slots
4-8?

 

My guess is that this isn't a requirement because the grant cards go in rows
C and D while the terminator is rows A and B, but I am hoping someone can
confirm this for me please?

 

Thanks

 

Rob

[cctalk] Attaching Test Clip to F-11 Chips on PDP 11/24 M7133 CPU

[Rob Jarratt via cctalk]

I am trying to work out why my PDP 11/24 CPU is not working. To do so I want
to use my logic analyser on the F-11 chips. I have a regular 40-pin test
clip, but it doesn't make a good contact with the pins on the F-11 chips.
The test clips I have are made for regular DIP chips where the legs come out
horizontally from the side of the chip before turning 90 degrees and going
down into the board. On the F-11 chips the pins are pretty much stuck to the
side of the chip body and so the clip can't make a good contact.

 

Does anyone have any tips for attaching a test clip to the F-11 chips?

 

Thanks

 

Rob

RE: Replacement for a DEC 7474 Chip

[Rob Jarratt via cctalk]

You are right, I thought I remembered someone else saying on this thread
that he couldn't find a datasheet for the 7474 so I didn't look, but
actually I found one quite easily!

 

From: Paul Koning  
Sent: 16 May 2022 22:08
To: r...@jarratt.me.uk; Robert Jarratt ;
cctalk@classiccmp.org
Cc: Rick Murphy 
Subject: Re: Replacement for a DEC 7474 Chip

 

 





On May 16, 2022, at 4:59 PM, Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote:

 

I have several 7474s, including one marked DEC 7474. Sadly, I fear that
shipping from the US is likely to be prohibitive.


I have ordered some 7474s. However I am getting a bit lost in the discussion
now. From a simple fan out point of view, would an S be sufficient to
replace it?

 

I can think of two ways to answer that: (1) ask and see if you get an
answer, hopefullly a correct one, (2) find the data sheet for the 74x74
you're looking at, and the devices it is driving, and do the calculation.
Sum up the max input currents of the driven devices, compare with the min
output current of the 74x74.  If the output current is >= the load current,
you have the needed fanout.

 

  paul

 

RE: Replacement for a DEC 7474 Chip

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Rick Murphy via
> cctalk
> Sent: 16 May 2022 17:28
> To: cctalk@classiccmp.org
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> On 5/15/2022 4:16 AM, Eric Smith via cctalk wrote:
> > On Sat, May 14, 2022, 16:09 ben via cctalk  wrote:
> >
> >> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
> >>> AFAIR LS can only drive one unit TTL load.
>   paul
> >> LS is 4 TTL, 4 ma low.
> >> Was there a trick of forcing the output of D flip flip to clear it? I
> >> was wondering if this is what kills all the 7474's?
> >>
> > I don't think that worked on any TTL (or CMOS) 74x74 flip flops,
> > except maybe by accident if you shorted the output enough to draw Vcc
> > down (or ground up) enough to disrupt the FF, and then you have other
> problems.
> >
> > Despite the logic diagram showing feedback from the outputs, all 74x74
> > have buffered outputs. The recent TI data sheets show an equivalent
> > schematic only for the 74LS74. I can't at the moment find one for the 7474.
> >
> > It seems likely to me that early pre-TTL logic families like RTL might
> > have had FFs with unbuffered outputs, but I haven't checked.
> 
> I have several 7474s, including one marked DEC 7474. Sadly, I fear that
> shipping from the US is likely to be prohibitive.

I have ordered some 7474s. However I am getting a bit lost in the discussion 
now. From a simple fan out point of view, would an S be sufficient to replace 
it?

> 
>  -Rick

RE: Replacement for a DEC 7474 Chip

[Rob Jarratt via cctalk]

Oh dear, while I was ordering an original 7474 I ordered some other parts
that were connected to the same bad chip in case other chips are damaged,
and I ordered a Fairchild 74LS08! I will ask them to change it for a
Motorola part they also have.

> -Original Message-
> From: cctalk  On Behalf Of dwight via
cctalk
> Sent: 14 May 2022 23:36
> To: Paul Koning via cctalk 
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> What ever you do, don't use a Fairchild part. When I worked for Intel in
the
> 80's, we finally band using Fairchild for any latching device. They failed
on
> pullup current, even when the parts were sent back and they claimed they
> were good. We just gave up on them, we couldn't hold production while
> they figured it out.
> We had a similar problem with PowerOne, a manufacture of power supplies.
> Since it was a custom supply, we had to send someone to their plant to fix
> their final test.
> Dwight
> 
> 
> 
> From: cctalk  on behalf of Nigel Johnson
> Ham via cctalk 
> Sent: Saturday, May 14, 2022 10:50 AM
> To: Paul Koning via cctalk 
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> AFAIR LS can only drive one unit TTL load.
> 
> I may have some 7474, even of that vintage, if you cannot find any
anywhere
> else.
> 
> cheers,
> 
> Nigel
> 
> 
> Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio,
> the origin of the open-source concept!
> Skype:  TILBURY2591
> 
> 
> On 2022-05-14 13:48, Paul Koning via cctalk wrote:
> >
> >> On May 14, 2022, at 1:41 PM, John Robertson via
> cctalk  wrote:
> >>
> >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:
> >>> Hello,
> >>>
> >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
> >>> 7474 D flip flop. The problem is I don't know which
> >>> modern series would be the best one to replace it with. I am sure I
> >>> have seen a list somewhere of modern equivalents for some DEC chip
> >>> numbers, but I can't remember where.
> >>>
> >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6
> >>> of the schematic (p157 of the PDF).
> >>>
> >>> Picture of the failed chip here:
> >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.j
> >>> pg
> >>>
> >>> Can anyone tell me what the best modern equivalent is likely to be?
> >>>
> >>> Thanks
> >>>
> >>> Rob
> >>>
> >> You are stuck with using an original 7474 family assuming this is
driving
> other early TTL. 74LS74, and others simply don't have the drive capability
to
> work.
> > I know LS has less fanout, but is it not able to drive plain 74xx at
all?  That
> doesn't sound right.  If the circuit in question runs near the fanout spec
of
> plain 74 the yes, 74LS won't work.
> >
> > Spec sheets and the actual schematic will give a definitive answer.
> >
> >paul
> >
> >

Replacement for a DEC 7474 Chip

[Rob Jarratt via cctalk]

Hello,

 

I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.

 

If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).

 

Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg

 

Can anyone tell me what the best modern equivalent is likely to be?

 

Thanks

 

Rob

RE: Testing a 74S240

[Rob Jarratt via cctalk]

I am embarrassed to report that after trying all the suggestions etc, I then
realised that I had connected the enable signal to pin 9 and not 19. I knew
it was 19, I knew where 19 was, but for some unfathomable reason I connected
it to 9 instead, despite checking multiple times. The original chip works. 

Sorry for the noise.

Regards

Rob

> -Original Message-
> From: Guy Dunphy 
> Sent: 01 May 2022 01:22
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: Testing a 74S240
> 
> At 11:25 PM 30/04/2022 +0100, you wrote:
> >In trying to fix my M7133 CPU from my 11/24 I thought I had identified
> >a failed 74S240. However, when I replaced it (while adding a socket),
> >the fault remained. So, I guess the original chip may not be faulty. I
> >decided to test the original chip on a breadboard to see if it is OK.
> >This is where I got rather confused.
> >
> >
> >
> >I used a bench PSU, obviously connected Vcc to +5V and GND to the
> >negative terminal. I connected pin 19 (the active low Enable ) to GND.
> >And then I tested the particular pair of pins, 13 and 7. I did not
> >connect any of the other pins. However, pin 7 seemed to hover around
> >0.6 to 0.8V, no matter what I did with pin 13. I tried it with the
> >replacement 74S240 and got the same result. I tried a second
> >replacement 74S240 which had never been installed on the M7133 in case
> >something on the CPU board was damaging it, and got the same result.
> >
> >
> >
> >I looked at the M7133 schematic and saw that pin 19 is connected to GND
> >by a 180R resistor. I don't have one of that value so I tried a 220R.
> >My understanding is that the resistor isn't completely necessary, but I
> >tried anyway. However, the results were identical. I added a 220R to
> >the input on pin 19 just in case, again to no avail.
> >
> >
> >
> >I noticed that the chip (original and replacement) was drawing
> >100-110mA from the bench PSU, which seems a bit high.
> 
> As others have mentioned, the supply current is normal. These are fast,
high
> power devices.
> 
> When you say 'breadboard', what do you mean? Is it one of those blocky
> things with rows of holes with metal connecting fingers inside?
> Those have a lot of capacitance between rows, and with fast 74S logic and
an
> inverting buffers chip like the 74S240 that can be a problem. Also I bet
you
> didn't bother with a supply decoupling 0.1uF ceramic directly between Vcc
> and Gnd at the IC.
> 
> So be aware that you may have a circuit oscillating at something above
> 20MHz, and your multimeter will just be showing averages.
> To avoid this, add the supply cap and tie all unused inputs directly to
Gnd.
> With the input you are interested in, tie to Gnd or to Vcc via a 1K
resistor.
> All with _short_ wires. Also with your multimeter (on Volts range) it's a
good
> idea to have a 1K resistor in series with the probe tip AT THE TIP.
Otherwise
> your meter lead is a nice radiating antenna, and can cause oscillations
with
> that less than ideal breadboard.
> The resistor won't affect voltage readings.
> 
> Old 74xx logic (mostly) doesn't have these problems, and people used to
that
> get confused when much faster logic seems to be behaving weirdly.
> 
> At least the IC won't blow up. I had an interesting learning experience
the
> first time I got hold of a 74AC series 20 pin buffer chip. I blithely
> breadboarded it with just the power rails and turned on +5V.
> BANG! the die exploded. Blew a nice big crater in the plastic.
> Turns out with the fully CMOS inputs, they will float around in the zone
> between 0 and 1, which causes the very powerful output drivers to draw
> huge current as both the upper and lower drivers turn somewhat on. Times
> eight... Instant silicon vaporization.
> 
> After tying all the inputs to valid logic levels, no more explosions.
> 
> Guy
> 

RE: Repairing a VT240.

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Chris Zach via
> cctalk
> Sent: 01 May 2022 02:57
> To: CCTalk mailing list 
> Subject: Repairing a VT240.
> 
> So I'm clearing out projects on my bench. The latest is a Dec VT240 which did
> not work. Has been dead for 20 years, the usual so I figured I would see if I
> could get it to run. This is a quick summary of what I did in the hopes it can
> help someone else out someday.
> 
> First I got the service manual. It's full of good information. Then I opened 
> it
> up and checked out the power supply. 5 volts was fine, -12 was fine, +12 was
> a bit off (10v) but I didn't have the unit under any load.
> Putting a small load on 5 and 12 got me a solid +12v and 5.1v so all was good
> there.
> 
> Then I hooked it up to the board and fired it up. Voltages were still good
> (nothing big shorted) and the VR201 would come up, display a screen filling
> up with scan lines 3 times, then go black with nothing. Keyboard had WAIT lit
> and didn't work. So something was wrong.
> 
> Looking through the manual I saw that this thing is nuts: It has a full
> pdp11 in there (a T11 CPU) with something like 256kw of memory, 32k RAM
> and the rest ROM chips. Which is impossible, however they built a bank
> switching system into the terminal so the T11 could access more than 32kw of
> memory. Yes, they literally built a MMU just so they could use a pdp11.
> 
> The 11 seems to control the basic functions and the graphics modes (4014
> and REGIS/VT220/VT125) through the graphics chip controller which looks like
> the same one on the Rainbow's expansion video board. However there is
> also an 8085 in there that apparently generates the shape of text characters.
> So they tossed in another processor because it's DEC
> 
> Anyway I got out the thermal camera to see if there were any unusual shorts
> or chips drawing a lot of current. And sure enough the 8085 was glowing
> cherry red in the center. Felt it, it was hot and probably blown.
> 
> So I bought a new CPU on Ebay, realized it was an 8085A instead of the
> 8085-2 on the unit, bought an 8085-2 that will come next week, got the 8085a
> and figured might as well try it.
> 
> Popped it in, and the unit comes up! Set up works, ports work, I haven't
> tested the 20ma current loop but that probably works too. This is pretty
> much the most super terminal I've seen, and is classic DEC over-engineering.
> But it works, and now I can spend some time thinking about what I might
> want to DO with it.
> 
> Moral: After testing the power supply try checking the board with an IR
> camera. You can see a lot of interesting things with one.
> 

Hello Chris,

That's interesting. What did you use for the thermal camera?

Regards

Rob

> Chris

Testing a 74S240

[Rob Jarratt via cctalk]

In trying to fix my M7133 CPU from my 11/24 I thought I had identified a
failed 74S240. However, when I replaced it (while adding a socket), the
fault remained. So, I guess the original chip may not be faulty. I decided
to test the original chip on a breadboard to see if it is OK. This is where
I got rather confused.

 

I used a bench PSU, obviously connected Vcc to +5V and GND to the negative
terminal. I connected pin 19 (the active low Enable ) to GND. And then I
tested the particular pair of pins, 13 and 7. I did not connect any of the
other pins. However, pin 7 seemed to hover around 0.6 to 0.8V, no matter
what I did with pin 13. I tried it with the replacement 74S240 and got the
same result. I tried a second replacement 74S240 which had never been
installed on the M7133 in case something on the CPU board was damaging it,
and got the same result.

 

I looked at the M7133 schematic and saw that pin 19 is connected to GND by a
180R resistor. I don't have one of that value so I tried a 220R. My
understanding is that the resistor isn't completely necessary, but I tried
anyway. However, the results were identical. I added a 220R to the input on
pin 19 just in case, again to no avail.

 

I noticed that the chip (original and replacement) was drawing 100-110mA
from the bench PSU, which seems a bit high.

 

I wondered if I might need a pull up resistor on the output, but my
understanding is that this is not necessary.

 

Is there a flaw in my testing method? Have I misunderstood something
fundamental?

 

Regards

 

Rob

RE: Advice on Desoldering an IC

[Rob Jarratt via cctalk]

I decided to invest in a Hakko FR-301. It worked almost immediately. Hours of 
trying before, I did it in 10 minutes!

Regards

Rob

> -Original Message-
> From: cctalk  On Behalf Of dwight via cctalk
> Sent: 16 April 2022 14:00
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: Advice on Desoldering an IC
> 
> Sometimes the IC has been installed with the pins under tension. This is
> typical of machine inserted ICs. When the solder is loose, bend the pin away
> from the side it is pressed against. Do this carefully, don't over bend. You
> want it to center in the hole. I recommend doing this with a separate iron
> than the desoldering tool, so you can see what you are doing. Once the pin is
> nicely centered in the hole use the desoldering tool to suck the solder out.
> Make sure to always use a clean tip. An oxidixed tip will require excess
> pressure to transfer heatand damage the trace. Keep the solder shinny with
> a spung or soft metal wool. Do mot use a hard metal to clean an iron clad tip
> or it will damage the iron and rot it from the inside 
> When not using the iron but leaving it hot, always leave a blob of solder so
> that it won't have a thin oxide coating that is hard to remove. KEEP A CLEAN
> TIP!
> After sucking the solder with the tool, with a small screw driver, give the 
> pin a
> slight sideways pressure and let the screw driver slip off the pin. It should
> make a plink sound or a momentary ring. This is something that you'll just
> have to learn the sound of. If it doesn't sound right it means it isn't free 
> of
> the sides. Add solder and try to bend the pin.
> Often the body side of the IC will have a tiny film of solder right where the 
> IC
> sits on the trace. If this is just the tiny amount to solder, one can break it
> loose with a pair of short needle nose pliers, By squeezing the two sides of
> the IC together. Don't expect to break loose a large blob.
> Of course, if you expect to throw the IC away, use sharp pointed dikes to cut
> the pins at the package and pull each pin individually while the solder is 
> hot.
> Use a small vice to hold the board so you can work from both sides. Tweezers
> are best but heat the solder first and when hot grab the pin from the top.
> Work quickly while the solder is hot.
> You may need to refill the pin with fresh clean solder. Old oxidized solder
> does not remove easily. Use separate rosin flux if you have it ( not plumber
> flux!! ).
> Like I said earlier, use a really clean tip. It should be shinny before 
> trying to
> heat the board. It is hard to do with the higher temperature solders. There is
> some low temperature stuff you can use to remove solder more quickly.
> I like using a large manual plastic solderpulit. Some like to use solder wick.
> The solder removal suckers are often hard to keep the tip clean. If you have
> to press hard on the tip to the work, the tip is not clean. It does help to 
> have
> some really tiny flux core solder to touch right at the junction of the iron 
> and
> work to start the heat transfer. Never use force to get the heat to start to
> transfer! Clean tip and a quick touch with solder is all that is needed.
> When you are not using the iron for some time, but leaving it on, add a
> thicker blob of solder on it so it doesn't get a thin hard to clean oxide on 
> it.
> KEEP YOUR TIP FREE OF THIN OXIDE!
> Dwight
> 
> 
> 
> 
> From: cctalk  on behalf of Rob Jarratt via
> cctalk 
> Sent: Friday, April 15, 2022 10:49 AM
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Advice on Desoldering an IC
> 
> I am trying to remove an IC from my PDP 11/24 CPU, a DS8641. I am really
> struggling to desolder it. I am using the technique of applying fresh solder
> and then removing it. But after multiple cycles of this I think I am starting 
> to
> damage the PCB.
> 
> 
> 
> I am using a fairly cheap desoldering station (this one
> https://cpc.farnell.com/duratool/d00672/desoldering-station-uk-eu-
> plug/dp/SD
> 01384?st=duratool%20desoldering). Its spec in terms of vacuum pressure is
> equivalent to that of the professional Hakko ones though. I am also trying a
> hand desoldering pump. None of these are able to clear many of the holes of
> solder, although some are doing better than others. Nevertheless, the IC
> remains stubbornly unmoving.
> 
> 
> 
> Are there any tips for removing ICs?
> 
> 
> 
> Thanks
> 
> 
> 
> Rob

RE: Advice on Desoldering an IC

[Rob Jarratt via cctalk]

I do have some diagonal cutters, but although small they seem to be still too 
bulky to reach the pins. I will have to try to find some finer ones. These seem 
to look OK: 
https://uk.farnell.com/klein-tools/d275-5/wire-cutter-diagonal-127mm/dp/2839543

 

Also, I have seen the recommendations regarding a Hakko 808. It looks like the 
modern successor is the FR301 
https://www.hakko.co.uk/product/fr-301-portable-desoldering-gun/. I think what 
may be better about this is the wider choice of nozzles.

 

Regards

 

Rob

 

From: Chris Zach  
Sent: 15 April 2022 18:51
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts ; Rob Jarratt 
via cctalk 
Subject: Re: Advice on Desoldering an IC

 

Cut the pins with a very sharp set of dykes then remove them one at a time. 
Then use flux and detailer braid to remove the solder

On April 15, 2022 1:49:33 PM EDT, Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote:

I am trying to remove an IC from my PDP 11/24 CPU, a DS8641. I am really
struggling to desolder it. I am using the technique of applying fresh solder
and then removing it. But after multiple cycles of this I think I am
starting to damage the PCB.

 

I am using a fairly cheap desoldering station (this one
https://cpc.farnell.com/duratool/d00672/desoldering-station-uk-eu-plug/dp/SD
01384?st=duratool%20desoldering). Its spec in terms of vacuum pressure is
equivalent to that of the professional Hakko ones though. I am also trying a
hand desoldering pump. None of these are able to clear many of the holes of
solder, although some are doing better than others. Nevertheless, the IC
remains stubbornly unmoving.

 

Are there any tips for removing ICs?

 

Thanks

 

Rob

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

Advice on Desoldering an IC

[Rob Jarratt via cctalk]

I am trying to remove an IC from my PDP 11/24 CPU, a DS8641. I am really
struggling to desolder it. I am using the technique of applying fresh solder
and then removing it. But after multiple cycles of this I think I am
starting to damage the PCB.

 

I am using a fairly cheap desoldering station (this one
https://cpc.farnell.com/duratool/d00672/desoldering-station-uk-eu-plug/dp/SD
01384?st=duratool%20desoldering). Its spec in terms of vacuum pressure is
equivalent to that of the professional Hakko ones though. I am also trying a
hand desoldering pump. None of these are able to clear many of the holes of
solder, although some are doing better than others. Nevertheless, the IC
remains stubbornly unmoving.

 

Are there any tips for removing ICs?

 

Thanks

 

Rob

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Noel Chiappa via
> cctalk
> Sent: 03 April 2022 03:41
> To: cctalk@classiccmp.org
> Cc: j...@mercury.lcs.mit.edu
> Subject: RE: PDP 11/24 - A Step Backwards
> 
> > It was quite a struggle to separate those nylon connectors, is there
a
> > trick to it?
> 
> You mean the Mate-n-lok's? Not really; just make sure the catch is
released.
> 
> What did you do about DCLO? (Oh, I think I see the answer, below looks
> like you're relying on the pullup on K3...)
> 
> > When I powered on, the CPU LEDs did not light up.
> 
> Two of them ('0' and '1') are just bits in a special register, and thus
only do
> anything when the bootstrap code fondles them. When you get ODT
> running, you can amuse yourself turning them off and on manually! :-)
> 
> > I did notice that the CLK LED flickered on briefly when I powered it
> > off.
> 
> Interesting. Not sure exactly what we can deduce from that; but
interesting.
> 
> > I put a scope probe on TP1 (p152 of the PDF), there was no activity,
> > the pin remained high.
> 
> Yes; the signal there (MCLK H) is more or less the same one that drives
the
> 'CLK' LED (MCLK L); so no big surprise there. Still, that reduces the
problem
> space to a small part of K1.
> 
> > The problem now is that I expect I will need to probe various pins
to
> > find out what is going on. But I don't have a Unibus extender and I
am
> > reluctant to remove the backplane. From what I can tell in the
> > Technical Manual you can't install the CPU in other slots
> 
> Basically right; the backplane and CPU are designed to have it go in slot
1.
> It _might_ work in other MUD slots, with some loss of functionality (e.g.
> slot 2 doesn't have grant lines; MUD slots won't have the 'UNIBUS Map
board
> pesent' line - pin FE1, on K11, UB TO MA VIA UBMAP) but I wouldn't want to
> chance it, there might be a clash.
> 
> > I am forced to tack solder probe wires to the chips, which works but
is
> > time consuming. Any other ways?
> 
> Sorry, I don't have any experience to suggest any; too well supplied with
> extender cards, so I've never had to resort to alternatives!
> 
> 
> > I *think* I have found something. There could be a fault in E52
(sheet
> > K6, p157 of the PDF). While K6 BUS DCLO L is +5V, I am measuring K6
BUF
> > DCLO H at an average 1.64V
> 
> Yeah, that's wrong. E52 is bad, and will have to be replaced.
> 
> (From the +5V on BUS DCLO, I guess you're relying on the pullup? DCLO on
> the UNIBUS, with the resistor network on the M9302, should be about 3.5V -
> but now I'm confused, even with the P/S connector unplugged, it should
still
> be 3.5V or so. Oh well, it's late, the brain is powering down... :-)

Sorry if I wasn't clear, I did plug the connector back in, so that DCLO and
LTC are connected, I just removed the ACLO pin.

> 
> The 'unused' gate in E52 is the one that the added wires from the ACLO ECO
> went to; I wonder if it was damaged by the -15V, somehow?

Oh, I forgot about that! That would seem highly likely.

> 
> > logical 0 output should be 0.4V max
> 
> Which is what you should be seeing.
> 
> > I also measured K6 BUF DCLO L to be always low, suggesting it thinks
> > the K6 BUF DCLO H is a logical 1.
> 
> Yup; and that definitely explains why the clock isn't running - BUF DCLO L
is
> clearing E41 on K1.
> 
> Anyway, you'll have to replace E52 (which will be a bit of a pain, with
the 3
> ECO eires tacked to it). The DS8641 is an old chip, no longer in
production, so
> the usual suppliers may not have it, but there are some on eBait.


I didn't look for replacements last night. Is there a modern equivalent? My
initial searches show there isn't but I may have found a source of NOS.

Worried about replacing it with the ECO wires as you say, I always marvel at
how neatly those wires are done, I wish I knew how to do such a neat job.

Thanks

Rob

> 
>   Noel

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Rob Jarratt via
> cctalk
> Sent: 02 April 2022 11:58
> To: 'Noel Chiappa' ; 'General Discussion:
On-Topic
> and Off-Topic Posts' 
> Subject: RE: PDP 11/24 - A Step Backwards
> 
> >
> > Disconnect the bad ACLO, power it on, and see if the CLK LED comes on.
> > if not, then we'll have to work out why not.
> 
> This is my plan for later today.

OK, so I disconnected ACLO only. It was quite a struggle to separate those
nylon connectors, is there a trick to it? In doing so I spotted two
backplane wire wrap pins touching and a couple of others that were quite
close too, so I separated them and inspected all the backplane pins. I
didn't see any other ones touching. When I powered on, the CPU LEDs did not
light up. Some random characters appeared on the console, but probably just
a bit of noise maybe? However, I did notice that the CLK LED flickered on
briefly when I powered it off. I put a scope probe on TP1 (p152 of the PDF),
there was no activity, the pin remained high.

The problem now is that I expect I will need to probe various pins to find
out what is going on. But I don't have a Unibus extender and I am reluctant
to remove the backplane. From what I can tell in the Technical Manual you
can't install the CPU in other slots to make room for attaching probes
either. I am forced to tack solder probe wires to the chips, which works but
is time consuming. Any other ways?

Using tack soldered wires, I have traced back and I *think* I have found
something. There could be a fault in E52 (sheet K6, p157 of the PDF). While
K6 BUS DCLO L is +5V, I am measuring K6 BUF DCLO H at an average 1.64V with
50us spikes at 2.08V. According to a NatSemi datasheet for the DS8641
(http://pdf.datasheetcatalog.com/datasheet/nationalsemiconductor/DS005806.PD
F) the logical 0 output should be 0.4V max and the logical 1 output should
be 2.4V min. I also measured K6 BUF DCLO L to be always low, suggesting it
thinks the K6 BUF DCLO H is a logical 1. This seems to suggest that E52 may
be faulty. Trace here:
https://rjarratt.files.wordpress.com/2022/04/e52-dclo-signal.jpg.

Regards

Rob

> 
> >
> > Noel

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: Brent Hilpert 
> Sent: 31 March 2022 22:48
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On 2022-Mar-31, at 2:14 PM, Rob Jarratt wrote:
> >> Those three comparators in the H777 are looking at a time-delay ramp
> >
> > Is that a typo? This is the H7140 not the H777.
> 
> Groan.
> 
> When this thread came up I went looking for the 11/24 schematic. I found
> the document I linked earlier for the 11/24 and found 'the' +5 power
supply.
> So apparently I've been looking at the wrong +5V supply (H777) because the
> rest of you are indeed looking at a different +5 supply (H7140), both of
which
> are in that same 11/24 pdf document.
> 
> And indeed, the ACLO control is Q15 in the H7140.
> 
> I really wish when people are asking for assistance or talking about a
> schematic or circuit they would include a link/reference to exactly what
they
> are looking at (a) so the reader doesn't have to go scratching around to
find it
> and (b) to avoid effort-wasting screw-ups like this.

Sorry about the wasted time. I think I did say at one point that it was the
H7140, but that will be way down the thread somewhere. I will use PDF page
numbers in future.

> 
> So yes, you can ignore a lot of the details I described, though some of
the
> principals I mentioned are still valid.

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> 
> Disconnect the bad ACLO, power it on, and see if the CLK LED comes on. if
> not, then we'll have to work out why not.

This is my plan for later today.

> 
>   Noel

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

I made some interesting discoveries this evening. See below.

> -Original Message-
> From: cctalk  On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 31 March 2022 21:03
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On 2022-Mar-31, at 12:36 AM, Noel Chiappa via cctalk wrote:
> >> From: Tony Duell
> >
> >> A short in FET Q15 on the bias/interface board in the PSU could do it.
> >> The gate of that FET is driven from an LM339 comparator the -ve
> >> supply of which is -15V.
> >
> > Ah; I hadn't even looked at the P/S prints.
> >
> > (Like I said, I'm really weak on analog: for digital, I have the
> > advantages that i) although I'm basically/mostly a software person,
> > the MIT CS department is part of the EE department, and they made sure
> > that all the CS people had a decent grounding in the fundamentals of
> > digital hardware; and
> > ii) in my early years, I was involved in a number of actual hardware
> > projects, including a UNIBUS DMA network interface that tuned into an
> > actual product. So I'm pretty good with a digital circuit diagram,
> > like these CPU prints. But analog stuff is still a mostly-closed book
> > to me! :-)
> >
> > Anyway, I'm happy to let you provide the analysis of the P/S... :-)
> >
> >> From: Rob Jarratt
> >> [Perhaps] something else on the CPU caused Q15 to fail (if indeed it
> >> did).
> >
> > I'd guess 'unlikely' (if Q15 has failed); UNIBUS ACLO is connected, on
> > the CPU card, to only a single gate (on K2), and that 383 ohm pull-up
> > (on K3), and the 1K pF cap there (the purpose of which I still don't
> > understand, unless it's just a smoother). Although I suppose that if
> > that cap failed, shorted, maybe that could have taken out Q15 somehow.
> 
> Note: It's Q14 that controls ACLO, not Q15, Q15 is involved in the +5
startup.
> Unless there are two versions of the schematic and I'm looking at a
different
> one than everyone else.
> 
>   pdfPg.30 of
> http://www.bitsavers.org/pdf/dec/pdp11/1124/MP01018_1124schem_Aug8
> 0.pdf
> 

Thanks for pointing that out, I had not noticed the other line going down to
Q13 and Q14.


> 
> >> Perhaps I should ... and disconnect ACLO, DCLO and LTC, they are all
> >> on the same connector
> >
> > Now why didn't I think of just un-plugging that whole connector!
> > Du! My only concern would be leaving inputs floating...
> >
> > DCLO, no problem; it has that pull-up on K3. (Ditto for ACLO, if the
> > buffering input gate isn't dead.) LTC, let's see... It's on K6, upper
> > left corner. I'm too lazy to work out what leaving that input floating
> > will do, and, if it has bad consequences, trace out all the places it
> > goes (it should be connected up to cause an interrupt, somewhere), but
> > there's no point; the KW11 has an 'interrupt enable' that has to be
> > set by software before it can do anything; so at the moment it's safe
> > to just ignore it for now, and stay with a focus on getting the main
> > CPU clock running. (LTC is not on the UNIBUS, so there's no pull-up on
> > the M9302 for it the way there is for ACLO & DCLO.)
> >
> > So unplug that connector, and see if E70 (on K2, lower right corner) is
OK.
> > (Remember, the pull-up will give it an Ok input with BUS ACLO
> > disconnected.) If yes, great, go check the main CPU clock.
> 
> Removing DCLO and ACLO from the PS to the bus may allow the CPU/clock to
> work. Or it may not.

Well I can tell you it didn't, disconnecting those connectors left the CPU
still not doing anything. However, there is a puzzle. On the CPU I found
that the track from the pull up resistor to E70 has been cut. This would
suggest that E70 pin 2 is floating, which I think means that K2 BUF ACLO H
is also floating (I haven't put a probe on it as yet). But as the cut is
deliberate, there must be a reason. The CPU did work for a while when I
first got the machine. K2 BUS ACLO L however has been patched to E52 pin 4,
which is the output of a gate on sheet K6. Can't say I understand why.
However, for whatever reason it would seem that perhaps the ACLO signal from
the PSU has always been considered bad?

> 
> DCLO & ACLO behave as power-on-reset signals to the system. If they are
> allowed to just float up as the power supply comes up you have no
> guarantees as to the end result ('end' meaning the state of things after
the
> power supply has come up), without doing an analysis of the pertinent
logic
> under their control.
> 
> JFETs are being used as the ACLO/DCLO control devices for a reason. In
> contrast to bipolars, the normal/no-gate-voltage state of a JFET is
Source-
> Drain conducting, thus the initial state at power-up of ACLO-L & DCLO-L
will
> be 0V/low-impedance-to-GND. The point is to maintain that state until the
> power supply levels are good so the logic can be forced into a known
state.
> 
> Those three comparators in the H777 are looking at a time-delay ramp

Is that a typo? This is the H7140 not the H777.

> 

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Tony Duell via
> cctalk
> Sent: 31 March 2022 04:26
> To: Noel Chiappa ; General Discussion: On-Topic
> and Off-Topic Posts 
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On Thu, Mar 31, 2022 at 3:52 AM Noel Chiappa via cctalk
>  wrote:
> 
> > If the machine then runs, it's up to you as to whether you get the P/S
> > repaired so that ACLO work properly - your call. (I wonder how the
> > -15V got to ACLO - I suspect a solder bridge from the prior repair -
> > but knowing the answer is not important to getting the machine
> > running.)
> 
> A short in FET Q15 on the bias/interface board in the PSU could do it.
> The gate of that FET is driven from an LM339 comparator the -ve supply of
> which is -15V.
> 

Yes I found that on the schematic and surmise that this is what has failed.

> Of course _why_ Q15 failed, if it has, is another matter.

Yes, a bit concerning, and as Noel says maybe this has damaged something else, 
or that something else on the CPU caused Q15 to fail (if indeed it did). 
Perhaps I should follow Noel Chiappa's suggestion and disconnect ACLO, DCLO and 
LTC, they are all on the same connector and see how far it gets. If that works 
then I think repair ACLO and see if anything on the CPU is bad or anything else 
that might cause a short on the ACLO signal of the bus.

> 
> -tony

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

I found these two signals and ACLO is low (-15V) so I guess this must be the 
problem and whatever blew inside the PSU is probably the reason this signal is 
low. DCLO is high and the DC ON light is illuminated, but the CPU doesn't do 
anything presumably because ACLO is asserted.

Regards

Rob

> -Original Message-
> From: Tony Duell 
> Sent: 27 March 2022 10:15
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On Sat, Mar 26, 2022 at 9:20 PM Rob Jarratt via cctalk 
> wrote:
> 
> > Can anyone suggest what else the CPU might need? Or is it LTC?
> >
> 
> I would check the ACLO and DCLO signals. These are both high (pulled up by
> the bus terminator) for normal running, a PSU can pull them low if it detects
> loss of mains or whatever. Normally that will halt the CPU
> 
> -tony

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

You were right, it is switching noise. As you said, I was zoomed in too far, I 
hadn't paid proper attention! The actual ripple is about 50mV peak to peak.

https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-averaged-ripple.jpg

Regards

Rob

> -Original Message-
> From: cctalk  On Behalf Of Matt Burke via
> cctalk
> Sent: 29 March 2022 01:31
> To: cctalk@classiccmp.org
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On 28/03/2022 23:22, Rob Jarratt via cctalk wrote:
> > Its 600mV, but it is more of a spike than a ripple. Here is a trace:
> > https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-ripple.jpg
> >
> 
> I think that's just switching noise. You appear to be zoomed in on the point
> where the main switching transistor is turning back on. Here is a trace from
> an H7100 power supply (connected to a 70A dummy load) for
> comparison:
> 
> http://www.9track.net/posts/h7100_trace.png
> 
> If you turn on averaging mode on the oscilloscope (acquire menu) then that
> should filter our some of the noise and you will be able to see the actual
> ripple a bit better. It should be noted though that a differential probe is
> required for accurate ripple and noise measurements.
> 
> Regards,
> 
> Matt

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

Its 600mV, but it is more of a spike than a ripple. Here is a trace: 
https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-ripple.jpg

Regards

Rob

> -Original Message-
> From: Wayne S 
> Sent: 28 March 2022 23:15
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Cc: Chris Zach 
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> How bad is the ripple?
> Anyone on the list know what’s acceptable?
> 
> 
> Sent from my iPhone
> 
> > On Mar 28, 2022, at 14:46, Rob Jarratt via cctalk 
> wrote:
> >
> > 
> >
> >> -Original Message-
> >> From: cctalk  On Behalf Of Chris Zach
> >> via cctalk
> >> Sent: 28 March 2022 20:57
> >> To: cctalk@classiccmp.org
> >> Subject: Re: PDP 11/24 - A Step Backwards
> >>
> >>> I don't think the CPU is working at all. The reason being that there
> >>> is absolutely no LED activity. Including an LED that is supposed to
> >>> indicate a clock. Having hopefully eliminated all the power voltages
> >>> it left me wondering if there was a fault on the CPU or in the PSU.
> >>> Having had activity on those LEDs recently it seems most likely that
> >>> it will be the PSU, particularly as *something* in there blew up.
> >>> The only signal that I can identify that seems likely to have this
> >>> kind of effect is LTC, but I don't know enough about LTC to know if
> >>> its absence could cause the CPU board not to work at all, although I
> >>> see above that you think it unlikely. I suppose the fault could be
> >>> something I can't see on the CPU board, particularly as there do
> >>> seem to be some quite large spikes, otherwise I am not sure if there
> >>> is anything else from the PSU that could prevent the CPU getting going.
> >>
> >> I'm on a nice long train trip right now but I recently got my 11/24
> >> running again. One thing that baffled me was it would not do anything
> >> on the serial port. No ODT, no nothing.
> >>
> >> Turns out you really need to make sure the slots are filled properly.
> >> The CPU top, then the memory map, then for the next 4 boards one has
> >> to be either a properly configured MS11-PL (the 128kw board) or the
> >> memory boards specific to that type of 11/24. Or you have to put
> >> G7273's in the CD slots.
> >>
> >
> > I have been reluctant to put everything back in, in case the PSU fries
> something. And the ripple I noticed is certainly something that bothers me.
> Previously I had a burning smell from the memory board. I have since
> replaced all the electrolytics on the memory board, but I have not tried
> putting it back in the machine since. Just checking my notes, it seems I have
> had *intermittent* lack of activity on the CPU LEDs before, so it may be
> worth trying to put everything back in, although the ripple makes me
> hesitant to do so. For the record, right now I have only the M7133, M7134
> and G7273 installed.
> >
> >
> >> Next you need proper devices or G7273's in the next two slots and a
> >> proper terminator in the left sockets of the last slots and a G7273 in the
> center slots.
> >> Only then will ODT work.
> >>
> >> Another oddity is that the 5.25 inch box has +5 and +12 I think and
> >> the
> >> 10.5 has +5 and +15. There are different memory boards that work in
> >> one and not the other, or both depending on jumper settings that have
> >> to be right. Unibus drives me bonkers sometimes with the number of
> >> different voltages requires (+5, +12, +15, +20, -15, etc) It
> >> doesn't help that the +15 and +12 are on the same pins.
> >>
> >> Plus it's possible someone screwed with some switches, make sure they
> >> are set properly (ie: default is a good start).
> >>
> >> If you're still stuck next week drop me a line and I'll fire up my
> >> 11/24 and see if I can replicate your failure.
> >>
> >>>
> >>>>
> >>>> The first will tell you that i) the CPU is basically functional,
> >>>> executing
> >>> micro-
> >>>> instructions, etc; ii) that the bus is basically functioning (for
> >>> master-slave
> >>>> cycles; DMA and interrupts will remain to be checked out); iii)
> >>>> that the console port is working. (Yes, on the KDF11-U, the console
> >>>> is on an
> >>> internal
> >>>> bus, and so in theory a machine could hav

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Chris Zach via
> cctalk
> Sent: 28 March 2022 20:57
> To: cctalk@classiccmp.org
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> > I don't think the CPU is working at all. The reason being that there
> > is absolutely no LED activity. Including an LED that is supposed to
> > indicate a clock. Having hopefully eliminated all the power voltages
> > it left me wondering if there was a fault on the CPU or in the PSU.
> > Having had activity on those LEDs recently it seems most likely that
> > it will be the PSU, particularly as *something* in there blew up. The
> > only signal that I can identify that seems likely to have this kind of
> > effect is LTC, but I don't know enough about LTC to know if its
> > absence could cause the CPU board not to work at all, although I see
> > above that you think it unlikely. I suppose the fault could be
> > something I can't see on the CPU board, particularly as there do seem
> > to be some quite large spikes, otherwise I am not sure if there is
> > anything else from the PSU that could prevent the CPU getting going.
> 
> I'm on a nice long train trip right now but I recently got my 11/24 running
> again. One thing that baffled me was it would not do anything on the serial
> port. No ODT, no nothing.
> 
> Turns out you really need to make sure the slots are filled properly.
> The CPU top, then the memory map, then for the next 4 boards one has to
> be either a properly configured MS11-PL (the 128kw board) or the memory
> boards specific to that type of 11/24. Or you have to put G7273's in the CD
> slots.
> 

I have been reluctant to put everything back in, in case the PSU fries 
something. And the ripple I noticed is certainly something that bothers me. 
Previously I had a burning smell from the memory board. I have since replaced 
all the electrolytics on the memory board, but I have not tried putting it back 
in the machine since. Just checking my notes, it seems I have had 
*intermittent* lack of activity on the CPU LEDs before, so it may be worth 
trying to put everything back in, although the ripple makes me hesitant to do 
so. For the record, right now I have only the M7133, M7134 and G7273 installed.


> Next you need proper devices or G7273's in the next two slots and a proper
> terminator in the left sockets of the last slots and a G7273 in the center 
> slots.
> Only then will ODT work.
> 
> Another oddity is that the 5.25 inch box has +5 and +12 I think and the
> 10.5 has +5 and +15. There are different memory boards that work in one
> and not the other, or both depending on jumper settings that have to be
> right. Unibus drives me bonkers sometimes with the number of different
> voltages requires (+5, +12, +15, +20, -15, etc) It doesn't help that the 
> +15
> and +12 are on the same pins.
> 
> Plus it's possible someone screwed with some switches, make sure they are
> set properly (ie: default is a good start).
> 
> If you're still stuck next week drop me a line and I'll fire up my 11/24 and 
> see
> if I can replicate your failure.
> 
> >
> >>
> >> The first will tell you that i) the CPU is basically functional,
> >> executing
> > micro-
> >> instructions, etc; ii) that the bus is basically functioning (for
> > master-slave
> >> cycles; DMA and interrupts will remain to be checked out); iii) that
> >> the console port is working. (Yes, on the KDF11-U, the console is on
> >> an
> > internal
> >> bus, and so in theory a machine could have the ODT 'front panel'
> >> working, _and_ still have a problem with the bus, but depending on
> >> the exact details of said problem, maybe not.)
> >>
> >> So, hook up a console, set the machine to 'halt', and power on. Is
> >> console ODT working? If so, congrats, you win, go to stage ii) below.
> >
> > I had a console attached. There is nothing on the console. When I
> > first got the machine I did get output on the console. But that was
> > before the PSU first failed on me, which is quite a few years ago now.
> >
> >>
> >> If not, you have a reduced area in which you have to investigate -
> >> and
> > you'll
> >> need a 'scope of some kind to make any progress. (If you don't have
> >> one, you're SOL. Get one.). In order i) is the CPU's internal clock
> >> (and thus, probably the microcode) running? (At this point you will
> >> need to consult
> > the
> >> "PDP-11/24 System Technical Manual", EK-11024-TM.) If so, is it
> >> trying to
> > talk
> >> to the console's registers? (See Section 4.6 of the TM, "Internal
> >> Address
> >> Decode".) If so, is the UART working properly? (4.7 of the TM,
> >> "Serial
> > Line
> >> Units".)
> >>
> >> If so, console ODT is running, you're now at stage ii): you can see
> >> if the
> > CPU
> >> will run. Deposit a 0777 ('BR .') in a likely location (I usually use
> >> 0100 or 01000); read it back to make sure the write succeeded. (If
> >> not,
> > likely
> >> either the UNIBUS or the main memory has a problem.) Start the
> >> 

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Antonio Carlini
> via cctalk
> Sent: 28 March 2022 07:50
> To: cctalk@classiccmp.org
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On 28/03/2022 04:57, Tony Duell via cctalk wrote:
> >
> > Very little of the stuff I've bought new has had such seals (with some
> > things, like my audio equipment, you are _expected_ to remove the
> > covers, the user manuals tell you how. They also include the full
> > schematics). Ditto test gear (if there is a seal it voids the
> > calibration only), computer stuff, etc.
> >
> > I don't think DEC ever put such seals on their machines when new.
> > Certainly not on things like power supplies,]
> >
> > -tony
> 
> The RZ28 I have right here has the usual "Warranty Void If Broken" seal on
> the side.
> 
> In this case the PSU was recently sent off for repair: I'm not surprised it 
> came
> back with a similar sticker.
> 
> They're not trying to stop you looking inside, they're trying not to have to 
> fix
> it again for free after you've fiddled.
> 
> What surprises me (a little) is that there is a commercial outfit willing to 
> work
> on something so old.

Me too, but they do!

> 
> 
> Antonio
> 
> 
> --
> Antonio Carlini
> anto...@acarlini.com

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Chris Zach via
> cctalk
> Sent: 27 March 2022 19:48
> To: cctalk@classiccmp.org
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> Bigger question is who repaired the power supply "under warranty"?

A company called Radwell International. Based near me. They succeeded in 
getting the PSU to work, which I have been unable to do as it is just beyond me.

> 
> On 3/27/2022 2:38 PM, Bill Gunshannon via cctalk wrote:
> > On 3/27/22 05:17, Tony Duell via cctalk wrote:
> >> On Sat, Mar 26, 2022 at 11:12 PM Rob Jarratt via cctalk
> >>  wrote:
> >>
> >>> Unfortunately, or fortunately, depending on how you look at it, the
> >>> PSU repair is under warranty, which means I can't do it myself
> >>> without invalidating the warranty, so I will have to send it back. I
> >>> don't know if the ripple is caused by the blown part, but I suppose
> >>> it is likely. I may be able to inspect it without breaking the seals.
> >>
> >> That sort of thing would make me very suspicious as to what they've
> >> done inside the PSU that they don't want you to see.
> >>
> >
> > Pretty much every electronic device I have ever bought had seals on it
> > and a notice that breaking the seals voided the warranty.  Even stuff
> > with easily replaceable (or upgradeable) components.  Nothing unusual
> > here.
> >
> > bill
> >
> >

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

Thanks for the lengthy reply. Some responses inline below.

> -Original Message-
> From: Noel Chiappa 
> Sent: 27 March 2022 21:09
> To: cctalk@classiccmp.org
> Cc: j...@mercury.lcs.mit.edu
> Subject: RE: PDP 11/24 - A Step Backwards
> 
> > From: Rob Jarratt
> 
> > today I went back to it to check things a bit more carefully. All
the
> > power outputs of the PSU appear nominal.
> > ...
> > Presumably, whatever the part is, it is stopping the CPU working,
> > because previously the CPU did appear to show some activity,
although
> > of course it could still be a failure on the CPU. I am not sure what
> > other outputs the CPU might depend on. There is the LTC signal for
the
> > line time clock, but I don't know if its absence would stop the CPU
> > working. I have not been able to test the LTC signal as yet.
> > Can anyone suggest what else the CPU might need? Or is it LTC?
> 
> I'm going to start with some meta-comments, and then add some practical
> suggestions for how to proceed.
> 
> Reading this, I'm guessing that you're a software person, right? 

Yes, that is correct.

> Not that
> there's anything wrong with that (_I_'m basically a sofware engineer), but
if
> one is going to collect and run (which inevitably means maintain/repair -
it
> was ever thus, including BITD) vintage computers, you need to have mildly
> decent hardware skills. Yes, to some degree, one can lay this off on
others
> (as has been done here with the power supply - something I'd do myself, as
> my analog skills are not very good), but I think developing some decent
> digital hardware understanding would really help.
> 
> For instance, take your question about the LTC. To some degree, a
complete,
> entirely accurate answer is dependent on the details of the software
> (bootstrap and/or OS). However, knowing how the LTC works, what the low-
> level details are of what the CPU hardware does with it, etc would tell
you
> whether it is a cost-effective (in terms of overall 'getting the hardware
> working'
> project) thing to spend time on looking at, to begin with.
> 
> (Parenthetical observation: I reckon that debugging _any_ issue, hardware
> _or_ software, is a process of 'what's the _cheapest_ [easiest, quickest,
etc]
> test I can do that will produce the _maximal_ reduction in the area that
the
> bug could be in. Rinse, repeat, until you've tracked the problem to its
> lair.)
> 
> (You may discover, once you get the machine mostly working, that the LTC
> _specifically_ isn't working - at which point you can dive into it in
detail.
> But until then, I'd ignore it. It's a relatively small aount of stuff, and
the
> chance of a problem in there is small. And even if it's broken, the likely
> effects are small. There are better things to look at - below. Having a
clear
> understanding of the machine's major functional units, and how they
> interact, would have made that clear.)
> 
> So, in addition that that overview of the major functional units, you
definitely
> need to know how the QBUS works (read the QBUS chapter in the
> "Microcomputer Products Handbook" or the "Microcomputer Processors"
> books). (Yes, I know, the -11/24 is a UNIBUS machine, but the two busses
> differ only in extremely minor details; if you fully understand one, you
can
> learn the other in half an hour. And the -11/24's CPU is a KDF11 CPU, and
> uses the microcode ODT 'front panel' of the QBUS CPUs.)
> 

I think I have been avoiding learning about the buses, but I think you are
right I will do some reading on them. I have a PDP11 Architecture Handbook
which talks about the Unibus, so I can read that. As an aside I have also
been trying to find a fault on a Pro 350 which uses the same CPU chipset. I
have a pinout but no datasheet. I tried asking here on cctalk a while ago,
but there doesn't seem to be a whole lot of documentation to help me
understand how the CPU chips actually work. So I do try to understand the
hardware when I can.

> 
> Having said that, and starting with the "All the power outputs of the PSU
> appear nominal" (which rules out a large area), this is the process I'd
follow to
> reduce the area the problem is in as quickly as possible. (And maybe I
should
> transform this into a 'fault analysis of QBUS (and some
> UNIBUS) PDP-11 systems' on the CHWiki.)
> 
> You need to see if the CPU is _basically working. Two stages to that: i)
is the
> ODT 'front panel' (in microcode) working, ii) is the CPU basically
functional -
> i.e. can it fetch and execute instructions. Answers to those will greatly
reduce
> the area in which the problem (if there's _only_ one - a possibility one
must
> keep in mind).

I don't think the CPU is working at all. The reason being that there is
absolutely no LED activity. Including an LED that is supposed to indicate a
clock. Having hopefully eliminated all the power voltages it left me
wondering if there was a fault on the CPU or in the PSU. Having had activity
on those 

RE: PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Toby Thain via
> cctalk
> Sent: 26 March 2022 22:07
> To: cctalk@classiccmp.org
> Subject: Re: PDP 11/24 - A Step Backwards
> 
> On 2022-03-26 5:20 p.m., Rob Jarratt via cctalk wrote:
> > I had the H7140 PSU in my PDP 11/24 repaired a little while ago and I
> > posted about it here:
> > https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/
> >
> > I have since had the PSU fixed again and it came back a couple of weeks ago.
> > When I installed it and applied power to the input, I heard a
> > reassuring relay click.
> >
> > So I powered it on. The fans turned, but there was a crackle and I
> > smelt something burning. I couldn't locate the smell, there were no
> > lights on the CPU board, but the fans continued to turn.
> >
> > I had to leave it a few days and today I went back to it to check
> > things a bit more carefully. All the power outputs of the PSU appear 
> > nominal.
> > However, the ripple seems quite high, with an amplitude of 600mV on
> > the +5V
> > output: https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-ripple.jpg.
> >
> > The DC ON light comes on, but the M7133 CPU LEDs show no activity
> > whatsoever.
> >
> > There is no apparent damage to the CPU or to the M7134 that was also
> > installed. So, I guess the component that blew up must be inside the PSU.
> > Presumably, whatever the part is, it is stopping the CPU working,
> > because
> 
> I think the next step would be to inspect the PSU and see if the ripple can be
> eliminated, perhaps by replacing whatever blew up :)
> 

Unfortunately, or fortunately, depending on how you look at it, the PSU repair 
is under warranty, which means I can't do it myself without invalidating the 
warranty, so I will have to send it back. I don't know if the ripple is caused 
by the blown part, but I suppose it is likely. I may be able to inspect it 
without breaking the seals.




> --T
> 
> > previously the CPU did appear to show some activity, although of
> > course it could still be a failure on the CPU. I am not sure what
> > other outputs the CPU might depend on. There is the LTC signal for the
> > line time clock, but I don't know if its absence would stop the CPU
> > working. I have not been able to test the LTC signal as yet.
> >
> > Can anyone suggest what else the CPU might need? Or is it LTC?
> >
> >
> >
> > Regards
> >
> >
> >
> > Rob
> >

PDP 11/24 - A Step Backwards

[Rob Jarratt via cctalk]

I had the H7140 PSU in my PDP 11/24 repaired a little while ago and I posted
about it here: https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/

I have since had the PSU fixed again and it came back a couple of weeks ago.
When I installed it and applied power to the input, I heard a reassuring
relay click.

So I powered it on. The fans turned, but there was a crackle and I smelt
something burning. I couldn't locate the smell, there were no lights on the
CPU board, but the fans continued to turn.

I had to leave it a few days and today I went back to it to check things a
bit more carefully. All the power outputs of the PSU appear nominal.
However, the ripple seems quite high, with an amplitude of 600mV on the +5V
output: https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-ripple.jpg.

The DC ON light comes on, but the M7133 CPU LEDs show no activity
whatsoever.

There is no apparent damage to the CPU or to the M7134 that was also
installed. So, I guess the component that blew up must be inside the PSU.
Presumably, whatever the part is, it is stopping the CPU working, because
previously the CPU did appear to show some activity, although of course it
could still be a failure on the CPU. I am not sure what other outputs the
CPU might depend on. There is the LTC signal for the line time clock, but I
don't know if its absence would stop the CPU working. I have not been able
to test the LTC signal as yet.

Can anyone suggest what else the CPU might need? Or is it LTC?

 

Regards

 

Rob

RE: Racking a PDP-11/24

[Rob Jarratt via cctalk]

> -Original Message-
> From: pbir...@gmail.com 
> Sent: 26 February 2022 20:22
> To: r...@jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Subject: RE: Racking a PDP-11/24
> 
> A top-mount corporate cabinet looks like this:
> http://www.cosam.org/images/pdp11-23/front2.jpg  The "DECDatasystem"
> front-bar in the photo is over the 1U strengthener that braces the upper
> portion of the rack ... since there is no brace at the top (as yours has).
> Your cabinet will work fine; in my experience RL02's are always tight and
> fiddly any place but the top-spot.
> 
> The "play" is because those immense-head screws are the pivot-points that
> allow the chassis to be pivoted up 90 degrees (when the slides are
extended)
> in order to more easily access the underside of the backplane.  They are
> intentionally slightly loose in order for the pivot to work.  You really
don't
> need, or want, to play with those screws.


Ah! Thanks for that advice.

> 
> -Original Message-
> From: Rob Jarratt 
> Sent: Saturday, February 26, 2022 3:04 PM
> To: pbir...@gmail.com; r...@jarratt.me.uk; 'General Discussion: On-Topic
> and Off-Topic Posts' 
> Subject: RE: Racking a PDP-11/24
> 
> Thank you for the reply.
> 
> > -Original Message-
> > From: pbir...@gmail.com 
> > Sent: 26 February 2022 08:48
> > To: r...@jarratt.me.uk; 'Rob Jarratt' ;
> 'General
> > Discussion: On-Topic and Off-Topic Posts' 
> > Subject: RE: Racking a PDP-11/24
> >
> > The conventional mounting for RL02 drives in a corporate cabinet puts
> > one
> at
> > the top with the 6U CPU in the middle *but* that assumes that you have
> > a corporate cabinet designed for that purpose -- in which case the top
> > is missing so that the RL02 disk-pack can be directly accessed, and
> > there is a 1U divider below the top RL02 that reinforces the rack (and
> > in effect
> replaces the
> > 1U lip on your rack top).
> 
> I saw something somewhere that suggested the RL02 should be at the top
> with the disk accessible without pulling out the drive. I don't think the
cabinet
> I have was intended to do that, but I will have a closer look.
> 
> >
> > AFAIK you should be able to make your rack plan work; it's just the
> > case that
> > RL02 are always "top snug" (at least in my experience).  Have you
> > tried raising the 6U CPU as high as possible *before* tightening the
> > bolts/screws on the mounting flanges on the slides (to the rack, not
> > to the chassis)?
> > There's usually ~1/8" of play there.  From your photo perhaps you have
> > already done that as the CPU-front looks to be snug to the rack-top.
> > If anything it looks as if your RL02 instead need to move down.
> >
> 
> I did try raising the CPU as high as possible. Will have another look to
see
> if the RL02s can be dropped a bit lower, but I don't think they can go
lower
> in terms of using lower slots, not unless I want to leave a gap and
possible
> interfere with the space at the bottom where there will be some cables I
> think. I would like to avoid filing though!
> 
> > I would start by moving the bottom RL02 down as far as possible, then
> repeat
> > with the second RL02.
> >
> > Your observation that there is "very little clearance between the CPU
> > and the
> > RL02 at the front but more at the back" suggests to me that you need
> > to fiddle move with the front and /or rear mounting flange positioning
> > on the various slides.  Don't assume that wherever "gravity drops
> > them" is going to
> > be correct.
> >
> > If none of the suggested adjustments are working then I would consider
> > getting out a rat-tail file and enlarging the slots on the mounting
> flanges on
> > the RL02 slides so as to let them drop a little lower (assuming that
> > you have
> > clearance at the bottom!).
> >
> > Regardless you shouldn't need to play with the slide-to-chassis
> > attachment points -- those are always "just so" and not really free to
> > fiddle with (no
> > "slop").
> 
> There *is* a bit of play in the big black sheet that is screwed onto CPU
> enclosure.
> 
> >
> > In my experience "racking" is a fiddling-time and clearances can be
> *really*
> > tight.  But ... it can be done :->.  Good Luck.
> >
> > -Original Message-
> > From: cctalk  On Behalf Of Rob Jarratt
> > via cctalk
> > Sent: Saturday, February 26, 2022 3:15 AM
> > To: General Discussion: On-Topic and Off-Topic Posts
> > 

RE: Racking a PDP-11/24

[Rob Jarratt via cctalk]

> -Original Message-
> From: Paul Koning 
> Sent: 26 February 2022 19:21
> To: r...@jarratt.me.uk; Robert Jarratt ;
> cctalk@classiccmp.org
> Subject: Re: Racking a PDP-11/24
> 
> 
> 
> > On Feb 26, 2022, at 3:14 AM, Rob Jarratt via cctalk

> wrote:
> >
> > I am wondering if I have racked my 11/24 correctly.
> >
> >
> >
> > As you can see here:
> > https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/ I have
> > put the CPU at the top and the two RL02 drives underneath.
> 
> That seems fine.  Others mentioned having them at the top of a low
cabinet,
> but the RL02s I used were in H960 (6 foot) racks, mid-level with stuff
above
> them.
> 
> > The problem is that the CPU enclosure catches on the RL02 underneath.
> > There is a bit of play in the mounting bracket:
> > https://rjarratt.files.wordpress.com/2022/02/cpu-mounting-bracket.jpg.
> > With a bit of manipulation I can get the CPU to slide in. However, I
> > am wondering if I have racked it correctly? I don't think there is
> > room to move the RL02s down and it would presumably leave a bit of a
> > gap below the CPU. There seems to be very little clearance between the
> > CPU and the RL02 at the front but more at the back, but I am sure that
> > the rails are mounted horizontally. Is it just a matter of tightening
> > the big screws that hold the mounting brackets to stop the play? If so
> > I am not sure I have a big enough screwdriver!
> 
> Hardware stores can fix that.  Or Brownells, where you can get really good
> screwdrivers that are less likely to damage screw heads than standard
> hardware store ones do.

I am not in the USA, but I am should be able to look for other screwdrivers
here in the UK. I already have one quite big one, but I think it is still
way too small for this purpose.

> 
> Something I observed on my H960 that wasn't all that obvious at first: the
> holes are NOT evenly spaced.  If I remember right, they come in groups of
> four where the spacing between groups is something like 1/8th of an inch
> more than the spacing within groups.  The consequence is that if you
attach
> your brackets using the wrong set of holes things may be 1/4 inch (or
> whatever the delta is) closer than they were meant to be.
> 
>   paul

RE: Racking a PDP-11/24

[Rob Jarratt via cctalk]

Thank you for the reply.

> -Original Message-
> From: pbir...@gmail.com 
> Sent: 26 February 2022 08:48
> To: r...@jarratt.me.uk; 'Rob Jarratt' ;
'General
> Discussion: On-Topic and Off-Topic Posts' 
> Subject: RE: Racking a PDP-11/24
> 
> The conventional mounting for RL02 drives in a corporate cabinet puts one
at
> the top with the 6U CPU in the middle *but* that assumes that you have a
> corporate cabinet designed for that purpose -- in which case the top is
> missing so that the RL02 disk-pack can be directly accessed, and there is
a 1U
> divider below the top RL02 that reinforces the rack (and in effect
replaces the
> 1U lip on your rack top).

I saw something somewhere that suggested the RL02 should be at the top with
the disk accessible without pulling out the drive. I don't think the cabinet
I have was intended to do that, but I will have a closer look.

> 
> AFAIK you should be able to make your rack plan work; it's just the case
that
> RL02 are always "top snug" (at least in my experience).  Have you tried
raising
> the 6U CPU as high as possible *before* tightening the bolts/screws on the
> mounting flanges on the slides (to the rack, not to the chassis)?
> There's usually ~1/8" of play there.  From your photo perhaps you have
> already done that as the CPU-front looks to be snug to the rack-top.  If
> anything it looks as if your RL02 instead need to move down.
> 

I did try raising the CPU as high as possible. Will have another look to see
if the RL02s can be dropped a bit lower, but I don't think they can go lower
in terms of using lower slots, not unless I want to leave a gap and possible
interfere with the space at the bottom where there will be some cables I
think. I would like to avoid filing though!

> I would start by moving the bottom RL02 down as far as possible, then
repeat
> with the second RL02.
> 
> Your observation that there is "very little clearance between the CPU and
the
> RL02 at the front but more at the back" suggests to me that you need to
> fiddle move with the front and /or rear mounting flange positioning on the
> various slides.  Don't assume that wherever "gravity drops them" is going
to
> be correct.
> 
> If none of the suggested adjustments are working then I would consider
> getting out a rat-tail file and enlarging the slots on the mounting
flanges on
> the RL02 slides so as to let them drop a little lower (assuming that you
have
> clearance at the bottom!).
> 
> Regardless you shouldn't need to play with the slide-to-chassis attachment
> points -- those are always "just so" and not really free to fiddle with
(no
> "slop").

There *is* a bit of play in the big black sheet that is screwed onto CPU
enclosure.

> 
> In my experience "racking" is a fiddling-time and clearances can be
*really*
> tight.  But ... it can be done :->.  Good Luck.
> 
> -Original Message-
> From: cctalk  On Behalf Of Rob Jarratt via
> cctalk
> Sent: Saturday, February 26, 2022 3:15 AM
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Racking a PDP-11/24
> 
> I am wondering if I have racked my 11/24 correctly.
> 
> As you can see here:
> https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/ I have put
> the CPU at the top and the two RL02 drives underneath.
> 
> The problem is that the CPU enclosure catches on the RL02 underneath.
> There is a bit of play in the mounting bracket:
> https://rjarratt.files.wordpress.com/2022/02/cpu-mounting-bracket.jpg.
> With a bit of manipulation I can get the CPU to slide in. However, I am
> wondering if I have racked it correctly? I don't think there is room to
move
> the RL02s down and it would presumably leave a bit of a gap below the CPU.
> There seems to be very little clearance between the CPU and the RL02 at
the
> front but more at the back, but I am sure that the rails are mounted
> horizontally. Is it just a matter of tightening the big screws that hold
the
> mounting brackets to stop the play? If so I am not sure I have a big
enough
> screwdriver!
> 
> Regards
> 
> Rob

Racking a PDP-11/24

[Rob Jarratt via cctalk]

I am wondering if I have racked my 11/24 correctly.

 

As you can see here:
https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/ I have put the
CPU at the top and the two RL02 drives underneath.

 

The problem is that the CPU enclosure catches on the RL02 underneath. There
is a bit of play in the mounting bracket:
https://rjarratt.files.wordpress.com/2022/02/cpu-mounting-bracket.jpg. With
a bit of manipulation I can get the CPU to slide in. However, I am wondering
if I have racked it correctly? I don't think there is room to move the RL02s
down and it would presumably leave a bit of a gap below the CPU. There seems
to be very little clearance between the CPU and the RL02 at the front but
more at the back, but I am sure that the rails are mounted horizontally. Is
it just a matter of tightening the big screws that hold the mounting
brackets to stop the play? If so I am not sure I have a big enough
screwdriver!

 

Regards

 

Rob

RE: 11/83 OS load - problem solved.

[Rob Jarratt via cctalk]

If images don't exist would it be possible to image them and get them on 
BitSavers?

> -Original Message-
> From: cctalk  On Behalf Of Rod Smallwood
> via cctalk
> Sent: 24 February 2022 08:05
> To: Bill Gunshannon via cctalk 
> Subject: 11/83 OS load - problem solved.
> 
> Hi
> 
>   Well I now have a full set of DEC orignal MicroRSX RX50 distribution 
> disks.
> 
> An old friend who I worked with at DEC had kept his install go bag and there
> they where. Not only that they are good and do boot.
> 
> Its not over, RT-11 would be a better fit so I'm looking at that.
> 
> Rod

RE: Is The M9312 Boot Module Essential?

[Rob Jarratt via cctalk]

I saw this reply later than the previous one. It confirms that I don't
*need* it for booting, but it would be useful.

I suspect some of the other cards that were in the machine might do the
necessary termination stuff.

Thanks

Rob

> -Original Message-
> From: cctalk  On Behalf Of Noel Chiappa via
> cctalk
> Sent: 19 February 2022 09:18
> To: cctalk@classiccmp.org
> Cc: j...@mercury.lcs.mit.edu
> Subject: Re: Is The M9312 Boot Module Essential?
> 
> > From: Rob Jarratt
> 
> > is the M9312 essential to ever get this machine to boot up an
operating
> > system?
> 
> Interesting question. I don't have my -11/24 running yet, so this reply is
> theoretical, not tried in practice (and as we all know, the difference
between
> theory and practice is even larger in practice than it is in theory), but
here
> goes.
> 
> The M9312 basically provides two things: 1) UNIBUS termination, and 2)
> boostrap ROM.
> 
> To further subdivide the former, it provides 1A) analog termination (i.e.
a
> resistance at the end of a transmission line that prevents reflections of
> signals passing down the otherwise un-terminated transmission lines of the
> bus), 1B) pullups (so those transmission lines normally float at roughly
3V,
> unless actively driven by one of the boards plugged into the bus) and 1C)
> 'SACK turnaround' (a start-up 'safety check' where an un-requested - and
> thus 'un-grabbed' by any device - bus grant from the CPU on start-up is
> 'turned around' by the terminator; this verifies that the grant lines are
un-
> broken between the CPU and the terminator - e.g. by someone forgetting to
> plug in a grant jumper).
> 
> 1A is not _absolutely_ necessary; this can be seen in small QBUS systems
> (the QBUS is, at the analog level, sort of identical to the UNIBUS; this
an be
> seen in the use of the same transceiver chips, such as 8641's, on both)
which
> can get away without 1A in small configurations. Whether it's needed on
your
> -11/24 is hard to predict, theoretically; the easiest thing is to just try
it and
> see. Note: it may 'work' without it, but not be as _reliable_ as with it.
> 
> 1B _is_ necessary, but can be provided anywhere on the bus; most
> UNIBUS/QBUS CPUs have it built in, and so does the KDF11-U of the -11/24:
> see pg.  of MP01028.
> 
> 1C is required by _some_ UNIBUS CPUs (ISTR that the -11/04 won't run
> without it), but the KDF11's in general don't; e.g. the -11/23 definitely
runs
> without it. The KDF11-U might have outboard circuitry to require it, but
I'm
> too lazy to grovel over the prints to see. Easiest to just try it and see.
> 
> 
> For 2, it all depends on what you're booting from. E.g. the RK11 has a
simple
> enough bootstrap that you can just enter it manually (although it gets old
> after a while - I remember re-'programming' (think 'soldering iron' :-) a
> castoff BM-792 someone gave us for our -11/40 so I wouldn't have to).
> 
> But if you're loading it over the console serial line, e.g. with PDP11GUI,
you
> don't need any ROM bootstrap - the built in console ODT will be enough.
> You can also load a bootstrap that way; I was booting off the QSIC RK11
with a
> boostrap loaded over the console serial line; that was faster than the
> bootstrap in the BDV11. This requires finding - or writing - a bootstrap,
which
> for later DEC mass storage controllers is not trivial.
> 
> YMMV.
> 
> 
> TLDR version - probably not!
> 
>   Noel

RE: Is The M9312 Boot Module Essential?

[Rob Jarratt via cctalk]

I am in the UK (Manchester). I suspect there are some people I know who would 
be able to lend me one.

 

The bit I am confused about is that I got a console prompt with just the CPU 
(M7133) and Unibus Map Module (M7134) installed. Presumably I could type in a 
bootstrap program from there?

 

Regards

 

Rob

 

From: Bill Degnan  
Sent: 19 February 2022 08:29
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: Re: Is The M9312 Boot Module Essential?

 

You need some way to initialize the system to the peripheral that contains 
the.OS media.  The m9312 is a general purpose co ntroller for that purpose.  
There are roms that install on the 9312 for almost any drive hardware.  When 
you get a system that comes with an M9312, it will have the appropriate 
peripheral Rom and console ROM installed from the factory.  The M9312 also had 
a console ROM monitor to allow a person to attach a terminal to enter bootstrap 
commands. 

 

But yes the *function* provided by the m9312 is necessary.  

 

Alternatively you can key in the bootstrap that tells your system how to boot 
your OS and communicate with your boot device.  

 

The m9312 is less useful without at least a console ROM

 

Where are you located?  Maybe someone nearby has in you could borrow just to 
see what you need.

Bill.

 

On Sat, Feb 19, 2022, 2:12 AM Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote:

I have a PDP-11/24. I have never got very far with it because of power
supply problems which I am hopeful will be resolved soon. Looking at the
technical manual, it describes an M9312 bootstrap/terminator module. The
machine did not come with one of these.



I am not sure how the machine could have been useful without it. It did work
briefly before the PSU failed and I remember getting a console prompt. So,
is the M9312 essential to ever get this machine to boot up an operating
system?



Thanks



Rob

Is The M9312 Boot Module Essential?

[Rob Jarratt via cctalk]

I have a PDP-11/24. I have never got very far with it because of power
supply problems which I am hopeful will be resolved soon. Looking at the
technical manual, it describes an M9312 bootstrap/terminator module. The
machine did not come with one of these.

 

I am not sure how the machine could have been useful without it. It did work
briefly before the PSU failed and I remember getting a console prompt. So,
is the M9312 essential to ever get this machine to boot up an operating
system?

 

Thanks

 

Rob

RE: Testing H745 Regulators

[Rob Jarratt via cctalk]

I found a 400V part on Mouser and ordered that. 

> -Original Message-
> From: Jonathan Chapman 
> Sent: 18 February 2022 21:23
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Subject: RE: Testing H745 Regulators
> 
> > I always forget that the VAC is the RMS and not the peak-to-peak. I
> > will look for a minimum rating of 100V.
> 
> 600V bridge modules are often more cost effective, and more likely to be in
> stock. That's why I went with the Vishay part I used.
> 
> If you can't find what you need due to shortages, you can also use a Faston
> tab rectifier and solder wire legs to it.
> 
> Thanks,
> Jonathan

RE: Testing H745 Regulators

[Rob Jarratt via cctalk]

> -Original Message-
> From: Brent Hilpert 
> Sent: 18 February 2022 06:30
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: Testing H745 Regulators
> 
> On 2022-Feb-17, at 2:38 PM, Rob Jarratt wrote:
> >> -Original Message-
> >> From: Brent Hilpert 
> >>
> >> 20V on a 10 ohm load: current = 2A.
> >> 15V, 1.5A.
> >>
> >> In this regulator design there is no path for more current than that
> >> which the load draws, aside from temporary peak currents to charge
> >> capacitors. If you're drawing 5A DC from the bench supply, something
> >> beyond 'failure to start' is wrong.
> >
> > That's interesting. On the H744s I have observed that if I have a high
> > load the bench PSU current limiter operates and the regulator cannot
> > output +5V, but if I start with a lower load and then add load, it can
> > continue to operate. Is the H745 different to the point that I
> > shouldn't expect this kind of behaviour? If it is the same, then why
> > do the H744s do this? I have tried waiting a few moments to allow the
> > input capacitor to charge up, but the regulator just does not start.
> 
> Presumably your high test load plus the initial cap-charge current is
pushing
> the bench PS into current limit, that is, with a high load there is less
available
> current to charge the caps before the bench PS starts current limiting.
This
> would slow down the cap charge rate, so it would take longer for the caps
to
> charge. I can't say I see it 'stopping starting', but it would lengthen
the time to
> 'start'. How long depends on the numbers. There may also be some
> dependance on how your bench PS responds in current limit.
> 

But if that was the case shouldn't it just take a bit of time to get going,
once the input cap has charged wouldn't it start regulating? It shouldn't
take more than a few seconds, but it never seems to start. Anyway with the
H745 the problem seems to be elsewhere.

> 
> On 2022-Feb-17, at 10:19 PM, Rob Jarratt via cctalk wrote:
> > Regarding the rating I am not clear what the rating of the original part
is, I
> haven't been able to find a datasheet for it, I have seen suggestions for
both
> 20A and 35A, I do know that the H745 regulator is fed 20-30VAC from a
> transformer.
> >
> > So presumably going for a 35A rating is the safer bet, and going for a
> minimum of 50V peak reverse voltage would be sufficient?
> 
> 
> At 30VAC input, peak V is ~ 44V, you're probably better off with
rectifiers
> higher than 50 PIV to provide some safety margin.

I always forget that the VAC is the RMS and not the peak-to-peak. I will
look for a minimum rating of 100V.

Thanks

Rob

RE: Testing H745 Regulators

[Rob Jarratt via cctalk]

Thanks Jonathan,

In my case height doesn't matter.

Regarding the rating I am not clear what the rating of the original part is, I 
haven't been able to find a datasheet for it, I have seen suggestions for both 
20A and 35A, I do know that the H745 regulator is fed 20-30VAC from a 
transformer.

So presumably going for a 35A rating is the safer bet, and going for a minimum 
of 50V peak reverse voltage would be sufficient?

Thanks

Rob

> -Original Message-
> From: Jonathan Chapman 
> Sent: 17 February 2022 23:37
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Subject: RE: Testing H745 Regulators
> 
> I used a Vishay GBPC3506W-E4/51, which is 600V 35A, 400A inrush rated.
> Looks like they're currently on-order at Mouser, I don't know about other
> suppliers.
> 
> The GBPC3506W-E4/51 is a little thinner than the bridge rectifier DEC used
> originally, which sometimes matters. For the 5411086 board in my PDP-11/10,
> height does matter, as it shares a heatsink with other parts. I stacked a few
> washers under the center of the bridge:
> 
> http://www.glitchwrks.com/images/dec/pdp1110_psu_repair/rectifier_spac
> er.jpg
> 
> Thanks,
> Jonathan
> 
> --- Original Message ---
> 
> On Thursday, February 17th, 2022 at 17:57, Rob Jarratt via cctalk
>  wrote:
> 
> > Hmmm this would suggest 200V 20A
> >
> >
> https://archive.org/stream/bitsavers_decfieldSeeTechnicalManual1972_191
> 26909
> >
> > 8/DEC_Field_Service_Technical_Manual_1972_djvu.txt rather than 35A.
> Given
> >
> > the 20-30VAC input though, I presume a 50V part would be OK still?
> >
> > > -Original Message-
> > >
> > > From: Rob Jarratt robert.jarr...@ntlworld.com
> > >
> > > Sent: 17 February 2022 22:38
> > >
> > > To: 'Brent Hilpert' bhilp...@shaw.ca; r...@jarratt.me.uk; 'General
> > >
> > > Discussion: On-Topic and Off-Topic Posts' cctalk@classiccmp.org
> > >
> > > Subject: RE: Testing H745 Regulators
> > >
> > > Sorry it has been a while responding. I have been looking further at my
> >
> > H745
> >
> > > regulators this evening. Below is what I have found and my responses to
> >
> > the
> >
> > > various questions.
> > >
> > > In summary, it looks like the rectifier may be faulty. It is marked
> >
> > NSS3514.
> >
> > > I believe it is a 35A part. Can anyone suggest a suitable replacement?
> >
> > Maybe
> >
> > > something like this:
> > >
> > > https://uk.farnell.com/taiwan-semiconductor/gbpc35005w-t0/bridge-
> > >
> > > rectifier-1
> > >
> > > ph-35a-50v-thd/dp/2677250?st=rectifier which is rated 50V and 35A?
> > >
> > > Thanks
> > >
> > > Rob
> > >
> > > > -Original Message-
> > > >
> > > > From: Brent Hilpert bhilp...@shaw.ca
> > > >
> > > > Sent: 27 January 2022 03:33
> > > >
> > > > To: r...@jarratt.me.uk; Rob Jarratt robert.jarr...@ntlworld.com;
> > > >
> > > > General
> > > >
> > > > Discussion: On-Topic and Off-Topic Posts cctalk@classiccmp.org
> > > >
> > > > Subject: Re: Testing H745 Regulators
> > > >
> > > > On 2022-Jan-26, at 3:41 PM, Rob Jarratt via cctalk wrote:
> > > >
> > > > > I am trying to test a couple of H745 regulators with a DC bench PSU
> > > > >
> > > > > and I am having some problems with testing them.
> > > > >
> > > > > My bench PSU is a twin unit so I can supply the +15V required as
> > > > >
> > > > > well as the "AC" input using 20VDC from the other half of the bench
> >
> > PSU.
> >
> > > > > The problem is that I don't think the bench PSU can supply enough
> > > > >
> > > > > startup current to allow the regulator to run. It can only supply 5A
> > > > >
> > > > > max.
> > > > >
> > > > > I have seen with the H744s that if I put too big a load on them,
> > > > >
> > > > > then they can't start because of the heavy startup current required.
> > > > >
> > > > > I can start them with a lower load and then add load once the
> > > > >
> > > > > regulator is running without breaching the current limit of the PSU.
> > > > >
> > > 

RE: Testing H745 Regulators

[Rob Jarratt via cctalk]

Hmmm this would suggest 200V 20A
https://archive.org/stream/bitsavers_decfieldSeeTechnicalManual1972_19126909
8/DEC_Field_Service_Technical_Manual_1972_djvu.txt rather than 35A. Given
the 20-30VAC input though, I presume a 50V part would be OK still?

> -Original Message-
> From: Rob Jarratt 
> Sent: 17 February 2022 22:38
> To: 'Brent Hilpert' ; r...@jarratt.me.uk; 'General
> Discussion: On-Topic and Off-Topic Posts' 
> Subject: RE: Testing H745 Regulators
> 
> Sorry it has been a while responding. I have been looking further at my
H745
> regulators this evening. Below is what I have found and my responses to
the
> various questions.
> 
> In summary, it looks like the rectifier may be faulty. It is marked
NSS3514.
> I believe it is a 35A part. Can anyone suggest a suitable replacement?
Maybe
> something like this:
> https://uk.farnell.com/taiwan-semiconductor/gbpc35005w-t0/bridge-
> rectifier-1
> ph-35a-50v-thd/dp/2677250?st=rectifier which is rated 50V and 35A?
> 
> Thanks
> 
> Rob
> 
> > -Original Message-
> > From: Brent Hilpert 
> > Sent: 27 January 2022 03:33
> > To: r...@jarratt.me.uk; Rob Jarratt ;
> > General
> > Discussion: On-Topic and Off-Topic Posts 
> > Subject: Re: Testing H745 Regulators
> >
> > On 2022-Jan-26, at 3:41 PM, Rob Jarratt via cctalk wrote:
> > > I am trying to test a couple of H745 regulators with a DC bench PSU
> > > and I am having some problems with testing them.
> > >
> > > My bench PSU is a twin unit so I can supply the +15V required as
> > > well as the "AC" input using 20VDC from the other half of the bench
PSU.
> > > The problem is that I don't think the bench PSU can supply enough
> > > startup current to allow the regulator to run. It can only supply 5A
> max.
> > >
> > > I have seen with the H744s that if I put too big a load on them,
> > > then they can't start because of the heavy startup current required.
> > > I can start them with a lower load and then add load once the
> > > regulator is running without breaching the current limit of the PSU.
> > >
> > > With the H745s I have tried reducing the load to see if I can get
> > > them to start, but a 10R load appears to be too much and the
> > > regulators draw the full 5A without outputting -15V.
> > >
> > > I have two H745s, both exhibit the same behaviour. I suppose they
> > > could both have the same fault, but I am inclined to think that
> > > perhaps they need a higher startup current than I can supply. Can
> > > anyone
> > confirm this?
> >
> >
> > 20V on a 10 ohm load: current = 2A.
> > 15V, 1.5A.
> >
> > In this regulator design there is no path for more current than that
> > which
> the
> > load draws, aside from temporary peak currents to charge capacitors.
> > If you're drawing 5A DC from the bench supply, something beyond
> > 'failure to start' is wrong. I would expect this supply to operate at
> > small load
> regardless.
> >
> 
> 
> That's interesting. On the H744s I have observed that if I have a high
load the
> bench PSU current limiter operates and the regulator cannot output +5V,
but
> if I start with a lower load and then add load, it can continue to
operate. Is
> the H745 different to the point that I shouldn't expect this kind of
behaviour?
> If it is the same, then why do the H744s do this? I have tried waiting a
few
> moments to allow the input capacitor to charge up, but the regulator just
> does not start.
> 
> 
> 
> > What is happening to the bench supply voltage? Does it go into current
> limit?
> > Does this bench supply have an adjustable current limit?, so that you
> could
> > run it up starting at a low current while taking measurements. Or,
> > does
> the
> > current respond with some linearity to varying the input voltage?
> >
> > What happens with no load R?
> 
> Yes, it hits the limit, and it does so even if I have no load at all.
> 
> >
> > Are you running it for any length of time at 5A? (Sounds like a bad
> > idea
> at this
> > point) Anything getting warm?
> 
> I daren't run it for long. I just tried running it for 15 seconds and
couldn't find
> anything hot.
> 
> >
> > Is the 723 socketed? Pull it and run it up while watching what happens
> > around the drive transistors and elsewhere.
> > If the 723 is not socketed, consider pulling Q5 or opening it's
> > emitter connection. With no drive to the drive transistors, input
> > current should
> be nil.
> >
> > Are any of the d

RE: Testing H745 Regulators

[Rob Jarratt via cctalk]

Sorry it has been a while responding. I have been looking further at my H745
regulators this evening. Below is what I have found and my responses to the
various questions.

In summary, it looks like the rectifier may be faulty. It is marked NSS3514.
I believe it is a 35A part. Can anyone suggest a suitable replacement? Maybe
something like this:
https://uk.farnell.com/taiwan-semiconductor/gbpc35005w-t0/bridge-rectifier-1
ph-35a-50v-thd/dp/2677250?st=rectifier which is rated 50V and 35A?

Thanks

Rob

> -Original Message-
> From: Brent Hilpert 
> Sent: 27 January 2022 03:33
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: Testing H745 Regulators
> 
> On 2022-Jan-26, at 3:41 PM, Rob Jarratt via cctalk wrote:
> > I am trying to test a couple of H745 regulators with a DC bench PSU
> > and I am having some problems with testing them.
> >
> > My bench PSU is a twin unit so I can supply the +15V required as well
> > as the "AC" input using 20VDC from the other half of the bench PSU.
> > The problem is that I don't think the bench PSU can supply enough
> > startup current to allow the regulator to run. It can only supply 5A
max.
> >
> > I have seen with the H744s that if I put too big a load on them, then
> > they can't start because of the heavy startup current required. I can
> > start them with a lower load and then add load once the regulator is
> > running without breaching the current limit of the PSU.
> >
> > With the H745s I have tried reducing the load to see if I can get them
> > to start, but a 10R load appears to be too much and the regulators
> > draw the full 5A without outputting -15V.
> >
> > I have two H745s, both exhibit the same behaviour. I suppose they
> > could both have the same fault, but I am inclined to think that
> > perhaps they need a higher startup current than I can supply. Can anyone
> confirm this?
> 
> 
> 20V on a 10 ohm load: current = 2A.
> 15V, 1.5A.
> 
> In this regulator design there is no path for more current than that which
the
> load draws, aside from temporary peak currents to charge capacitors. If
> you're drawing 5A DC from the bench supply, something beyond 'failure to
> start' is wrong. I would expect this supply to operate at small load
regardless.
> 


That's interesting. On the H744s I have observed that if I have a high load
the bench PSU current limiter operates and the regulator cannot output +5V,
but if I start with a lower load and then add load, it can continue to
operate. Is the H745 different to the point that I shouldn't expect this
kind of behaviour? If it is the same, then why do the H744s do this? I have
tried waiting a few moments to allow the input capacitor to charge up, but
the regulator just does not start. 



> What is happening to the bench supply voltage? Does it go into current
limit?
> Does this bench supply have an adjustable current limit?, so that you
could
> run it up starting at a low current while taking measurements. Or, does
the
> current respond with some linearity to varying the input voltage?
> 
> What happens with no load R?

Yes, it hits the limit, and it does so even if I have no load at all.

> 
> Are you running it for any length of time at 5A? (Sounds like a bad idea
at this
> point) Anything getting warm?

I daren't run it for long. I just tried running it for 15 seconds and
couldn't find anything hot.

> 
> Is the 723 socketed? Pull it and run it up while watching what happens
> around the drive transistors and elsewhere.
> If the 723 is not socketed, consider pulling Q5 or opening it's emitter
> connection. With no drive to the drive transistors, input current should
be nil.
> 
> Are any of the drive transistors socketed, so they could be measured out
of
> circuit? and other R measurements made without them in circuit?
> 
> Pull F1 to isolate circuitry. Still draws current?

Well, that is interesting. I pulled the fuse and it does indeed pull in all
the current. According to the schematic, the only thing it could be is C1 or
R1. I tested C1 out of circuit not too long ago and that seemed fine. I took
it out again to be sure and C1 seems fine. R1 also seems to be fine. Which
leads me to suspect the rectifier may be bad, although some in circuit tests
seem to show it is OK. So, to be clear, I tested with the fuse out, with the
rectifier feeding C1, with R1 (3.9K) across C1. The bench PSU hits 5A when
trying to supply 20VDC to the rectifier. As the fuse was removed there
couldn't be anything else that is shorted. The capacitor does not charge up.
This does suggest the rectifier is the problem.


> 
> Have you looked for shorts/leaks?, especially leaky junctions in
transistors
> Q2::Q5.
> e.g. R meas

RE: Testing H745 Regulators

[Rob Jarratt via cctalk]

> -Original Message-
> From: W2HX 
> Sent: 27 January 2022 00:11
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts 
> Subject: RE: Testing H745 Regulators
> 
> Any chance you have a second supply you can parallel with the one you're
> using? Some bench supplies are dual supplies and have a parallel mode so
> you can benefit from both sides of the supply and they can track the same
> voltage. Or just put two supplies in parallel with the same voltage set on
both
> 

It is already a twin PSU, I am using one half to power the AC input and the
other half to power the +15V input.

I do have a separate bench PSU I could use for the +15V and twin up the
actual twin PSU, so I might try that next.


> 73 Eugene W2HX
> Subscribe to my Youtube Channel: https://www.youtube.com/c/w2hx-
> channel/videos
> 
> 
> 
> -----Original Message-----
> From: cctalk  On Behalf Of Rob Jarratt via
> cctalk
> Sent: Wednesday, January 26, 2022 6:42 PM
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Testing H745 Regulators
> 
> I am trying to test a couple of H745 regulators with a DC bench PSU and I
am
> having some problems with testing them.
> 
> 
> 
> My bench PSU is a twin unit so I can supply the +15V required as well as
the
> "AC" input using 20VDC from the other half of the bench PSU. The problem
is
> that I don't think the bench PSU can supply enough startup current to
allow
> the regulator to run. It can only supply 5A max.
> 
> 
> 
> I have seen with the H744s that if I put too big a load on them, then they
> can't start because of the heavy startup current required. I can start
them
> with a lower load and then add load once the regulator is running without
> breaching the current limit of the PSU.
> 
> 
> 
> With the H745s I have tried reducing the load to see if I can get them to
start,
> but a 10R load appears to be too much and the regulators draw the full 5A
> without outputting -15V.
> 
> 
> 
> I have two H745s, both exhibit the same behaviour. I suppose they could
> both have the same fault, but I am inclined to think that perhaps they
need a
> higher startup current than I can supply. Can anyone confirm this?
> 
> 
> 
> Thanks
> 
> 
> 
> Rob

Testing H745 Regulators

[Rob Jarratt via cctalk]

I am trying to test a couple of H745 regulators with a DC bench PSU and I am
having some problems with testing them.

 

My bench PSU is a twin unit so I can supply the +15V required as well as the
"AC" input using 20VDC from the other half of the bench PSU. The problem is
that I don't think the bench PSU can supply enough startup current to allow
the regulator to run. It can only supply 5A max.

 

I have seen with the H744s that if I put too big a load on them, then they
can't start because of the heavy startup current required. I can start them
with a lower load and then add load once the regulator is running without
breaching the current limit of the PSU.

 

With the H745s I have tried reducing the load to see if I can get them to
start, but a 10R load appears to be too much and the regulators draw the
full 5A without outputting -15V.

 

I have two H745s, both exhibit the same behaviour. I suppose they could both
have the same fault, but I am inclined to think that perhaps they need a
higher startup current than I can supply. Can anyone confirm this?

 

Thanks

 

Rob

Testing the H745 Regulator

[Rob Jarratt via cctalk]

I am going to be moving on to testing some H745 regulators. If I understand
the schematic correctly the +15V input is the reference voltage used to
regulate the -15V output, is that correct? If so, then presumably it doesn't
need a whole lot of current, is that right?

 

Thanks

 

Rob

RE: DEC 3000/600 Alphaserver problems

[Rob Jarratt via cctalk]

> 
> The hot tweezer sounds very nice but I think I'll stick with what I have
for
> now.  These machines have other problems and won't ever do anything
> useful unless I get the cache issues sorted out so I don't necessarily
want to
> invest in new tools for them to end up not getting them repaired.
> 

I love an excuse to buy a tool. I think *good* tools are always a worthwhile
investment. After a buying a tool that I think I will only use once, I tend
to find other uses for it over time. Maybe you will find yourself needing to
replace an SMT component on another machine in the future?


> Many thanks.
> 
> Regards,
> Peter.
> 
> >
> > Richard Schauer
> > KF9VP
> >

Mate-n-lok connector for H744 Regulator

[Rob Jarratt via cctalk]

To make testing of the H744 a bit easier I would like to try to make up some
connectors for the mate-n-lok connector to make it easier to connect power
and load. I know the H744 uses a mate-n-lok connector, but there seem to be
a lot of different types and I don't seem to be able to find a type that
would work. Does anyone know what the correct one is?

 

Thanks

 

Rob

RE: Source for replacement caps in H744 regulators

[Rob Jarratt via cctalk]

Weirdly, the whine has disappeared. This is after I put the suspect capacitors 
back in. Because I had previously removed them for reforming, I wonder if one 
of them was not properly screwed in?

Regards

Rob

> -Original Message-
> From: cctalk  On Behalf Of Jay Jaeger via
> cctalk
> Sent: 07 January 2022 16:02
> To: cctalk@classiccmp.org
> Subject: Re: Source for replacement caps in H744 regulators
> 
> On 1/6/2022 7:03 PM, W2HX via cctalk wrote:
> > My 2c. I am not familiar with a "whine" but certainly a "hum." Sometimes if
> a power supply has seen a lot of heavy load over its lifetime, the heat
> generated can begin to do things to the transformer. And once that heat has
> done its "thing" to the transformer, it stays that way. And no replacing
> external components will change the hum. However, there are some
> transformers with bolts and nuts that hold the laminations together.
> Sometimes they can be tightened to reduce the hum. I don’t know this PS
> specifically and whether it falls into this category or not.
> >
> > I don’t know if what you are hearing is transformer hum, but if it is, you
> may just have to live with it.
> >
> > 73 Eugene W2HX
> > Subscribe to my Youtube Channel:
> > https://www.youtube.com/c/w2hx-channel/videos
> 
> This does not fall "into this category".  This is typically high frequency (in
> excess of 10KHz) whine, not 60 cycle, 120 cycle or even
> 400 cycle "hum".
> 
> My experience with many PDP-11 machines going back to the mid 1970s, and
> those in my collection, is that this whine from the *switching* power
> supplies is very common. For some people, it may be above the frequency
> that they can hear.  For me it is not (I could also hear burglar alarms in 
> excess
> of 20KHz back in the day, though I doubt I could now, at age 70.)
> 
> My *guess* is that it comes from the inductors in the switching circuit, and 
> is
> *mechanical*, induced by the switching waveform, which in turn is
> dependent upon load.  If I had one that was really bad, I'd be tempted to put
> on a glove for insulation and hold those to see if the mechanical pressure
> made any difference.
> 
> JRJ