[cctalk] Re: NRAO Data tapes

[Warner Losh via cctalk]

On Tue, May 9, 2023, 9:47 PM Mark Linimon via cctalk 
wrote:

> I wonder if those date to the first time I visited, where the PDP-11s
> were still installed :-)
>
> Are there still T-shirts available?  Mine from that trip is trash :-)
>

The pdp-11s were Gove by the time I got to tech in 85 I think


mcl
>

[cctalk] Re: NRAO Data tapes

[Mark Linimon via cctalk]

I wonder if those date to the first time I visited, where the PDP-11s
were still installed :-)

Are there still T-shirts available?  Mine from that trip is trash :-)

mcl

[cctalk] Re: Altair 8800 question

[Tarek Hoteit via cctalk]

Not to complicate the haunt for the first Altair, but there is no guarantee 
that it ever existed. Maybe it was an urban legend that it was shipped. If it 
did, did it work? Might just buy the remaining issue of Popular Electronics 
featuring the fake Altair. 

Regards,
Tarek Hoteit

> On May 9, 2023, at 6:43 PM, Sellam Abraham via cctalk  
> wrote:
> 
> On Tue, May 9, 2023 at 4:01 PM Fred Cisin via cctalk 
> wrote:
> 
>> 
>> Both UPS and Fedex will sometimes falsely claim that they had made a
>> delivery attempt.
> 
> 
> I've experienced this.
> 
> 
>>  Does REA claim to have delivered the Altair?
>> 
> 
> I don't know that anyone can ever know at this point.
> 
> I'm thinking to put out a small bounty for any verifiable information
> leading to knowledge of the fate of the shipment, and a larger bounty
> leading to the prototype itself.  Anonimity guaranteed, no questions asked.
> 
> Sellam

[cctalk] Re: Altair 8800 question

[Sellam Abraham via cctalk]

On Tue, May 9, 2023 at 4:01 PM Fred Cisin via cctalk 
wrote:

>
> Both UPS and Fedex will sometimes falsely claim that they had made a
> delivery attempt.


I've experienced this.


>   Does REA claim to have delivered the Altair?
>

I don't know that anyone can ever know at this point.

I'm thinking to put out a small bounty for any verifiable information
leading to knowledge of the fate of the shipment, and a larger bounty
leading to the prototype itself.  Anonimity guaranteed, no questions asked.

Sellam

[cctalk] Re: Altair 8800 question

[Fred Cisin via cctalk]

On Tue, May 9, 2023 at 1:03 PM steve shumaker via cctalk <
cctalk@classiccmp.org> wrote:

A mostly similar variation has it that they shipped the unit via Railway
Express Agency   (remember REA??)  and as the story goes, it was in
transit when REA closed their doors unexpectedly and with no warning,
went bankrupt.


On Tue, 9 May 2023, Sellam Abraham via cctalk wrote:

I don't believe I ever heard that story.  Then again, I'm not sure where I
got "UPS" from.  Perhaps just an assumption on my part.
I'm too young to remember REA, let alone having ever even heard of it.
Interestingly, there is a tidbit in the REA Stupidpedia article about the
Altair 8800 prototype:
*In November 1975, REA Express terminated operations and filed for
bankruptcy. During the railroad strike of October 1974, the first Altair
8800 microcomputer was lost. It had been shipped from Albuquerque to
Popular Electronics magazine in New York via REA and never arrived.*
https://en.wikipedia.org/wiki/Railway_Express_Agency
Very interesting.  So it's now more plausible that a former REA employee
might have the prototype stuffed away in their garage somewhere.


Think of a big brown UPS step-van, but GREEN.

One more hypothesis: :-)
It's still awaiting dellivery, but REA's deliveries are running a little 
late.  They sent an email just last year, asking for delivery address 
confirmation, but the recipient assumed that it was phishing. :-)



I don't know REA's late package algorithm.
Back when REA was active (early 1970s),

If UPS was unable to deliver a package on time, they would do it next, 
risking making all subsequent packages alsobe a day late.


If Fedex was unable to deliver a package on time, they would put it at the 
END of the queue!  That way, all of the subsequent packages would be on 
time, with just one package being horribly late.  Their advertising was 
what PERCENTAGE OF THEIR packages were on time.  I had something that 
really had to be on time, so I had it shipped to me Fedex.  It was a WEEK 
late!  (so that only one package was late)


Both UPS and Fedex will sometimes falsely claim that they had made a 
delivery attempt.  Does REA claim to have delivered the Altair?


--
Grumpy Ol' Fred

[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: NRAO Data tapes {External}

[Dani Werts via cctalk]

Eric, 

More than happy to send a reel or two your way. 
Just give me some information and I can make it happen. 

Danielle


-Original Message-
From: Eric Moore via cctalk  
Sent: Tuesday, May 9, 2023 3:08 PM
To: General Discussion: On-Topic and Off-Topic Posts 
Cc: Eric Moore 
Subject: [cctalk] Re: NRAO Data tapes {External}

I have an SEL 810A operating, which some of these tapes were likely written 
with. If any of the tapes are labelled I can quickly identify if it is an SEL 
product.

-Eric

On Tue, May 9, 2023, 1:21 PM Chuck Guzis via cctalk 
wrote:

> On 5/9/23 09:36, Dani Werts via cctalk wrote:
> > Hello,
> >
> > I'm an employee with the National radio astronomy observatory here 
> > in
> Socorro New Mexico.
> > As part of our NGVLA upgrades, we are seeking to get rid of old data
> tapes from the tape reel days of Computing. These contain things such 
> as the boot loaders, OS, specific collection programs and antenna 
> movement programs.
> > I personally would hate to see these just wind up in the literal
> dumpster and would like to see them sent out to a museum or an 
> archiving body that can preserve them and keep them safe as a dynamic part of 
> history.
>
> Funny coincidence--I just processed a 1981 9 track tape from Mauna Kea 
> dealing with some Neptune observations.  It's all binary, which makes 
> me wonder if the client has the program to digest it--I wonder if said 
> program might be in your collection of tapes.
>
> --Chuck
>
>
>

[cctalk] Re: NRAO Data tapes

[Eric Moore via cctalk]

I have an SEL 810A operating, which some of these tapes were likely written
with. If any of the tapes are labelled I can quickly identify if it is an
SEL product.

-Eric

On Tue, May 9, 2023, 1:21 PM Chuck Guzis via cctalk 
wrote:

> On 5/9/23 09:36, Dani Werts via cctalk wrote:
> > Hello,
> >
> > I'm an employee with the National radio astronomy observatory here in
> Socorro New Mexico.
> > As part of our NGVLA upgrades, we are seeking to get rid of old data
> tapes from the tape reel days of Computing. These contain things such as
> the boot loaders, OS, specific collection programs and antenna movement
> programs.
> > I personally would hate to see these just wind up in the literal
> dumpster and would like to see them sent out to a museum or an archiving
> body that can preserve them and keep them safe as a dynamic part of history.
>
> Funny coincidence--I just processed a 1981 9 track tape from Mauna Kea
> dealing with some Neptune observations.  It's all binary, which makes me
> wonder if the client has the program to digest it--I wonder if said
> program might be in your collection of tapes.
>
> --Chuck
>
>
>

[cctalk] Re: Altair 8800 question

[Henry Bent via cctalk]

On Tue, 9 May 2023 at 16:53, Sellam Abraham via cctalk <
cctalk@classiccmp.org> wrote:

>
> *In November 1975, REA Express terminated operations and filed for
> bankruptcy. During the railroad strike of October 1974, the first Altair
> 8800 microcomputer was lost. It had been shipped from Albuquerque to
> Popular Electronics magazine in New York via REA and never arrived.*
>
> https://en.wikipedia.org/wiki/Railway_Express_Agency
>
> Very interesting.  So it's now more plausible that a former REA employee
> might have the prototype stuffed away in their garage somewhere.
>

I know that it's fun to believe that the prototype might still exist almost
50 years later, but given my experience with package shipping services I'd
say that it's infinitely more probable that it somehow ended up so damaged
that the shipping service just claimed that they "lost" it.  After all,
what's the probability that someone working for a small-time shipping
service in the mid-'70s was going to have any idea about the value of a
hobbyist computer, let alone any inclination to save it for 50 years?

-Henry

[cctalk] Re: Altair 8800 question

[Sellam Abraham via cctalk]

On Tue, May 9, 2023 at 1:03 PM steve shumaker via cctalk <
cctalk@classiccmp.org> wrote:

> A mostly similar variation has it that they shipped the unit via Railway
> Express Agency   (remember REA??)  and as the story goes, it was in
> transit when REA closed their doors unexpectedly and with no warning,
> went bankrupt.
>

I don't believe I ever heard that story.  Then again, I'm not sure where I
got "UPS" from.  Perhaps just an assumption on my part.

I'm too young to remember REA, let alone having ever even heard of it.

Interestingly, there is a tidbit in the REA Stupidpedia article about the
Altair 8800 prototype:

*In November 1975, REA Express terminated operations and filed for
bankruptcy. During the railroad strike of October 1974, the first Altair
8800 microcomputer was lost. It had been shipped from Albuquerque to
Popular Electronics magazine in New York via REA and never arrived.*

https://en.wikipedia.org/wiki/Railway_Express_Agency

Very interesting.  So it's now more plausible that a former REA employee
might have the prototype stuffed away in their garage somewhere.

Sellam

[cctalk] Re: Altair 8800 question

[steve shumaker via cctalk]

A mostly similar variation has it that they shipped the unit via Railway 
Express Agency   (remember REA??)  and as the story goes, it was in 
transit when REA closed their doors unexpectedly and with no warning, 
went bankrupt.


Steve

On 5/9/23 11:47 AM, Sellam Abraham via cctalk wrote:

On Tue, May 9, 2023 at 11:29 AM Brent Hilpert via cctalk <
cctalk@classiccmp.org> wrote:


Might note also that the Altair on/in the Pop Electronics issue is
physically vastly different than what shipped, it's not even the same case.


Yes.  According to lore, that unit got lost in transit to Popular
Electronics.

My theories:

1) It actually did get lost in transit
2) MITS lied and said they sent it, but didn't, because they weren't ready
with the prototype for the photoshoot and needed to buy more time to finish
it
3) The UPS driver, being an aspiring if unscrupulous electronics and
computer nerd, and tipped off as to what was in the package, "lost" it and
took it home for himself

To the extent that prototype is still out there, it would definitely be one
of the few truly "holy grails" of the computer collecting hobby, along with
the missing chunk of the Apple 1 prototype.

Sellam

[cctalk] Re: Altair 8800 question

[W2HX via cctalk]

Ok thanks everyone for the clarification. 


73 Eugene W2HX
Subscribe to my Youtube Channel: https://www.youtube.com/@w2hx/videos
 

-Original Message-
From: Brent Hilpert via cctalk  
Sent: Tuesday, May 9, 2023 2:29 PM
To: General Discussion: On-Topic and Off-Topic Posts 
Cc: Brent Hilpert 
Subject: [cctalk] Re: Altair 8800 question 

On 2023-May-09, at 11:08 AM, W2HX via cctalk wrote:
> I see some altairs have a metal escutcheon on the bottom with the stylized 
> words "MITS ALTAIR 8800 COMPUTER" whereas others, the front panel is just the 
> dark faceplate top to bottom. What is the difference? Would one have been a 
> kit and the other sold fully assembled? Or maybe later units vs earlier units?


I'm currently working on restoration of an 8800. On this unit, as Jonathan 
mentioned, the strip is glued/stuck on and some could easily be getting lost 
over the years.

Might note also that the Altair on/in the Pop Electronics issue is physically 
vastly different than what shipped, it's not even the same case.

[cctalk] Re: Altair 8800 question

[Sellam Abraham via cctalk]

On Tue, May 9, 2023 at 11:29 AM Brent Hilpert via cctalk <
cctalk@classiccmp.org> wrote:

>
> Might note also that the Altair on/in the Pop Electronics issue is
> physically vastly different than what shipped, it's not even the same case.


Yes.  According to lore, that unit got lost in transit to Popular
Electronics.

My theories:

1) It actually did get lost in transit
2) MITS lied and said they sent it, but didn't, because they weren't ready
with the prototype for the photoshoot and needed to buy more time to finish
it
3) The UPS driver, being an aspiring if unscrupulous electronics and
computer nerd, and tipped off as to what was in the package, "lost" it and
took it home for himself

To the extent that prototype is still out there, it would definitely be one
of the few truly "holy grails" of the computer collecting hobby, along with
the missing chunk of the Apple 1 prototype.

Sellam

[cctalk] Re: Altair 8800 question

[Brent Hilpert via cctalk]

On 2023-May-09, at 11:08 AM, W2HX via cctalk wrote:
> I see some altairs have a metal escutcheon on the bottom with the stylized 
> words "MITS ALTAIR 8800 COMPUTER" whereas others, the front panel is just the 
> dark faceplate top to bottom. What is the difference? Would one have been a 
> kit and the other sold fully assembled? Or maybe later units vs earlier units?


I'm currently working on restoration of an 8800. On this unit, as Jonathan 
mentioned, the strip is glued/stuck on and some could easily be getting lost 
over the years.

Might note also that the Altair on/in the Pop Electronics issue is physically 
vastly different than what shipped, it's not even the same case.

[cctalk] Re: Altair 8800 question

[Chuck Guzis via cctalk]

On 5/9/23 11:11, Jonathan Chapman via cctalk wrote:
> Everyone lost the metal strip! It's sandwiched in between the trim of the 
> outer case and the dress panel. Some folks stuck it to the dress panel with 
> glue, double-sided tape, etc. Those are usually the machines which still have 
> the metal strip.
> 
> Thanks,
> Jonathan

Yup, lost mine sometime in the 1980s.  The stickum on the strip dried
out pretty quickly.

--Chuck

[cctalk] Re: NRAO Data tapes

[Chuck Guzis via cctalk]

On 5/9/23 09:36, Dani Werts via cctalk wrote:
> Hello,
> 
> I'm an employee with the National radio astronomy observatory here in Socorro 
> New Mexico.
> As part of our NGVLA upgrades, we are seeking to get rid of old data tapes 
> from the tape reel days of Computing. These contain things such as the boot 
> loaders, OS, specific collection programs and antenna movement programs.
> I personally would hate to see these just wind up in the literal dumpster and 
> would like to see them sent out to a museum or an archiving body that can 
> preserve them and keep them safe as a dynamic part of history.

Funny coincidence--I just processed a 1981 9 track tape from Mauna Kea
dealing with some Neptune observations.  It's all binary, which makes me
wonder if the client has the program to digest it--I wonder if said
program might be in your collection of tapes.

--Chuck

[cctalk] Re: Altair 8800 question

[Jonathan Chapman via cctalk]

Everyone lost the metal strip! It's sandwiched in between the trim of the outer 
case and the dress panel. Some folks stuck it to the dress panel with glue, 
double-sided tape, etc. Those are usually the machines which still have the 
metal strip.

Thanks,
Jonathan

--- Original Message ---
On Tuesday, May 9th, 2023 at 14:08, W2HX via cctalk  
wrote:


> 
> 
> I see some altairs have a metal escutcheon on the bottom with the stylized 
> words "MITS ALTAIR 8800 COMPUTER" whereas others, the front panel is just the 
> dark faceplate top to bottom. What is the difference? Would one have been a 
> kit and the other sold fully assembled? Or maybe later units vs earlier units?
> 
> 
> 73 Eugene W2HX
> Check out my Youtube Channel: https://www.youtube.com/@w2hx/videos

[cctalk] Altair 8800 question

[W2HX via cctalk]

I see some altairs have a metal escutcheon on the bottom with the stylized 
words "MITS ALTAIR 8800 COMPUTER" whereas others, the front panel is just the 
dark faceplate top to bottom. What is the difference? Would one have been a kit 
and the other sold fully assembled? Or maybe later units vs earlier units?


73 Eugene W2HX
Check out my Youtube Channel: https://www.youtube.com/@w2hx/videos

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> 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.

Regards,
Peter.

[cctalk] Re: NRAO Data tapes

[Chuck Guzis via cctalk]

It would be useful to know what systems these tapes were used on.

All the best,
--Chuck




On 5/9/23 09:36, Dani Werts via cctalk wrote:
> Hello,
> 
> I'm an employee with the National radio astronomy observatory here in Socorro 
> New Mexico.
> As part of our NGVLA upgrades, we are seeking to get rid of old data tapes 
> from the tape reel days of Computing. These contain things such as the boot 
> loaders, OS, specific collection programs and antenna movement programs.
> I personally would hate to see these just wind up in the literal dumpster and 
> would like to see them sent out to a museum or an archiving body that can 
> preserve them and keep them safe as a dynamic part of history.
> 
> If anyone is interested or knows someone who would be interested in the VLAs 
> data tape library please let me know.
> 
> 
> V/R
> Danielle Werts
> Front end engineer
> VLA Socorro New Mexico

[cctalk] Re: NRAO Data tapes

[The Doctor via cctalk]

--- Original Message ---
On Tuesday, May 9th, 2023 at 09:36, Dani Werts via cctalk 
 wrote:


> As part of our NGVLA upgrades, we are seeking to get rid of old data tapes 
> from the tape
> reel days of Computing. These contain things such as the boot loaders, OS, 
> specific
> collection programs and antenna movement programs.

Reach out to the Computer History Museum (https://computerhistory.org/), they 
might be
interested.  The Internet Archive might also be interested in copies.

The Doctor [412/724/301/703/415/510]
WWW: https://drwho.virtadpt.net/
Don't be mean. You don't have to be mean.

[cctalk] Re: Rainbow H7842 PSU Fault

[Brent Hilpert via cctalk]

On 2023-May-09, at 12:28 AM, Brent Hilpert via cctalk wrote:
> I thought I must be off somewhere by 10^n when first doing the calc. The 51Ω 
> is 3 orders of magnitude away from the 0.01Ω on the other outputs, so a 
> similar diff could be anticipated on the current sense.
> 
> The low current sense might be explained as follows:
> 
> ...

> I'm speculating the unusual shunt regulator in the -12V output is actually a 
> 'controlled freewheeling diode' for a -12V bucking step-down. The choke would 
> be stepping down the voltage from the -12 output secondary from something 
> higher down to the actual -12V output. With a higher V from the secondary, a 
> lower current is needed for the same energy throughput, and thus sense on a 
> lower max current from the secondary. However, one would anticipate the 
> regulation V sense point in such operation to be on the other side of the 
> choke than what's shown in the schematic.

Thinking further this morning. The TIP121 is the wrong orientation or polarity 
to be acting as a freewheeling conductor (--explanation). However, that's the 
transistor section. The 121 has a built-in flyback diode which is the right 
orientation (++explanation). Still some things that don't make sense though, 
like the high EI ratio needed from the order 100 I difference (--explanation), 
and how the load current would be supplied during the charge portion of the 
cycle (--explanation).

[cctalk] NRAO Data tapes

[Dani Werts via cctalk]

Hello,

I'm an employee with the National radio astronomy observatory here in Socorro 
New Mexico.
As part of our NGVLA upgrades, we are seeking to get rid of old data tapes from 
the tape reel days of Computing. These contain things such as the boot loaders, 
OS, specific collection programs and antenna movement programs.
I personally would hate to see these just wind up in the literal dumpster and 
would like to see them sent out to a museum or an archiving body that can 
preserve them and keep them safe as a dynamic part of history.

If anyone is interested or knows someone who would be interested in the VLAs 
data tape library please let me know.


V/R
Danielle Werts
Front end engineer
VLA Socorro New Mexico

[cctalk] Re: Rainbow H7842 PSU Fault

[Brent Hilpert via cctalk]

On 2023-May-08, at 12:27 PM, Peter Coghlan via cctalk wrote:
> 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.


I thought I must be off somewhere by 10^n when first doing the calc. The 51Ω is 
3 orders of magnitude away from the 0.01Ω on the other outputs, so a similar 
diff could be anticipated on the current sense.

The low current sense might be explained as follows:

This power supply does not follow the more-common design of flyback switching 
mains supplies.
This supply is actually 2 (or 3) bucking regulators being fed from a 
mains-isolation transformer, with the bucking regulation going to the PWM 
oscillator then looping back to the driver on the primary side of the 
mains-isolation transformer. This is discernible in the presence of the 
freewheeling diodes beside the rectifier diodes of the +5 & +12 outputs. DEC 
seemed to like doing this as the VaxMate supply from jarratt last year was of 
similar design. A guess is it may have made the inductor design easier, 
separating the flyback operation out to separated inductors rather than in the 
all in the same mains transformer.

I'm speculating the unusual shunt regulator in the -12V output is actually a 
'controlled freewheeling diode' for a -12V bucking step-down. The choke would 
be stepping down the voltage from the -12 output secondary from something 
higher down to the actual -12V output. With a higher V from the secondary, a 
lower current is needed for the same energy throughput, and thus sense on a 
lower max current from the secondary. However, one would anticipate the 
regulation V sense point in such operation to be on the other side of the choke 
than what's shown in the schematic.

[cctalk] Re: Rainbow H7842 PSU Fault

[Brent Hilpert via cctalk]

On 2023-May-08, at 11:24 PM, Rob Jarratt via cctalk wrote:
> 
> 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?

The TIP121 would/could be conducting because under your test setup with no 
VStart-12 it's biased in the reverse-from-normal direction and could be 
conducting in reverse through the BC junction. The 0.016V across the 10Ω R from 
C-to-GND is nonetheless higher than would be expected from the trickle current 
available.

Which brings me to this, in reply to earlier message:

On 2023-May-08, at 5:03 AM, Rob Jarratt wrote:
>> From: Brent Hilpert 
>> 
>> 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. ...

Yes, this was acknowledged. I was explaining how it might have been the source 
of the mystery 0.08V, so if/as it's not then here's another possibility:
 It's conceivable there is an oscillation being generated around this -12V 
output circuitry, perhaps by a bad semiconductor junction or because some of 
the junctions are operating in reverse to normal under the test setup in 
conjunction with the caps and inductors, the mystery voltage being generated in 
consequence. Have you poked around the -12V area with a scope, as opposed to 
multi-meter V measurements? 

[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.
> > > >
> > > >
> > > >
> > >