Re: VAXmate PSU fixed, but no video

[Peter Coghlan via cctalk]

>
> There were two VT220 designs (I think), using either an onboard or
> offboard flyback. The part number of the onboard flyback is 16-26299-01,
> and there are some available on eBay if you are feeling rich.
>

I don't want to hijack Rob's thread too much :-)

I actually have one of each design and the one with the offboard transformer
(I think) works. The board layouts are different but the components seem
to be the same.  It is hard to be sure whether the only differences between
the transformers relate to their mounting arrangements.  I tried swapped some
of the HV components back and forth between them to eliminate them from
suspicion.  I didn't dare try swapping the transformers in case they are
not electrically identical and because I didn't want a fault elsewhere in
the bad terminal damaging what is probably my only good transformer.

I have the part number for the other type of transformer somewhere but
I can't locate it right now...

Regards,
Peter Coghlan

>
> Cheers,
> Aaron
> 

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

This is the circuit: 
https://rjarratt.files.wordpress.com/2020/04/transformerringingtester.png

Regards

Rob

> -Original Message-
> From: Rob Jarratt 
> Sent: 18 April 2020 17:25
> To: 'Toby Thain' ; r...@jarratt.me.uk; 'General
> Discussion: On-Topic and Off-Topic Posts' ; 'Mattis
> Lind' 
> Subject: RE: VAXmate PSU fixed, but no video
> 
> It is not my design, Tony Duell sent it to me and I think he got it from a
> magazine many years ago. I will try to find it.
> 
> Regards
> 
> Rob
> 
> > -Original Message-
> > From: Toby Thain 
> > Sent: 18 April 2020 16:51
> > To: r...@jarratt.me.uk; Rob Jarratt ;
> > General
> > Discussion: On-Topic and Off-Topic Posts ;
> > 'Mattis Lind' 
> > Subject: Re: VAXmate PSU fixed, but no video
> >
> > On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> > > I checked Q6 (in circuit), it is not shorted. I have already checked
> > > all the
> > electrolytic capacitors for a bad ESR and replaced those where it was high.
> > There are no shorts on the electrolytic capacitors.
> > >
> > >
> > >
> > > I will have to desolder the transformer and do a ring test on it (I
> > > built a ring
> > test circuit some years ago). I don’t think you can do ring tests in
> > circuit can you?
> > >
> >
> > I think more than one of us would be curious about your ring test circuit
> design.
> >
> > I've been slowly learning to design a pwm flyback controller which is
> > more or less the same thing in principle?
> >
> > --Toby
> >
> > >
> > >
> > > Regards
> > >
> > >
> > >
> > > Rob
> > >
> > >
> > >
> > > From: Mattis Lind 
> > > Sent: 18 April 2020 13:31
> > > To: r...@jarratt.me.uk; Rob Jarratt ;
> > > General Discussion: On-Topic and Off-Topic Posts
> > > 
> > > Subject: Re: VAXmate PSU fixed, but no video
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
> > mailto:cctalk@classiccmp.org> >:
> > >
> > > Some of you may recall seeing me post about the VAXmate PSU failure.
> > > Thanks to members of this list I found the failed part in the PSU
> > > and the PSU is now working again. However, it looks like the PSU
> > > failed because of a failure on the monitor board. There is a burning
> > > smell coming from it, possibly the flyback transformer, but I am not
> > > 100% sure. I don't see physical damage, but of course that doesn't
> > > mean there isn't a problem. When I took the monitor board out again
> > > after this, I wasn't sure if the EHT lead was making good contact
> > > with the CRT anode. The monitor board is described in section 4.4 of this
> document:
> > > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_D
> > > es
> > > cripti
> > > <http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_
> > > De
> > > scription_1987.pdf>
> > > on_1987.pdf
> > >
> > >
> > >
> > >
> > >
> > > I need some advice on diagnosing the problem, I have a few questions:
> > >
> > >
> > >
> > > 1.  If the EHT lead was not properly connected to the CRT anode, could
> > > that cause problems?
> > >
> > >
> > >
> > > No, that is unlikely in my opinion
> > >
> > >
> > >
> > > 2.  Is there anything I can safely do with a bench power supply to
> > > isolate the problem?
> > >
> > > 3.  Any other suggestions for diagnosing the problem?
> > >
> > >
> > >
> > > Check Q6 transistor. Looking at the picture 4-14, I think it is
> > > wrong. But
> > nevertheless check the capacitors C47 and C53.
> > >
> > > It happens that the EHT transformer get short-circuit turns. But it
> > > has only
> > occurred to me once. More often a capacitor is bad, a rectifier or the
> > switch transistor.
> > >
> > >
> > >
> > > If the EHT transformer is bad it performs bad when doing a ringing test.
> > >
> > >
> > >
> > > Try to find out what is smelling. Check all power semiconductors.
> > >
> > >
> > >
> > > 4.  There is an outline spec of the flyback transformer in the section
> > > 4.4.3.2 of the VAXmate technical description, what chance of finding
> > > a "modern" replacement?
> > >
> > >
> > >
> > > Usually very low. There are companies selling replacements but there
> > > are so
> > many variants so the chances to find one is very low. And there are
> > less and less of these on the market. But it is worth a try to check
> > what they have. It might be the case that they used the same monitor
> > design as some other DEC product. VT420? Then it could possible a
> > better chance finding one. donberg.ie <http://donberg.ie>  seem to have
> VT420 LOPT.
> > >
> > >
> > >
> > > /Mattis
> > >
> > >
> > >
> > >
> > >
> > >
> > > I have posted about the PSU repair here:
> > >
> > >
> > >
> > > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-bu
> > > t-
> > > no-vid
> > > <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-b
> > > ut
> > > -no-video/>
> > > eo/
> > >
> > >
> > >
> > > Thanks
> > >
> > >
> > >
> > > Rob
> > >
> > >
> > >
> > >

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

It is not my design, Tony Duell sent it to me and I think he got it from a 
magazine many years ago. I will try to find it.

Regards

Rob

> -Original Message-
> From: Toby Thain 
> Sent: 18 April 2020 16:51
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts ; 'Mattis
> Lind' 
> Subject: Re: VAXmate PSU fixed, but no video
> 
> On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> > I checked Q6 (in circuit), it is not shorted. I have already checked all the
> electrolytic capacitors for a bad ESR and replaced those where it was high.
> There are no shorts on the electrolytic capacitors.
> >
> >
> >
> > I will have to desolder the transformer and do a ring test on it (I built a 
> > ring
> test circuit some years ago). I don’t think you can do ring tests in circuit 
> can
> you?
> >
> 
> I think more than one of us would be curious about your ring test circuit 
> design.
> 
> I've been slowly learning to design a pwm flyback controller which is more or
> less the same thing in principle?
> 
> --Toby
> 
> >
> >
> > Regards
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Mattis Lind 
> > Sent: 18 April 2020 13:31
> > To: r...@jarratt.me.uk; Rob Jarratt ;
> > General Discussion: On-Topic and Off-Topic Posts
> > 
> > Subject: Re: VAXmate PSU fixed, but no video
> >
> >
> >
> >
> >
> >
> >
> > Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
> mailto:cctalk@classiccmp.org> >:
> >
> > Some of you may recall seeing me post about the VAXmate PSU failure.
> > Thanks to members of this list I found the failed part in the PSU and
> > the PSU is now working again. However, it looks like the PSU failed
> > because of a failure on the monitor board. There is a burning smell
> > coming from it, possibly the flyback transformer, but I am not 100%
> > sure. I don't see physical damage, but of course that doesn't mean
> > there isn't a problem. When I took the monitor board out again after
> > this, I wasn't sure if the EHT lead was making good contact with the
> > CRT anode. The monitor board is described in section 4.4 of this document:
> > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Des
> > cripti
> > <http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_De
> > scription_1987.pdf>
> > on_1987.pdf
> >
> >
> >
> >
> >
> > I need some advice on diagnosing the problem, I have a few questions:
> >
> >
> >
> > 1.  If the EHT lead was not properly connected to the CRT anode, could
> > that cause problems?
> >
> >
> >
> > No, that is unlikely in my opinion
> >
> >
> >
> > 2.  Is there anything I can safely do with a bench power supply to
> > isolate the problem?
> >
> > 3.  Any other suggestions for diagnosing the problem?
> >
> >
> >
> > Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But
> nevertheless check the capacitors C47 and C53.
> >
> > It happens that the EHT transformer get short-circuit turns. But it has only
> occurred to me once. More often a capacitor is bad, a rectifier or the switch
> transistor.
> >
> >
> >
> > If the EHT transformer is bad it performs bad when doing a ringing test.
> >
> >
> >
> > Try to find out what is smelling. Check all power semiconductors.
> >
> >
> >
> > 4.  There is an outline spec of the flyback transformer in the section
> > 4.4.3.2 of the VAXmate technical description, what chance of finding a
> > "modern" replacement?
> >
> >
> >
> > Usually very low. There are companies selling replacements but there are so
> many variants so the chances to find one is very low. And there are less and
> less of these on the market. But it is worth a try to check what they have. It
> might be the case that they used the same monitor design as some other DEC
> product. VT420? Then it could possible a better chance finding one. donberg.ie
> <http://donberg.ie>  seem to have VT420 LOPT.
> >
> >
> >
> > /Mattis
> >
> >
> >
> >
> >
> >
> > I have posted about the PSU repair here:
> >
> >
> >
> > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-
> > no-vid
> > <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but
> > -no-video/>
> > eo/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >
> >
> >

Re: VAXmate PSU fixed, but no video

[Toby Thain via cctalk]

On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> I checked Q6 (in circuit), it is not shorted. I have already checked all the 
> electrolytic capacitors for a bad ESR and replaced those where it was high. 
> There are no shorts on the electrolytic capacitors.
> 
>  
> 
> I will have to desolder the transformer and do a ring test on it (I built a 
> ring test circuit some years ago). I don’t think you can do ring tests in 
> circuit can you?
> 

I think more than one of us would be curious about your ring test
circuit design.

I've been slowly learning to design a pwm flyback controller which is
more or less the same thing in principle?

--Toby

>  
> 
> Regards
> 
>  
> 
> Rob
> 
>  
> 
> From: Mattis Lind  
> Sent: 18 April 2020 13:31
> To: r...@jarratt.me.uk; Rob Jarratt ; General 
> Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: VAXmate PSU fixed, but no video
> 
>  
> 
>  
> 
>  
> 
> Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk 
> mailto:cctalk@classiccmp.org> >:
> 
> Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
> to members of this list I found the failed part in the PSU and the PSU is
> now working again. However, it looks like the PSU failed because of a
> failure on the monitor board. There is a burning smell coming from it,
> possibly the flyback transformer, but I am not 100% sure. I don't see
> physical damage, but of course that doesn't mean there isn't a problem. When
> I took the monitor board out again after this, I wasn't sure if the EHT lead
> was making good contact with the CRT anode. The monitor board is described
> in section 4.4 of this document:
> http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti 
> <http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Description_1987.pdf>
>  
> on_1987.pdf
> 
> 
> 
> 
> 
> I need some advice on diagnosing the problem, I have a few questions:
> 
> 
> 
> 1.  If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
> 
>  
> 
> No, that is unlikely in my opinion 
> 
>  
> 
> 2.  Is there anything I can safely do with a bench power supply to
> isolate the problem? 
> 
> 3.  Any other suggestions for diagnosing the problem?
> 
>  
> 
> Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But 
> nevertheless check the capacitors C47 and C53.
> 
> It happens that the EHT transformer get short-circuit turns. But it has only 
> occurred to me once. More often a capacitor is bad, a rectifier or the switch 
> transistor.
> 
>  
> 
> If the EHT transformer is bad it performs bad when doing a ringing test.
> 
>  
> 
> Try to find out what is smelling. Check all power semiconductors.
> 
>  
> 
> 4.  There is an outline spec of the flyback transformer in the section
> 4.4.3.2 of the VAXmate technical description, what chance of finding a
> "modern" replacement?
> 
>  
> 
> Usually very low. There are companies selling replacements but there are so 
> many variants so the chances to find one is very low. And there are less and 
> less of these on the market. But it is worth a try to check what they have. 
> It might be the case that they used the same monitor design as some other DEC 
> product. VT420? Then it could possible a better chance finding one. 
> donberg.ie <http://donberg.ie>  seem to have VT420 LOPT.
> 
>  
> 
> /Mattis
> 
>  
> 
> 
> 
> 
> I have posted about the PSU repair here:
> 
> 
> 
> https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid 
> <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/>
>  
> eo/
> 
> 
> 
> Thanks
> 
> 
> 
> Rob
> 
> 
> 
> 

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

I don't have an LCR meter but I think it might be a good idea to get one.

Regards

Rob

> -Original Message-
> From: cctalk  On Behalf Of Aaron Jackson
via
> cctalk
> Sent: 18 April 2020 15:13
> To: Peter Coghlan ; General Discussion: On-Topic
> and Off-Topic Posts 
> Subject: Re: VAXmate PSU fixed, but no video
> 
> >> 1. If the EHT lead was not properly connected to the CRT anode, could
> >> that cause problems?
> >
> > Possibly.  I have VT220 terminal which was making a smell of ozone
> > when it was running which I should have done something about but never
got
> around to.
> > This could have been due to corona discharge around the CRT anode
> > connection or around the flyback transformer but I never found out.
> > Eventually, it stopped working, drawing excess current from the 12V
> > power supply.  The flyback transformer appears to have been damaged.
> 
> I recently decided to take another look at a VT220 I've got which appears
to
> have a bad FBT. Q202 switching transistor has been replaced and is
outputting
> a 14.7KHz signal but the flyback also seems to draw too much current and
> causes the terminal to hiccup.
> 
> It appears to be a problem with the primary winding, which has an
inductance
> of 5.6uH, although has now increased to 6.4uH after smoking again (I left
it on
> while hiccuping to check that the transistor was still switching)... If
you have an
> LCR meter I'd be curious to know what inductance you measure on the
primary
> winding.
> 
> I did eventually notice a small crack in the plastic on the primary
winding side.
> I'm not sure if it is superficial but I suspect this is where the smoke
escaped.
> 
> >> 2. Is there anything I can safely do with a bench power supply to
> >> isolate the problem?
> >> 3. Any other suggestions for diagnosing the problem?
> >
> > One approach to testing flyback transformers seems to be to use a
> > circuit that causes them to ring and observing whether the ringing is
> > damped by shorted turns.  I've never got around to trying this myself.
> 
> If you want to check the secondary winding, there is a diode which has a
high
> forward bias voltage. If you pass 20v or so through the secondary and look
at it
> with a volt meter you should see the voltage drop. It won't conduct at all
if the
> voltage is too low.
> 
> >> 4. There is an outline spec of the flyback transformer in the section
> >> 4.4.3.2 of the VAXmate technical description, what chance of finding
> >> a "modern" replacement?
> >
> > I wish you good luck with this.  I never had any luck locating one for
> > my
> > VT220 :-(
> 
> There were two VT220 designs (I think), using either an onboard or
offboard
> flyback. The part number of the onboard flyback is 16-26299-01, and there
are
> some available on eBay if you are feeling rich.
> 
> Cheers,
> Aaron
> 
> (Sorry about the ridiculous footer that will appear below.)
> 
> 
> This message and any attachment are intended solely for the addressee and
> may contain confidential information. If you have received this message in
> error, please contact the sender and delete the email and attachment.
> 
> Any views or opinions expressed by the author of this email do not
necessarily
> reflect the views of the University of Nottingham. Email communications
with
> the University of Nottingham may be monitored where permitted by law.
> 
> 

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

I checked Q6 (in circuit), it is not shorted. I have already checked all the 
electrolytic capacitors for a bad ESR and replaced those where it was high. 
There are no shorts on the electrolytic capacitors.

 

I will have to desolder the transformer and do a ring test on it (I built a 
ring test circuit some years ago). I don’t think you can do ring tests in 
circuit can you?

 

Regards

 

Rob

 

From: Mattis Lind  
Sent: 18 April 2020 13:31
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: Re: VAXmate PSU fixed, but no video

 

 

 

Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk 
mailto:cctalk@classiccmp.org> >:

Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
to members of this list I found the failed part in the PSU and the PSU is
now working again. However, it looks like the PSU failed because of a
failure on the monitor board. There is a burning smell coming from it,
possibly the flyback transformer, but I am not 100% sure. I don't see
physical damage, but of course that doesn't mean there isn't a problem. When
I took the monitor board out again after this, I wasn't sure if the EHT lead
was making good contact with the CRT anode. The monitor board is described
in section 4.4 of this document:
http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti 
<http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Description_1987.pdf>
 
on_1987.pdf





I need some advice on diagnosing the problem, I have a few questions:



1.  If the EHT lead was not properly connected to the CRT anode, could
that cause problems?

 

No, that is unlikely in my opinion 

 

2.  Is there anything I can safely do with a bench power supply to
isolate the problem? 

3.  Any other suggestions for diagnosing the problem?

 

Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But 
nevertheless check the capacitors C47 and C53.

It happens that the EHT transformer get short-circuit turns. But it has only 
occurred to me once. More often a capacitor is bad, a rectifier or the switch 
transistor.

 

If the EHT transformer is bad it performs bad when doing a ringing test.

 

Try to find out what is smelling. Check all power semiconductors.

 

4.  There is an outline spec of the flyback transformer in the section
4.4.3.2 of the VAXmate technical description, what chance of finding a
"modern" replacement?

 

Usually very low. There are companies selling replacements but there are so 
many variants so the chances to find one is very low. And there are less and 
less of these on the market. But it is worth a try to check what they have. It 
might be the case that they used the same monitor design as some other DEC 
product. VT420? Then it could possible a better chance finding one. donberg.ie 
<http://donberg.ie>  seem to have VT420 LOPT.

 

/Mattis

 




I have posted about the PSU repair here:



https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid 
<https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/>
 
eo/



Thanks



Rob

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Peter Coghlan
via
> cctalk
> Sent: 18 April 2020 12:57
> To: General Discussion: On-Topic and Off-Topic Posts

> Subject: Re: VAXmate PSU fixed, but no video
> 
> >
> > Some of you may recall seeing me post about the VAXmate PSU failure.
> > Thanks to members of this list I found the failed part in the PSU and
> > the PSU is now working again. However, it looks like the PSU failed
> > because of a failure on the monitor board. There is a burning smell
> > coming from it, possibly the flyback transformer, but I am not 100%
> > sure. I don't see physical damage, but of course that doesn't mean
> > there isn't a problem. When I took the monitor board out again after
> > this, I wasn't sure if the EHT lead was making good contact with the
> > CRT anode. The monitor board is described in section 4.4 of this
document:
> > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Des
> > cripti
> > on_1987.pdf
> >
> 
> Congratulations on getting your PSU failure sorted out.  I suppose you
realise
> this means there will be a line of people wanting you to look at our PSU
> problems the next time we meet up :-)

Given how long it took me to fix this one, I don't think anyone will want to
ask me!


> 
> >
> > I need some advice on diagnosing the problem, I have a few questions:
> >
> > 1.  If the EHT lead was not properly connected to the CRT anode, could
> > that cause problems?
> >
> 
> Possibly.  I have VT220 terminal which was making a smell of ozone when it
> was running which I should have done something about but never got around
> to.
> This could have been due to corona discharge around the CRT anode
> connection or around the flyback transformer but I never found out.
> Eventually, it stopped working, drawing excess current from the 12V power
> supply.  The flyback transformer appears to have been damaged.
> 
> >
> > 2.  Is there anything I can safely do with a bench power supply to
> > isolate the problem?
> > 3.  Any other suggestions for diagnosing the problem?
> >
> 
> One approach to testing flyback transformers seems to be to use a circuit
that
> causes them to ring and observing whether the ringing is damped by shorted
> turns.  I've never got around to trying this myself.
> 
> >
> > 4.  There is an outline spec of the flyback transformer in the section
> > 4.4.3.2 of the VAXmate technical description, what chance of finding a
> > "modern" replacement?
> >
> 
> I wish you good luck with this.  I never had any luck locating one for my
> VT220 :-(
> 
> Regards,
> Peter Coghlan.
> 
> >
> > I have posted about the PSU repair here:
> >
> >
> >
> > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-
> > no-video/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >

Re: VAXmate PSU fixed, but no video

[Mattis Lind via cctalk]

>
>
> One approach to testing flyback transformers seems to be to use a circuit
> that causes them to ring and observing whether the ringing is damped by
> shorted turns.  I've never got around to trying this myself.
>
>
I have a VT100 that failed with some smoke coming out of the center of the
transformer coil. I tried the ringing test. Here are my results:
http://www.datormuseum.se/peripherals/terminals/vt100

/Mattis


>
>

Re: VAXmate PSU fixed, but no video

[Aaron Jackson via cctalk]

>> 1.   If the EHT lead was not properly connected to the CRT anode, could
>> that cause problems?
>
> Possibly.  I have VT220 terminal which was making a smell of ozone when it was
> running which I should have done something about but never got around to.
> This could have been due to corona discharge around the CRT anode connection
> or around the flyback transformer but I never found out.  Eventually, it
> stopped working, drawing excess current from the 12V power supply.  The
> flyback transformer appears to have been damaged.

I recently decided to take another look at a VT220 I've got which
appears to have a bad FBT. Q202 switching transistor has been replaced
and is outputting a 14.7KHz signal but the flyback also seems to draw
too much current and causes the terminal to hiccup.

It appears to be a problem with the primary winding, which has an
inductance of 5.6uH, although has now increased to 6.4uH after smoking
again (I left it on while hiccuping to check that the transistor was
still switching)... If you have an LCR meter I'd be curious to know what
inductance you measure on the primary winding.

I did eventually notice a small crack in the plastic on the primary
winding side. I'm not sure if it is superficial but I suspect this is
where the smoke escaped.

>> 2.   Is there anything I can safely do with a bench power supply to
>> isolate the problem?
>> 3.   Any other suggestions for diagnosing the problem?
>
> One approach to testing flyback transformers seems to be to use a circuit
> that causes them to ring and observing whether the ringing is damped by
> shorted turns.  I've never got around to trying this myself.

If you want to check the secondary winding, there is a diode which has a
high forward bias voltage. If you pass 20v or so through the secondary
and look at it with a volt meter you should see the voltage drop. It
won't conduct at all if the voltage is too low.

>> 4.   There is an outline spec of the flyback transformer in the section
>> 4.4.3.2 of the VAXmate technical description, what chance of finding a
>> "modern" replacement?
>
> I wish you good luck with this.  I never had any luck locating one for my
> VT220 :-(

There were two VT220 designs (I think), using either an onboard or
offboard flyback. The part number of the onboard flyback is 16-26299-01,
and there are some available on eBay if you are feeling rich.

Cheers,
Aaron

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Re: VAXmate PSU fixed, but no video

[Mattis Lind via cctalk]

Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk <
cctalk@classiccmp.org>:

> Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
> to members of this list I found the failed part in the PSU and the PSU is
> now working again. However, it looks like the PSU failed because of a
> failure on the monitor board. There is a burning smell coming from it,
> possibly the flyback transformer, but I am not 100% sure. I don't see
> physical damage, but of course that doesn't mean there isn't a problem.
> When
> I took the monitor board out again after this, I wasn't sure if the EHT
> lead
> was making good contact with the CRT anode. The monitor board is described
> in section 4.4 of this document:
>
> http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti
> on_1987.pdf
> 
>
>
>
>
>
> I need some advice on diagnosing the problem, I have a few questions:
>
>
>
> 1.  If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
>

No, that is unlikely in my opinion


> 2.  Is there anything I can safely do with a bench power supply to
> isolate the problem?

3.  Any other suggestions for diagnosing the problem?
>

Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But
nevertheless check the capacitors C47 and C53.
It happens that the EHT transformer get short-circuit turns. But it has
only occurred to me once. More often a capacitor is bad, a rectifier or the
switch transistor.

If the EHT transformer is bad it performs bad when doing a ringing test.

Try to find out what is smelling. Check all power semiconductors.


> 4.  There is an outline spec of the flyback transformer in the section
> 4.4.3.2 of the VAXmate technical description, what chance of finding a
> "modern" replacement?
>

Usually very low. There are companies selling replacements but there are so
many variants so the chances to find one is very low. And there are less
and less of these on the market. But it is worth a try to check what they
have. It might be the case that they used the same monitor design as some
other DEC product. VT420? Then it could possible a better chance finding
one. donberg.ie seem to have VT420 LOPT.

/Mattis


>
>
> I have posted about the PSU repair here:
>
>
>
>
> https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid
> eo/
> 
>
>
>
> Thanks
>
>
>
> Rob
>
>
>
>

Re: VAXmate PSU fixed, but no video

[Peter Coghlan via cctalk]

> 
> Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
> to members of this list I found the failed part in the PSU and the PSU is
> now working again. However, it looks like the PSU failed because of a
> failure on the monitor board. There is a burning smell coming from it,
> possibly the flyback transformer, but I am not 100% sure. I don't see
> physical damage, but of course that doesn't mean there isn't a problem. When
> I took the monitor board out again after this, I wasn't sure if the EHT lead
> was making good contact with the CRT anode. The monitor board is described
> in section 4.4 of this document:
> http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti
> on_1987.pdf
>   

Congratulations on getting your PSU failure sorted out.  I suppose you realise
this means there will be a line of people wanting you to look at our PSU
problems the next time we meet up :-)
 
> 
> I need some advice on diagnosing the problem, I have a few questions:  
> 
> 1.If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
>

Possibly.  I have VT220 terminal which was making a smell of ozone when it was
running which I should have done something about but never got around to.
This could have been due to corona discharge around the CRT anode connection
or around the flyback transformer but I never found out.  Eventually, it
stopped working, drawing excess current from the 12V power supply.  The
flyback transformer appears to have been damaged.

>
> 2.Is there anything I can safely do with a bench power supply to
> isolate the problem?
> 3.Any other suggestions for diagnosing the problem?
>

One approach to testing flyback transformers seems to be to use a circuit
that causes them to ring and observing whether the ringing is damped by
shorted turns.  I've never got around to trying this myself.

>
> 4.There is an outline spec of the flyback transformer in the section
> 4.4.3.2 of the VAXmate technical description, what chance of finding a
> "modern" replacement?
> 

I wish you good luck with this.  I never had any luck locating one for my
VT220 :-(

Regards,
Peter Coghlan.

>
> I have posted about the PSU repair here:
> 
>  
> 
> https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/
> 
>  
> 
> Thanks
> 
>  
> 
> Rob
> 
>  

Re: VAXmate PSU

[Eric Smith via cctalk]

On Fri, Apr 10, 2020 at 10:14 AM Rob Jarratt via cctalk <
cctalk@classiccmp.org> wrote:

> D12 is an MBR3045PT. It tests correctly as a common cathode diode network.
> However, the forward voltage seems to be 0.19V. The datasheet (
> https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html)
> would suggest it should be 0.76V at room temperature. I can see no physical
> damage to it though.
>

Even when they are good, the forward drop of a diode can be FAR lower than
the typical forward drop (around 0.7 for normal silicon diodes) if you're
putting significantly less that the rated current through it. 0.19V forward
drop is only slightly lower than typical characteristic at e.g. 10mA
current. See figure 3. I've seen more than enough variation of diodes from
typical curves for that alone to convince me that the diode is bad (though
obviously it may be).

The real test is how much it conducts in reverse. All diodes will pass a
small amount of reverse current. This one shouldn't pass more than 1 mA in
the reverse direction at room temperature, even with near the rated reverse
voltage (45V) applied.

Re: VAXmate PSU

[Holm Tiffe via cctalk]

Rob Jarratt via cctalk wrote:

> Thanks Eric, I had a private reply that said pretty much the same thing.
> 
>  
> 
> Last night I replaced the shorted diode and another electrolytic capacitor 
> and the PSU started up on the bench, powering a load board and an RD53 hard 
> disk. Ripple was good except on the -9V output, but that output doesn’t seem 
> to have any capacitors after the final transformer, so I think it is OK.
> 
>  
> 
> I need to replace a few marginal capacitors on the video module now, while I 
> have the machine in bits. Then I will put it all back together to see that it 
> still works. I have a lingering worry that the failed diode was on the +28V 
> supply to the video module and that there might be a fault on the video 
> module, but it doesn’t appear to present a short circuit, so I am hopeful.
> 
>  
> 
> I will do a blog post once it is all back up and running.
> 
>  
> 
> Thanks
> 
>  
> 
> Rob
> 

Congratulations Rob, you made it.

Regards,

Holm
-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
i...@tsht.de Fax +49 3731 74200 Tel +49 3731 74222 Mobil: 0172 8790 741

RE: VAXmate PSU

[Rob Jarratt via cctalk]

Thanks Eric, I had a private reply that said pretty much the same thing.

 

Last night I replaced the shorted diode and another electrolytic capacitor and 
the PSU started up on the bench, powering a load board and an RD53 hard disk. 
Ripple was good except on the -9V output, but that output doesn’t seem to have 
any capacitors after the final transformer, so I think it is OK.

 

I need to replace a few marginal capacitors on the video module now, while I 
have the machine in bits. Then I will put it all back together to see that it 
still works. I have a lingering worry that the failed diode was on the +28V 
supply to the video module and that there might be a fault on the video module, 
but it doesn’t appear to present a short circuit, so I am hopeful.

 

I will do a blog post once it is all back up and running.

 

Thanks

 

Rob

 

 

From: Eric Smith  
Sent: 16 April 2020 00:05
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: Re: VAXmate PSU

 

On Fri, Apr 10, 2020 at 10:14 AM Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote:

D12 is an MBR3045PT. It tests correctly as a common cathode diode network. 
However, the forward voltage seems to be 0.19V. The datasheet 
(https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html)
 would suggest it should be 0.76V at room temperature. I can see no physical 
damage to it though.

 

Even when they are good, the forward drop of a diode can be FAR lower than the 
typical forward drop (around 0.7 for normal silicon diodes) if you're putting 
significantly less that the rated current through it. 0.19V forward drop is 
only slightly lower than typical characteristic at e.g. 10mA current. See 
figure 3. I've seen more than enough variation of diodes from typical curves 
for that alone to convince me that the diode is bad (though obviously it may 
be).

 

The real test is how much it conducts in reverse. All diodes will pass a small 
amount of reverse current. This one shouldn't pass more than 1 mA in the 
reverse direction at room temperature, even with near the rated reverse voltage 
(45V) applied.

 

RE: VAXmate PSU

[Rob Jarratt via cctalk]

I took a break and while I was doing that I thought the same thing, I am going 
to see if I can see a reason for the failure.

 

I did wonder if the tester’s current would be too low to show the nominal 
forward voltage on D11 and D12. I won’t change those.

 

Thanks

 

Rob

 

From: Mattis Lind  
Sent: 10 April 2020 15:16
To: r...@jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts ; 
Matt Burke 
Subject: Re: VAXmate PSU

 

 

 




I have found the problem and a possible other problem too.

 

But that is great! 


I have discovered that D24 is shorted (no visible damage), although its twin 
D23 is not. I will replace both. I know it is shorted because I lifted one end.

 

Well, sometime there are no visible damage but still a smell. 


I have also checked the rectifiers, D11 and D12. I am not sure if there is a 
possible problem here too.

D12 is an MBR3045PT. It tests correctly as a common cathode diode network. 
However, the forward voltage seems to be 0.19V. The datasheet 
(https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html)
 would suggest it should be 0.76V at room temperature. I can see no physical 
damage to it though.

 

The point is that the measurement in the data sheet is with 20A forward 
current. You meter perhaps give 10 mA forward current. This is a Schottky 
rectifier it has very low Vf.

0.19 is completely normal.

 


D11 is marked PHS 2402. It too tests correctly as a common cathode diode 
network, with a forward voltage of 0.43V. I can't find a datasheet for it 
though, but I believe the equivalent is MUR1610CT 
(https://pdf1.alldatasheet.com/datasheet-pdf/view/82128/ONSEMI/MUR1610CT.html) 
and I think the forward voltage for that should be 0.9V. Again no physical 
damage that I can see.

Is it possible both rectifiers are marginal? Or at least D12? Should I replace 
one or both?

 

No. At this point replace just D24 and possibly D23. Actually I don't think it 
is necessary. But to be on the safe side it might be a good idea.

 

But the question is why it failed. Sometimes they fail because of just nothing. 
But it could be something in the network following it that had made it fail. 
Check that there are no short circuit in the LM317 regulator. Check output 
capacitors in this area. ESR and value.

 

/Mattis

 

Thanks

Rob

> > I did check Q2 before, but perhaps I should check again. I have
> > checked for shorts on the actual outputs, but there don't seem to be
> > any. I have checked one of those rectifiers, I think one of my next
> > tasks is to desolder all of them and check them.
> 
> 
> I have put probes on Q2 with the probes connected as follows:
> 
> Ch1. D19 anode (for triggering)
> Ch2. D19 gate
> Ch3. Q2 anode
> Ch4. Q2 gate
> 
> The result is here:
> 
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-
> secondary-scr-detail.png
> 
> It doesn't look like Q2 is being triggered.
> 
> >
> > >
> > > Matt

Re: VAXmate PSU

[Mattis Lind via cctalk]

>
>
> I have found the problem and a possible other problem too.
>

But that is great!

>
> I have discovered that D24 is shorted (no visible damage), although its
> twin D23 is not. I will replace both. I know it is shorted because I lifted
> one end.
>

Well, sometime there are no visible damage but still a smell.

>
> I have also checked the rectifiers, D11 and D12. I am not sure if there is
> a possible problem here too.
>
> D12 is an MBR3045PT. It tests correctly as a common cathode diode network.
> However, the forward voltage seems to be 0.19V. The datasheet (
> https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html)
> would suggest it should be 0.76V at room temperature. I can see no physical
> damage to it though.
>

The point is that the measurement in the data sheet is with 20A forward
current. You meter perhaps give 10 mA forward current. This is a Schottky
rectifier it has very low Vf.
0.19 is completely normal.


>
> D11 is marked PHS 2402. It too tests correctly as a common cathode diode
> network, with a forward voltage of 0.43V. I can't find a datasheet for it
> though, but I believe the equivalent is MUR1610CT (
> https://pdf1.alldatasheet.com/datasheet-pdf/view/82128/ONSEMI/MUR1610CT.html)
> and I think the forward voltage for that should be 0.9V. Again no physical
> damage that I can see.
>
> Is it possible both rectifiers are marginal? Or at least D12? Should I
> replace one or both?
>
>
No. At this point replace just D24 and possibly D23. Actually I don't think
it is necessary. But to be on the safe side it might be a good idea.

But the question is why it failed. Sometimes they fail because of just
nothing. But it could be something in the network following it that had
made it fail. Check that there are no short circuit in the LM317 regulator.
Check output capacitors in this area. ESR and value.

/Mattis


> Thanks
>
> Rob
>
> > > I did check Q2 before, but perhaps I should check again. I have
> > > checked for shorts on the actual outputs, but there don't seem to be
> > > any. I have checked one of those rectifiers, I think one of my next
> > > tasks is to desolder all of them and check them.
> >
> >
> > I have put probes on Q2 with the probes connected as follows:
> >
> > Ch1. D19 anode (for triggering)
> > Ch2. D19 gate
> > Ch3. Q2 anode
> > Ch4. Q2 gate
> >
> > The result is here:
> >
> > https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-
> > secondary-scr-detail.png
> >
> > It doesn't look like Q2 is being triggered.
> >
> > >
> > > >
> > > > Matt
>
>

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: Rob Jarratt 
> Sent: 09 April 2020 20:34
> To: r...@jarratt.me.uk; 'Rob Jarratt' ; 'General
> Discussion: On-Topic and Off-Topic Posts' ; 'Matt
> Burke' ; 'Mattis Lind' 
> Subject: RE: VAXmate PSU
> 
> > -Original Message-
> > From: cctalk  On Behalf Of Rob Jarratt
> > via cctalk
> > Sent: 09 April 2020 19:52
> > To: 'Matt Burke' ; 'Mattis Lind'
> > ; 'General Discussion: On-Topic and Off-Topic
> > Posts' 
> > Subject: RE: VAXmate PSU
> >
> >
> >
> > > -Original Message-
> > > From: cctalk  On Behalf Of Matt Burke
> > > via cctalk
> > > Sent: 09 April 2020 09:40
> > > To: cctalk@classiccmp.org
> > > Subject: Re: VAXmate PSU
> > >
> > > On 08/04/2020 22:24, Rob Jarratt via cctalk wrote:
> > > > I will look at all the suggestions, particularly of a failure on
> > > > the secondary
> > > side. Something must have burned up, because there was a distinct
> > > burning smell after the initial failure, although I have never been
> > > able to see any physical damage to anything, despite looking many times.
> > > >
> > > >
> > > >
> > > > But the thing that really puzzles me is that, after correcting the
> > > > probes to
> > > include the D19 anode, there doesn’t seem to be anything that would
> > > cause
> > > D19 to trigger. Am I reading the trace wrong?
> > > >
> > >
> > > The only paths to ground for VCC are C12, E3 and D19. The fact that
> > > VCC is stable for a period of time and then drops suddenly suggests
> > > it must be D19. It looks like there could be a pulse on D19 gate but
> > > it is in the order of 1-2 uS before VCC drops. There are more pulses
> > > prior to that but I wonder if some of that is just induced noise?
> >
> >
> > Here is a trace taken using a 5us timebase.
> >
> > https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-det
> > ail-
> > high-resolution.png
> >
> > As a reminder the probes are connected as follows:
> >
> > Ch1. 555 timer.
> > Ch2. D19 Anode
> > Ch3. D19 Gate.
> > Ch4. Q1 Source.
> >
> > It doesn't look like there are any glitches on the D19 gate. I don't
> > think it is clear why D19 is triggering, because all the previous
> > oscillations are about the same.
> >
> > >
> > > It will be interesting to see what's happening on the secondary
> > > side, particularly with Q2. I guess you have already checked the
> > > obvious things like a short circuit on one of the outputs? Also
> > > worth checking the rectifier diodes D11, D12, D22 and D23 (I think).
> >

I have found the problem and a possible other problem too.

I have discovered that D24 is shorted (no visible damage), although its twin 
D23 is not. I will replace both. I know it is shorted because I lifted one end.

I have also checked the rectifiers, D11 and D12. I am not sure if there is a 
possible problem here too.

D12 is an MBR3045PT. It tests correctly as a common cathode diode network. 
However, the forward voltage seems to be 0.19V. The datasheet 
(https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html)
 would suggest it should be 0.76V at room temperature. I can see no physical 
damage to it though.

D11 is marked PHS 2402. It too tests correctly as a common cathode diode 
network, with a forward voltage of 0.43V. I can't find a datasheet for it 
though, but I believe the equivalent is MUR1610CT 
(https://pdf1.alldatasheet.com/datasheet-pdf/view/82128/ONSEMI/MUR1610CT.html) 
and I think the forward voltage for that should be 0.9V. Again no physical 
damage that I can see.

Is it possible both rectifiers are marginal? Or at least D12? Should I replace 
one or both?

Thanks

Rob

> > I did check Q2 before, but perhaps I should check again. I have
> > checked for shorts on the actual outputs, but there don't seem to be
> > any. I have checked one of those rectifiers, I think one of my next
> > tasks is to desolder all of them and check them.
> 
> 
> I have put probes on Q2 with the probes connected as follows:
> 
> Ch1. D19 anode (for triggering)
> Ch2. D19 gate
> Ch3. Q2 anode
> Ch4. Q2 gate
> 
> The result is here:
> 
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-
> secondary-scr-detail.png
> 
> It doesn't look like Q2 is being triggered.
> 
> >
> > >
> > > Matt

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Rob Jarratt via
> cctalk
> Sent: 09 April 2020 19:52
> To: 'Matt Burke' ; 'Mattis Lind' ;
> 'General Discussion: On-Topic and Off-Topic Posts' 
> Subject: RE: VAXmate PSU
> 
> 
> 
> > -Original Message-
> > From: cctalk  On Behalf Of Matt Burke
> > via cctalk
> > Sent: 09 April 2020 09:40
> > To: cctalk@classiccmp.org
> > Subject: Re: VAXmate PSU
> >
> > On 08/04/2020 22:24, Rob Jarratt via cctalk wrote:
> > > I will look at all the suggestions, particularly of a failure on the
> > > secondary
> > side. Something must have burned up, because there was a distinct
> > burning smell after the initial failure, although I have never been
> > able to see any physical damage to anything, despite looking many times.
> > >
> > >
> > >
> > > But the thing that really puzzles me is that, after correcting the
> > > probes to
> > include the D19 anode, there doesn’t seem to be anything that would
> > cause
> > D19 to trigger. Am I reading the trace wrong?
> > >
> >
> > The only paths to ground for VCC are C12, E3 and D19. The fact that
> > VCC is stable for a period of time and then drops suddenly suggests it
> > must be D19. It looks like there could be a pulse on D19 gate but it
> > is in the order of 1-2 uS before VCC drops. There are more pulses
> > prior to that but I wonder if some of that is just induced noise?
> 
> 
> Here is a trace taken using a 5us timebase.
> 
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-
> high-resolution.png
> 
> As a reminder the probes are connected as follows:
> 
> Ch1. 555 timer.
> Ch2. D19 Anode
> Ch3. D19 Gate.
> Ch4. Q1 Source.
> 
> It doesn't look like there are any glitches on the D19 gate. I don't think it 
> is
> clear why D19 is triggering, because all the previous oscillations are about 
> the
> same.
> 
> >
> > It will be interesting to see what's happening on the secondary side,
> > particularly with Q2. I guess you have already checked the obvious
> > things like a short circuit on one of the outputs? Also worth checking
> > the rectifier diodes D11, D12, D22 and D23 (I think).
> 
> I did check Q2 before, but perhaps I should check again. I have checked for
> shorts on the actual outputs, but there don't seem to be any. I have checked
> one of those rectifiers, I think one of my next tasks is to desolder all of 
> them
> and check them.


I have put probes on Q2 with the probes connected as follows:

Ch1. D19 anode (for triggering)
Ch2. D19 gate
Ch3. Q2 anode
Ch4. Q2 gate

The result is here:

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-secondary-scr-detail.png

It doesn't look like Q2 is being triggered.

> 
> >
> > Matt

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Matt Burke via
> cctalk
> Sent: 09 April 2020 09:40
> To: cctalk@classiccmp.org
> Subject: Re: VAXmate PSU
> 
> On 08/04/2020 22:24, Rob Jarratt via cctalk wrote:
> > I will look at all the suggestions, particularly of a failure on the 
> > secondary
> side. Something must have burned up, because there was a distinct burning
> smell after the initial failure, although I have never been able to see any
> physical damage to anything, despite looking many times.
> >
> >
> >
> > But the thing that really puzzles me is that, after correcting the probes to
> include the D19 anode, there doesn’t seem to be anything that would cause
> D19 to trigger. Am I reading the trace wrong?
> >
> 
> The only paths to ground for VCC are C12, E3 and D19. The fact that VCC is
> stable for a period of time and then drops suddenly suggests it must be D19. 
> It
> looks like there could be a pulse on D19 gate but it is in the order of 1-2 uS
> before VCC drops. There are more pulses prior to that but I wonder if some of
> that is just induced noise?


Here is a trace taken using a 5us timebase.

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-high-resolution.png

As a reminder the probes are connected as follows:

Ch1. 555 timer.
Ch2. D19 Anode
Ch3. D19 Gate.
Ch4. Q1 Source.

It doesn't look like there are any glitches on the D19 gate. I don't think it 
is clear why D19 is triggering, because all the previous oscillations are about 
the same.

> 
> It will be interesting to see what's happening on the secondary side, 
> particularly
> with Q2. I guess you have already checked the obvious things like a short 
> circuit
> on one of the outputs? Also worth checking the rectifier diodes D11, D12, D22
> and D23 (I think).

I did check Q2 before, but perhaps I should check again. I have checked for 
shorts on the actual outputs, but there don't seem to be any. I have checked 
one of those rectifiers, I think one of my next tasks is to desolder all of 
them and check them.

> 
> Matt

Re: VAXmate PSU

[Matt Burke via cctalk]

On 08/04/2020 22:24, Rob Jarratt via cctalk wrote:
> I will look at all the suggestions, particularly of a failure on the 
> secondary side. Something must have burned up, because there was a distinct 
> burning smell after the initial failure, although I have never been able to 
> see any physical damage to anything, despite looking many times.
>
>  
>
> But the thing that really puzzles me is that, after correcting the probes to 
> include the D19 anode, there doesn’t seem to be anything that would cause D19 
> to trigger. Am I reading the trace wrong?
>

The only paths to ground for VCC are C12, E3 and D19. The fact that VCC
is stable for a period of time and then drops suddenly suggests it must
be D19. It looks like there could be a pulse on D19 gate but it is in
the order of 1-2 uS before VCC drops. There are more pulses prior to
that but I wonder if some of that is just induced noise?

It will be interesting to see what's happening on the secondary side,
particularly with Q2. I guess you have already checked the obvious
things like a short circuit on one of the outputs? Also worth checking
the rectifier diodes D11, D12, D22 and D23 (I think).

Matt

RE: VAXmate PSU

[Rob Jarratt via cctalk]

I will repeat it tonight (or tomorrow) with the faster timebase and get the 
last two cycles. The switching frequency is 100KHz.

 

Thanks

 

Rob

 

From: Mattis Lind  
Sent: 09 April 2020 08:47
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: VAXmate PSU

 



torsdag 9 april 2020 skrev Rob Jarratt mailto:robert.jarr...@ntlworld.com> >:

I did use a faster timebase, 10us, triggering on the negative edge of channel 
2, and it looked identical to this:

 

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.png

 

I thought the switching frequency was in the range of 50 kHz. But it might be 
higher. What would be interesting is to see the last two switch cycles in 
detail. What is the switching frequency?

 

Admittedly the trace you see was taken on the slower timebase and then zoomed 
in, but there was no difference, the result was identical.

 

You can loose detail since the sampling rate is a function of sweep frequency/ 
time base setting.  

 

Regards

 

Rob

 

From: Mattis Lind mailto:mattisl...@gmail.com> > 
Sent: 09 April 2020 05:52
To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; General Discussion: 
On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> >; Rob Jarratt mailto:robert.jarr...@ntlworld.com> >
Subject: VAXmate PSU

 



onsdag 8 april 2020 skrev Rob Jarratt mailto:robert.jarr...@ntlworld.com> >:

I will look at all the suggestions, particularly of a failure on the secondary 
side. Something must have burned up, because there was a distinct burning smell 
after the initial failure, although I have never been able to see any physical 
damage to anything, despite looking many times.

 

Aha. Don’t think I seen you writing about that before, or did you? It might be 
very hard to find the source some times. Even just a small burn will give quite 
some smell. Check ALL semiconductors very carefully.  

 

But the thing that really puzzles me is that, after correcting the probes to 
include the D19 anode, there doesn’t seem to be anything that would cause D19 
to trigger. Am I reading the trace wrong?

 

It is very hard to tell from the traces what is going on since the resolution 
is too low.  Use a faster timebase. 5 or 10 microseconds. Find out if you can 
trigger on something that happen only when it stops. Like channel 2 negative 
slope. 

 

/Mattis

 

Thanks

 

Rob

 

From: Mattis Lind < <mailto:mattisl...@gmail.com> mattisl...@gmail.com> 
Sent: 08 April 2020 07:42
To:  <mailto:r...@jarratt.me.uk> r...@jarratt.me.uk; Rob Jarratt < 
<mailto:robert.jarr...@ntlworld.com> robert.jarr...@ntlworld.com>; General 
Discussion: On-Topic and Off-Topic Posts < <mailto:cctalk@classiccmp.org> 
cctalk@classiccmp.org>
Subject: Re: VAXmate PSU

 

 

 

Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk 
mailto:cctalk@classiccmp.org> >:



> -Original Message-
> From: cctalk  <mailto:cctalk-boun...@classiccmp.org> > On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 06 April 2020 21:07
> To: General Discussion: On-Topic and Off-Topic Posts
mailto:cctalk@classiccmp.org> >
> Subject: Re: VAXmate PSU
> 
> On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
> >>
> >>> I have obtained a scope trace as you suggest. R32 is still lifted so
> >>> the
> >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC
> >>> (no variac). The channels are:
> >>> 1.Ch1. 555 timer.
> >>> 2.Ch2. D19 Anode
> >>> 3.Ch3. D19 Gate.
> >>> 4.Ch4. Q1 Source.
> >
> > Sorry, that looks like a cut and paste error, here is the link to the
> > scope picture
> > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger
> > .png
> >
> > I used a 100ms timebase for the capture and then "zoomed in" a bit
> 
> 
> You would need to zoom in far more to see what's going on when the SCR
> triggers, to cover just a few cycles around the trigger time.
> 
> Once an SCR has been triggerred, the gate becomes a voltage/current
supply, a
> diode drop above 0.
> You see this on your trace in that after triggerring the gate sits at
something +V
> above 0.
> The spike you see may just be an artifact of the internal SCR trigger
action.
> I presume you see some increased current draw from your bench supply for
the
> 3842 after the SCR triggers.
> 
> What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
but
> it shows on the trace as just noise around 0V.
> 
> I would still suggest that you scope the state of the secondary-side
crowbar -
> the gate of Q2, and base of Q4.
> Should be simple to do, before trying to remove or disconnect the main
&g

RE: VAXmate PSU

[Rob Jarratt via cctalk]

I did use a faster timebase, 10us, triggering on the negative edge of channel 
2, and it looked identical to this:

 

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.png

 

Admittedly the trace you see was taken on the slower timebase and then zoomed 
in, but there was no difference, the result was identical.

 

Regards

 

Rob

 

From: Mattis Lind  
Sent: 09 April 2020 05:52
To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 
; Rob Jarratt 
Subject: VAXmate PSU

 



onsdag 8 april 2020 skrev Rob Jarratt mailto:robert.jarr...@ntlworld.com> >:

I will look at all the suggestions, particularly of a failure on the secondary 
side. Something must have burned up, because there was a distinct burning smell 
after the initial failure, although I have never been able to see any physical 
damage to anything, despite looking many times.

 

Aha. Don’t think I seen you writing about that before, or did you? It might be 
very hard to find the source some times. Even just a small burn will give quite 
some smell. Check ALL semiconductors very carefully.  

 

But the thing that really puzzles me is that, after correcting the probes to 
include the D19 anode, there doesn’t seem to be anything that would cause D19 
to trigger. Am I reading the trace wrong?

 

It is very hard to tell from the traces what is going on since the resolution 
is too low.  Use a faster timebase. 5 or 10 microseconds. Find out if you can 
trigger on something that happen only when it stops. Like channel 2 negative 
slope. 

 

/Mattis

 

Thanks

 

Rob

 

From: Mattis Lind < <mailto:mattisl...@gmail.com> mattisl...@gmail.com> 
Sent: 08 April 2020 07:42
To:  <mailto:r...@jarratt.me.uk> r...@jarratt.me.uk; Rob Jarratt < 
<mailto:robert.jarr...@ntlworld.com> robert.jarr...@ntlworld.com>; General 
Discussion: On-Topic and Off-Topic Posts < <mailto:cctalk@classiccmp.org> 
cctalk@classiccmp.org>
Subject: Re: VAXmate PSU

 

 

 

Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk 
mailto:cctalk@classiccmp.org> >:



> -Original Message-
> From: cctalk  <mailto:cctalk-boun...@classiccmp.org> > On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 06 April 2020 21:07
> To: General Discussion: On-Topic and Off-Topic Posts
mailto:cctalk@classiccmp.org> >
> Subject: Re: VAXmate PSU
> 
> On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
> >>
> >>> I have obtained a scope trace as you suggest. R32 is still lifted so
> >>> the
> >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC
> >>> (no variac). The channels are:
> >>> 1.Ch1. 555 timer.
> >>> 2.Ch2. D19 Anode
> >>> 3.Ch3. D19 Gate.
> >>> 4.Ch4. Q1 Source.
> >
> > Sorry, that looks like a cut and paste error, here is the link to the
> > scope picture
> > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger
> > .png
> >
> > I used a 100ms timebase for the capture and then "zoomed in" a bit
> 
> 
> You would need to zoom in far more to see what's going on when the SCR
> triggers, to cover just a few cycles around the trigger time.
> 
> Once an SCR has been triggerred, the gate becomes a voltage/current
supply, a
> diode drop above 0.
> You see this on your trace in that after triggerring the gate sits at
something +V
> above 0.
> The spike you see may just be an artifact of the internal SCR trigger
action.
> I presume you see some increased current draw from your bench supply for
the
> 3842 after the SCR triggers.
> 
> What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
but
> it shows on the trace as just noise around 0V.
> 
> I would still suggest that you scope the state of the secondary-side
crowbar -
> the gate of Q2, and base of Q4.
> Should be simple to do, before trying to remove or disconnect the main
> transformer.

Oh dear! After Brent's question about D19 anode, I realise that the probe
was connected to the cathode! I have now done it again with the probe
connected to the anode. I have taken two images of the same capture, one at
low resolution to show the overall behaviour

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png

And one zoomed in to show what happens when the SCR shuts down.

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2. 
<https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.png>
 
png

The channels are the same as before, namely:
Ch1. 555 timer.
Ch2. D19 Anode (now corrected as it was previously the cathode!)
Ch3. D19 Gate.
Ch4. Q1 Source.

I got an earlier trace which showed the D19 anode at 9V, which is under the
Undervoltage Lockout threshol

RE: VAXmate PSU

[Rob Jarratt via cctalk]

I will look at all the suggestions, particularly of a failure on the secondary 
side. Something must have burned up, because there was a distinct burning smell 
after the initial failure, although I have never been able to see any physical 
damage to anything, despite looking many times.

 

But the thing that really puzzles me is that, after correcting the probes to 
include the D19 anode, there doesn’t seem to be anything that would cause D19 
to trigger. Am I reading the trace wrong?

 

Thanks

 

Rob

 

From: Mattis Lind  
Sent: 08 April 2020 07:42
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: Re: VAXmate PSU

 

 

 

Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk 
mailto:cctalk@classiccmp.org> >:



> -Original Message-
> From: cctalk  <mailto:cctalk-boun...@classiccmp.org> > On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 06 April 2020 21:07
> To: General Discussion: On-Topic and Off-Topic Posts
mailto:cctalk@classiccmp.org> >
> Subject: Re: VAXmate PSU
> 
> On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
> >>
> >>> I have obtained a scope trace as you suggest. R32 is still lifted so
> >>> the
> >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC
> >>> (no variac). The channels are:
> >>> 1.Ch1. 555 timer.
> >>> 2.Ch2. D19 Anode
> >>> 3.Ch3. D19 Gate.
> >>> 4.Ch4. Q1 Source.
> >
> > Sorry, that looks like a cut and paste error, here is the link to the
> > scope picture
> > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger
> > .png
> >
> > I used a 100ms timebase for the capture and then "zoomed in" a bit
> 
> 
> You would need to zoom in far more to see what's going on when the SCR
> triggers, to cover just a few cycles around the trigger time.
> 
> Once an SCR has been triggerred, the gate becomes a voltage/current
supply, a
> diode drop above 0.
> You see this on your trace in that after triggerring the gate sits at
something +V
> above 0.
> The spike you see may just be an artifact of the internal SCR trigger
action.
> I presume you see some increased current draw from your bench supply for
the
> 3842 after the SCR triggers.
> 
> What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
but
> it shows on the trace as just noise around 0V.
> 
> I would still suggest that you scope the state of the secondary-side
crowbar -
> the gate of Q2, and base of Q4.
> Should be simple to do, before trying to remove or disconnect the main
> transformer.

Oh dear! After Brent's question about D19 anode, I realise that the probe
was connected to the cathode! I have now done it again with the probe
connected to the anode. I have taken two images of the same capture, one at
low resolution to show the overall behaviour

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png

And one zoomed in to show what happens when the SCR shuts down.

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2. 
<https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.png>
 
png

The channels are the same as before, namely:
Ch1. 555 timer.
Ch2. D19 Anode (now corrected as it was previously the cathode!)
Ch3. D19 Gate.
Ch4. Q1 Source.

I got an earlier trace which showed the D19 anode at 9V, which is under the
Undervoltage Lockout threshold, but I have not been able to repeat it.

I don't fully understand the debate about using the variac. 

 

I am not going to debate this either since I know what I have been doing for 
years and it works perfectly well for me. I have fixed the bigger PSUs in a VAX 
11/750 (one broken switch transistor and multiple broken output rectifiers). 
PSU in NORD-10/S (most carbon composition resistors had gone out of spec). PSUs 
in many smaller machines as well.

 

I prefer to work in circuits where I can fiddle around without the danger of 
getting killed all the time. Regardless of use of HV differential probe it can 
be dangerous. Running it on 50VAC with a protection transformer do expose a lot 
of problems already and you can poke around safely in the PSU.

I have not yet seen a problem that wasn't seen at low voltage, but I expect 
there could be semiconductors that experience breakdown that occur at lower 
than specified voltage.

 

 

However, my
measurements appear to suggest that when I use the variac the SCR triggers
because of what appears to be a genuine overcurrent detected by R13. I think
this is because the duty cycle at low AC input voltages is 50% (rather than
about 10% or less as per the trace I have just taken), and I measured 2V
across R13, which does seem to be enough to trigger the SCR. When I 

Re: VAXmate PSU

[Mattis Lind via cctalk]

Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk <
cctalk@classiccmp.org>:

>
>
> > -Original Message-
> > From: cctalk  On Behalf Of Brent Hilpert
> via
> > cctalk
> > Sent: 06 April 2020 21:07
> > To: General Discussion: On-Topic and Off-Topic Posts
> 
> > Subject: Re: VAXmate PSU
> >
> > On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
> > >>
> > >>> I have obtained a scope trace as you suggest. R32 is still lifted so
> > >>> the
> > >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC
> > >>> (no variac). The channels are:
> > >>> 1.Ch1. 555 timer.
> > >>> 2.Ch2. D19 Anode
> > >>> 3.Ch3. D19 Gate.
> > >>> 4.Ch4. Q1 Source.
> > >
> > > Sorry, that looks like a cut and paste error, here is the link to the
> > > scope picture
> > > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger
> > > .png
> > >
> > > I used a 100ms timebase for the capture and then "zoomed in" a bit
> >
> >
> > You would need to zoom in far more to see what's going on when the SCR
> > triggers, to cover just a few cycles around the trigger time.
> >
> > Once an SCR has been triggerred, the gate becomes a voltage/current
> supply, a
> > diode drop above 0.
> > You see this on your trace in that after triggerring the gate sits at
> something +V
> > above 0.
> > The spike you see may just be an artifact of the internal SCR trigger
> action.
> > I presume you see some increased current draw from your bench supply for
> the
> > 3842 after the SCR triggers.
> >
> > What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
> but
> > it shows on the trace as just noise around 0V.
> >
> > I would still suggest that you scope the state of the secondary-side
> crowbar -
> > the gate of Q2, and base of Q4.
> > Should be simple to do, before trying to remove or disconnect the main
> > transformer.
>
> Oh dear! After Brent's question about D19 anode, I realise that the probe
> was connected to the cathode! I have now done it again with the probe
> connected to the anode. I have taken two images of the same capture, one at
> low resolution to show the overall behaviour
>
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png
>
> And one zoomed in to show what happens when the SCR shuts down.
>
>
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.
> png
> <https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.png>
>
> The channels are the same as before, namely:
> Ch1. 555 timer.
> Ch2. D19 Anode (now corrected as it was previously the cathode!)
> Ch3. D19 Gate.
> Ch4. Q1 Source.
>
> I got an earlier trace which showed the D19 anode at 9V, which is under the
> Undervoltage Lockout threshold, but I have not been able to repeat it.
>
> I don't fully understand the debate about using the variac.


I am not going to debate this either since I know what I have been doing
for years and it works perfectly well for me. I have fixed the bigger PSUs
in a VAX 11/750 (one broken switch transistor and multiple broken output
rectifiers). PSU in NORD-10/S (most carbon composition resistors had gone
out of spec). PSUs in many smaller machines as well.

I prefer to work in circuits where I can fiddle around without the danger
of getting killed all the time. Regardless of use of HV differential probe
it can be dangerous. Running it on 50VAC with a protection transformer do
expose a lot of problems already and you can poke around safely in the PSU.
I have not yet seen a problem that wasn't seen at low voltage, but I expect
there could be semiconductors that experience breakdown that occur at lower
than specified voltage.



> However, my
> measurements appear to suggest that when I use the variac the SCR triggers
> because of what appears to be a genuine overcurrent detected by R13. I
> think
> this is because the duty cycle at low AC input voltages is 50% (rather than
> about 10% or less as per the trace I have just taken), and I measured 2V
> across R13, which does seem to be enough to trigger the SCR. When I use
> 220VAC, the voltage across R13 does rise to 6V, which should also trigger
> the SCR I think, except that the peak last a lot less and so perhaps the
> fact that the 6V last for a brief period is insufficient to trigger it?
>


On the issue of duty cycle. If we look at this from the start up
perspective rather than the steady state perspective. At startup there are
no stor

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Matt Burke via
> cctalk
> Sent: 08 April 2020 01:25
> To: cctalk@classiccmp.org
> Subject: Re: VAXmate PSU
> 
> On 07/04/2020 23:34, Rob Jarratt via cctalk wrote:
> > I have seen the suggestions to study the waveforms at a much higher
> > resolution. What I am doing is setting the overall timebase in the
> > 100ms range so that I can trigger on when the 555 starts to oscillate
> > and capture the whole period of operation until the SCR triggers. I
> > can then zoom in, as can be seen from the trace provided in this
> > email. I hope that is good enough, or am I missing some problem with doing
> it this way?
> >
> 
> The timebase will also affect the sampling rate (due to the finite memory
> depth) so some finer details of the waveform may not be captured when
> running at a slower rate.
> 

I did have this in mind but it seemed to me that the sampling was fine enough. 
To be sure though, I just now changed my trigger to capture the drop in the D19 
anode voltage and set the timebase to 10us, but the results looked identical. I 
couldn't see any spikes that were different to the pictures I posted last night.

Thanks

Rob
 
> Matt

Re: VAXmate PSU

[Matt Burke via cctalk]

On 07/04/2020 23:34, Rob Jarratt via cctalk wrote:
> I have seen the suggestions to study the waveforms at a much higher
> resolution. What I am doing is setting the overall timebase in the 100ms
> range so that I can trigger on when the 555 starts to oscillate and capture
> the whole period of operation until the SCR triggers. I can then zoom in, as
> can be seen from the trace provided in this email. I hope that is good
> enough, or am I missing some problem with doing it this way?
>

The timebase will also affect the sampling rate (due to the finite
memory depth) so some finer details of the waveform may not be captured
when running at a slower rate.

Matt

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 06 April 2020 21:07
> To: General Discussion: On-Topic and Off-Topic Posts

> Subject: Re: VAXmate PSU
> 
> On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
> >>
> >>> I have obtained a scope trace as you suggest. R32 is still lifted so
> >>> the
> >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC
> >>> (no variac). The channels are:
> >>> 1.Ch1. 555 timer.
> >>> 2.Ch2. D19 Anode
> >>> 3.Ch3. D19 Gate.
> >>> 4.Ch4. Q1 Source.
> >
> > Sorry, that looks like a cut and paste error, here is the link to the
> > scope picture
> > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger
> > .png
> >
> > I used a 100ms timebase for the capture and then "zoomed in" a bit
> 
> 
> You would need to zoom in far more to see what's going on when the SCR
> triggers, to cover just a few cycles around the trigger time.
> 
> Once an SCR has been triggerred, the gate becomes a voltage/current
supply, a
> diode drop above 0.
> You see this on your trace in that after triggerring the gate sits at
something +V
> above 0.
> The spike you see may just be an artifact of the internal SCR trigger
action.
> I presume you see some increased current draw from your bench supply for
the
> 3842 after the SCR triggers.
> 
> What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
but
> it shows on the trace as just noise around 0V.
> 
> I would still suggest that you scope the state of the secondary-side
crowbar -
> the gate of Q2, and base of Q4.
> Should be simple to do, before trying to remove or disconnect the main
> transformer.

Oh dear! After Brent's question about D19 anode, I realise that the probe
was connected to the cathode! I have now done it again with the probe
connected to the anode. I have taken two images of the same capture, one at
low resolution to show the overall behaviour

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png

And one zoomed in to show what happens when the SCR shuts down.

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.
png

The channels are the same as before, namely:
Ch1. 555 timer.
Ch2. D19 Anode (now corrected as it was previously the cathode!)
Ch3. D19 Gate.
Ch4. Q1 Source.

I got an earlier trace which showed the D19 anode at 9V, which is under the
Undervoltage Lockout threshold, but I have not been able to repeat it.

I don't fully understand the debate about using the variac. However, my
measurements appear to suggest that when I use the variac the SCR triggers
because of what appears to be a genuine overcurrent detected by R13. I think
this is because the duty cycle at low AC input voltages is 50% (rather than
about 10% or less as per the trace I have just taken), and I measured 2V
across R13, which does seem to be enough to trigger the SCR. When I use
220VAC, the voltage across R13 does rise to 6V, which should also trigger
the SCR I think, except that the peak last a lot less and so perhaps the
fact that the 6V last for a brief period is insufficient to trigger it?

I have seen the suggestions to study the waveforms at a much higher
resolution. What I am doing is setting the overall timebase in the 100ms
range so that I can trigger on when the 555 starts to oscillate and capture
the whole period of operation until the SCR triggers. I can then zoom in, as
can be seen from the trace provided in this email. I hope that is good
enough, or am I missing some problem with doing it this way?

I would like to follow Mattis's suggestions (and other people have said it
too) to break the feedback loop, but it does look difficult to know how best
to do it.

I also understand Brent's suggestion that the gate spike is just the result
of the SCR triggering, rather then the cause of the trigger. I had wondered
if that might be the case.

Regards

Rob

Re: VAXmate PSU

[Brent Hilpert via cctalk]

On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
>> 
>>> I have obtained a scope trace as you suggest. R32 is still lifted so
>>> the
>>> UC3842 is powered by the bench PSU, but I am using the full 240VAC (no
>>> variac). The channels are:
>>> 1.  Ch1. 555 timer.
>>> 2.  Ch2. D19 Anode
>>> 3.  Ch3. D19 Gate.
>>> 4.  Ch4. Q1 Source.
> 
> Sorry, that looks like a cut and paste error, here is the link to the scope
> picture
> https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger.png
> 
> I used a 100ms timebase for the capture and then "zoomed in" a bit 


You would need to zoom in far more to see what's going on when the SCR 
triggers, to cover just a few cycles around the trigger time.

Once an SCR has been triggerred, the gate becomes a voltage/current supply, a 
diode drop above 0.
You see this on your trace in that after triggerring the gate sits at something 
+V above 0.
The spike you see may just be an artifact of the internal SCR trigger action.
I presume you see some increased current draw from your bench supply for the 
3842 after the SCR triggers.

What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc but it 
shows on the trace as just noise around 0V.

I would still suggest that you scope the state of the secondary-side crowbar - 
the gate of Q2, and base of Q4.
Should be simple to do, before trying to remove or disconnect the main 
transformer.

Re: VAXmate PSU

[Mattis Lind via cctalk]

Den sön 5 apr. 2020 kl 23:53 skrev Rob Jarratt via cctalk <
cctalk@classiccmp.org>:

>
>
> > -Original Message-
> > From: cctalk  On Behalf Of Brent Hilpert
> via
> > cctalk
> > Sent: 05 April 2020 21:18
> > To: Rob Jarratt ; General Discussion:
> On-Topic
> > and Off-Topic Posts 
> > Subject: Re: VAXmate PSU
> >
> >
> > Switching power supplies are, to coin a phrase, voltage/current-ratio
> power
> > translators.
> > They will attempt to adjust the (cycle-averaged) input-current demand in
> > inverse proportion to the input voltage, to meet the power demand of the
> load.
> >
> > When you load a switching supply, and run it with a low input voltage, it
> will
> > attempt to increase the input-current demand, either with increased peak
> > current or increased duty-cycle (ON-time of primary switching
> transistor(s)).
> >
> > Suppose you have a load demand of 100W. At 100V input the input current
> > needed is 1A.
> > At 10V input, the input current needed is 10A.
> >
> > If a supply is not explicitly designed for low supply voltages, it can
> lead to
> > excessive primary-side currents.
> > This is why it is a bad idea to 'run up' switching supplies from a variac
> or
> > otherwise run them outside their specced input voltage range.
> >
> > You don't say what the observed duty-cycle (ON-time) is. What would be
> > expected is it's running 'wide-open' because it's trying to get enough
> energy
> > through the transformer to meet the load demand while gasping for
> resources
> > from the input because the input voltage is so low.
> >
> > So from the scenario you've set up, it's difficult to discern whether the
> > behaviour is normal or faulty (the scenario masks the otherwise-observed
> > faulty behaviour).
> >
>
>
>
> I have to say that when I was thinking about this, I did wonder if the
> problem was that it was trying to raise the output voltage with not a lot
> of
> input, and that therefore the duty cycle would be too high. I will remove
> the variac from the equation. For the record I was seeing a duty cycle of
> about 50%. In later testing at 240VAC the duty cycle does seem a lot lower.
>

When you use a variac the developed voltage on the secondary side will be
much lower due to the turn ratio of the transformer. You measured 0.4V on
the 5V line if I understand you correctly. That means approx 1/12 of
nominal output. That would then lead to 1/12 current on the secondary side
and 1/144th of the power developed. With this little power developed on the
secondary side there is no chance you can exceed the power transistor
ratings.

I do this all the time. I measure transistor current when doing this and
there are not a ten fold or so increase in current as Brent claim.

By using a variac and you can measure and study waveforms and detect
problems in driving circuits without killing the main switch transistor.
You can study actions of crowbar functions.

There are two different basic variants for SMPS mode PSUs. Forward or
Flyback. Forward are normally used in higher power designs while flyback is
lower power.

A forward mode PSU is like creating a square wave AC current and feeding it
to a transformer. In forward mode the duty cycle cannot be more than 50%
since the core need to reset before next cycle. The output will be a in
relation to the input by the turn ratio of the transformer. A forward
converter need a way to reset the transformer, either by various diodes on
the primary diode or using various bridge topologies for the switching
system. There are also filtering inductors on the secondary side.

However your PSU look very much like a flyback design. It means that the
transformer has an airgap.  It is actually both and inductor and
transformer. A flyback converter stores energy in the inductor when the
transistor is turned on and then when turned off the energy is
transfered to the load. When the transistor is switched on the current
will increase linearly in the primary winding.  This will continue until
the transistor is switched off or the core is saturated. When saturated the
current will rise sharply. This also means that the duty cycle can be
almost 100% only limited by the design of the transformer so that it does
not saturate.

Saturation is a condition you don't want to have. What could be interesting
is to study the waveform of the drain current in more detail. You need to
set timebase of your oscilloscope to a much shorter time span to in
detail study the increase of current in the drain.

The UC3842 does allow up to 100% duty cycle while the other cousins in the
same family, UC3844 and UC3845, only go up to 50%, thus made for
forward converters.

It can be interesting to investigate if t

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> I would agree that a check for shorts in the output stages can be of interest.
> Possibly disconnect one output stage at a time and see if that make a
> difference.
> 
> 

I like this idea, I will have to see how best to do that.

Regards

Rob

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: Brent Hilpert 
> Sent: 06 April 2020 00:12
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: VAXmate PSU
> 
> On 2020-Apr-05, at 2:53 PM, Rob Jarratt wrote:
> > I have obtained a scope trace as you suggest. R32 is still lifted so
> > the
> > UC3842 is powered by the bench PSU, but I am using the full 240VAC (no
> > variac). The channels are:
> > 1.  Ch1. 555 timer.
> > 2.  Ch2. D19 Anode
> > 3.  Ch3. D19 Gate.
> > 4.  Ch4. Q1 Source.
> > The picture is here:
> > https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width
> > -modul
> > ator.png
> >
> >
> > You are correct, I have mixed up the values with R30 and R31, the
> > correct value is 15K. I have updated the schematic, and rearranged it
> > to look diagrammatically more like a sample diagram in the TI
> > datasheet for the UC3842. The updated schematic is here:
> > https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width
> > -modul
> > ator.png
> 
> 
> The links for pic and schematic don't seem to be correct.
> I picked up a modified 15K+15K schematic from the blog web page but
haven't
> found the scope pic.
> 

Sorry, that looks like a cut and paste error, here is the link to the scope
picture
https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger.png

I used a 100ms timebase for the capture and then "zoomed in" a bit 


> Keep in mind at a low scope timebase resolution (long timebase) (as per
your
> previous scope pics), a brief spike on R13 may not show up in the scope
display
> (too brief for the discrete per pixel display interval, but it could have
charged
> C18 enough where it will linger longer and show up on the display. DSOs
can
> present some vagaries like this. If you haven't done so, examine the
traces at a
> higher timebase resolution (it could have been recorded in the sampling,
just
> not being seen).

Re: VAXmate PSU

[Mattis Lind via cctalk]

Den sön 5 apr. 2020 kl 22:18 skrev Brent Hilpert via cctalk <
cctalk@classiccmp.org>:

> On 2020-Apr-05, at 6:05 AM, Rob Jarratt via cctalk wrote:
> > I found time to follow Mattis’s suggestion today and I got some
> interesting results.
> >
> > I powered the UC3842 with about 16V from a bench power supply. I lifted
> R32 so that the transformer would not supply it. I then used an isolating
> transformer to power a variac and applied the variac to the AC inlet. I
> also used a load board from a MicroVAX 2000 and an old RD53 disk as the
> load, so there should be enough load.
> >
> > I found that I can vary the AC input up to a maximum of about 40VAC
> before the SCR triggers, the  5V output reaches about 400mV. If I raise the
> AC input more slowly, it will usually cut out before that, around 30VAC. I
> noticed that the inrush thermistors also get quite hot at these low AC
> voltages, I don’t know if this is because of the relatively low AC supply
> voltage, or if this indicates a problem of some kind.
> >
> > The voltage coming out of L3 into the T1 “bounces” somewhat. I guess
> this is because the AC input is only 20V or so, or it may be expected
> ripple from the smoothing capacitors? In the description below, the peaks
> of the bounces are used. Throughout the variation from 0VAC to 40VAC the
> duty cycle of the oscillation of the UC3842 output does not change, I guess
> because the output voltage has not reached its target value.
> >
> > With the AC input at about 25VAC the circuit seems to be stable (apart
> from the bounces mentioned above). At this supply voltage, the voltage at
> the source of Q1 reaches 2V. The current sense resistor is 1 Ohm, which
> means 2A must be flowing through it at that time.
> >
> > When the Q1 source is at 2V, the other end of R14 is at about 0.5V,
> which is just below the trigger voltage for the SCR. This makes sense
> because R14 and R15 form a voltage divider that looks to be nominally 25%
> of the Q1 source. Given the SCR nominally triggers at about 0.8V, this
> means that the current sense resistor is set to trigger the SCR at about
> 2.5A, I think. This would suggest that the duty cycle on Q1 is too high and
> causing too much current to be drawn. So presumably the feedback to the
> UC3842 is not working correctly.
> >
> > I tried setting the AC input at 120V and using a one-shot sample. Q1 is
> switched for about 30ms and then there is a spike on the SCR gate to 2V and
> it triggers. The gate voltage then remains at 1V. However, there is no
> spike across the current sense resistor (R13), so I don’t know if the spike
> is because the SCR is being turned for some other reason. There is nothing
> unusual on the anode of D19 to cause it to trigger due to avalanche
> breakdown. I got the same result when the AC input was 220V. I wonder if
> the SCR is behaving slightly differently because I have lifted R32?
> >
> > Since there might be a feedback problem, I looked at the VFB input to
> the UC3842 when doing a one-shot test at 240VAC. I can see VFB steadily
> rise over the period when Q1 switched, up to a maximum of 4V. I don’t
> really know if this is how it should behave though, but it seems to make
> logical sense. During all that time the duty cycle of Q1 does not change.
> >
> > I am not too sure where to go from here. I hope the above makes sense. I
> would appreciate any further thoughts.
>
>
> Switching power supplies are, to coin a phrase, voltage/current-ratio
> power translators.
> They will attempt to adjust the (cycle-averaged) input-current demand in
> inverse proportion to the input voltage, to meet the power demand of the
> load.
>

I am not sure what you mean here. A SMMPS mode chop up the input voltage
and feed it through a transformer. Then it can vary the duty cycle to to
regulate the output voltage in case of load variation or input voltage
variation.


>
> When you load a switching supply, and run it with a low input voltage, it
> will attempt to increase the input-current demand, either with increased
> peak current or increased duty-cycle (ON-time of primary switching
> transistor(s)).
>
> Suppose you have a load demand of 100W. At 100V input the input current
> needed is 1A.
> At 10V input, the input current needed is 10A.
>

NO, that is not how it works. I think you are confusing things. All SMPS
has a certain turn ratio. There is nothing magic with a SMPS PSU rather
than a normal iron core transformer. It does transform the primary side
voltage into a secondary side voltage based on turn ratio like any standard
iron core transfomer. But at a higher frequency since then we can have a
smaller transformer. For your calculation to hold some kind of magic duty
cycle will be needed.
The advantage of a SMPS mode PSU is that you also can very the duty cycle
and thus be able to regulate the output voltage as it is feed back to the
control circuitry.

Your statement only holds for the interval the SMPS PSU is designed to
operate in. If it is 

Re: VAXmate PSU

[Brent Hilpert via cctalk]

On 2020-Apr-05, at 2:53 PM, Rob Jarratt wrote:
> I have obtained a scope trace as you suggest. R32 is still lifted so the
> UC3842 is powered by the bench PSU, but I am using the full 240VAC (no
> variac). The channels are:
> 1.Ch1. 555 timer.
> 2.Ch2. D19 Anode
> 3.Ch3. D19 Gate.
> 4.Ch4. Q1 Source.
> The picture is here:
> https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width-modul
> ator.png
> 
> 
> You are correct, I have mixed up the values with R30 and R31, the correct
> value is 15K. I have updated the schematic, and rearranged it to look
> diagrammatically more like a sample diagram in the TI datasheet for the
> UC3842. The updated schematic is here:
> https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width-modul
> ator.png


The links for pic and schematic don't seem to be correct.
I picked up a modified 15K+15K schematic from the blog web page but haven't 
found the scope pic.

Keep in mind at a low scope timebase resolution (long timebase) (as per your 
previous scope pics), a brief spike on R13 may not show up in the scope display 
(too brief for the discrete per pixel display interval, but it could have 
charged C18 enough where it will linger longer and show up on the display. DSOs 
can present some vagaries like this. If you haven't done so, examine the traces 
at a higher timebase resolution (it could have been recorded in the sampling, 
just not being seen).

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 05 April 2020 21:18
> To: Rob Jarratt ; General Discussion:
On-Topic
> and Off-Topic Posts 
> Subject: Re: VAXmate PSU
> 
> 
> Switching power supplies are, to coin a phrase, voltage/current-ratio
power
> translators.
> They will attempt to adjust the (cycle-averaged) input-current demand in
> inverse proportion to the input voltage, to meet the power demand of the
load.
> 
> When you load a switching supply, and run it with a low input voltage, it
will
> attempt to increase the input-current demand, either with increased peak
> current or increased duty-cycle (ON-time of primary switching
transistor(s)).
> 
> Suppose you have a load demand of 100W. At 100V input the input current
> needed is 1A.
> At 10V input, the input current needed is 10A.
> 
> If a supply is not explicitly designed for low supply voltages, it can
lead to
> excessive primary-side currents.
> This is why it is a bad idea to 'run up' switching supplies from a variac
or
> otherwise run them outside their specced input voltage range.
> 
> You don't say what the observed duty-cycle (ON-time) is. What would be
> expected is it's running 'wide-open' because it's trying to get enough
energy
> through the transformer to meet the load demand while gasping for
resources
> from the input because the input voltage is so low.
> 
> So from the scenario you've set up, it's difficult to discern whether the
> behaviour is normal or faulty (the scenario masks the otherwise-observed
> faulty behaviour).
> 



I have to say that when I was thinking about this, I did wonder if the
problem was that it was trying to raise the output voltage with not a lot of
input, and that therefore the duty cycle would be too high. I will remove
the variac from the equation. For the record I was seeing a duty cycle of
about 50%. In later testing at 240VAC the duty cycle does seem a lot lower.


> All this is also dependant on how large your dummy load is (as a % of the
rated
> max power output of the supply).
> 
> If you want to run at a low input voltage, remove or very lightly load the
output.
> From your schematic, there is a small load presented internally from
various
> voltage dividers around the outputs, although not all the R values are in
the
> schematic, so can't calc the current.
> If you still get the over-current SCR triggering, suspicion could lean
towards a
> short somewhere - a winding in the main transformer, secondary rectifiers
or
> caps - anything presenting an excessive energy sink to the main switcher,
> including over-sensitivity of the crowbar circuits on the secondary side.
The
> secondary crowbar circuit monitors the output voltages relative to a
reference.
> You could scope-monitor the gate of the SCR over there.


I have already done a ringing test on the main transformer, and I think that
it is OK. One of the windings does not ring very well, but I think it is one
that has few windings and supplies the on-going power to the primary side
once the PSU has started up. I have also done a bit of testing on the
secondary rectifiers, but not found anything so far. I will look at
secondary side again more closely.

> 
> The spike you mention on the primary-side SCR gate without a corresponding
> spike on R13 does seem odd, seeing scope traces pic could be interesting,
> perhaps scope the anode, the gate and R13. Possibility of some odd trigger
> fault in the SCR.

I have obtained a scope trace as you suggest. R32 is still lifted so the
UC3842 is powered by the bench PSU, but I am using the full 240VAC (no
variac). The channels are:
1.  Ch1. 555 timer.
2.  Ch2. D19 Anode
3.  Ch3. D19 Gate.
4.  Ch4. Q1 Source.
The picture is here:
https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width-modul
ator.png

> There is a small amount of filtering on the SCR-gate/over-current voltage
> divider (C18/2.2nF) so you would expect to a slightly averaged version of
the
> voltage at R13 after the voltage divider (at the cap/gate).
> 
> Aside: You have R27 & R28 at 20+20 ohms in your schematic. This is an
awfully
> low R for dropping the hundreds of supply V down to the 16V/low-current of
the
> 3842 supply. For schematic accuracy, you might double-check the value of
> those.

You are correct, I have mixed up the values with R30 and R31, the correct
value is 15K. I have updated the schematic, and rearranged it to look
diagrammatically more like a sample diagram in the TI datasheet for the
UC3842. The updated schematic is here:
https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width-modul
ator.png


> 
> 10/10 for your tenacity in this repair attempt.

Thank you, I am sure it will be simple when I find the problem, and I am
learning a lot.

Re: VAXmate PSU

[Brent Hilpert via cctalk]

On 2020-Apr-05, at 6:05 AM, Rob Jarratt via cctalk wrote:
> I found time to follow Mattis’s suggestion today and I got some interesting 
> results.
> 
> I powered the UC3842 with about 16V from a bench power supply. I lifted R32 
> so that the transformer would not supply it. I then used an isolating 
> transformer to power a variac and applied the variac to the AC inlet. I also 
> used a load board from a MicroVAX 2000 and an old RD53 disk as the load, so 
> there should be enough load.
> 
> I found that I can vary the AC input up to a maximum of about 40VAC before 
> the SCR triggers, the  5V output reaches about 400mV. If I raise the AC input 
> more slowly, it will usually cut out before that, around 30VAC. I noticed 
> that the inrush thermistors also get quite hot at these low AC voltages, I 
> don’t know if this is because of the relatively low AC supply voltage, or if 
> this indicates a problem of some kind.
> 
> The voltage coming out of L3 into the T1 “bounces” somewhat. I guess this is 
> because the AC input is only 20V or so, or it may be expected ripple from the 
> smoothing capacitors? In the description below, the peaks of the bounces are 
> used. Throughout the variation from 0VAC to 40VAC the duty cycle of the 
> oscillation of the UC3842 output does not change, I guess because the output 
> voltage has not reached its target value.
> 
> With the AC input at about 25VAC the circuit seems to be stable (apart from 
> the bounces mentioned above). At this supply voltage, the voltage at the 
> source of Q1 reaches 2V. The current sense resistor is 1 Ohm, which means 2A 
> must be flowing through it at that time.
> 
> When the Q1 source is at 2V, the other end of R14 is at about 0.5V, which is 
> just below the trigger voltage for the SCR. This makes sense because R14 and 
> R15 form a voltage divider that looks to be nominally 25% of the Q1 source. 
> Given the SCR nominally triggers at about 0.8V, this means that the current 
> sense resistor is set to trigger the SCR at about 2.5A, I think. This would 
> suggest that the duty cycle on Q1 is too high and causing too much current to 
> be drawn. So presumably the feedback to the UC3842 is not working correctly.
> 
> I tried setting the AC input at 120V and using a one-shot sample. Q1 is 
> switched for about 30ms and then there is a spike on the SCR gate to 2V and 
> it triggers. The gate voltage then remains at 1V. However, there is no spike 
> across the current sense resistor (R13), so I don’t know if the spike is 
> because the SCR is being turned for some other reason. There is nothing 
> unusual on the anode of D19 to cause it to trigger due to avalanche 
> breakdown. I got the same result when the AC input was 220V. I wonder if the 
> SCR is behaving slightly differently because I have lifted R32?
> 
> Since there might be a feedback problem, I looked at the VFB input to the 
> UC3842 when doing a one-shot test at 240VAC. I can see VFB steadily rise over 
> the period when Q1 switched, up to a maximum of 4V. I don’t really know if 
> this is how it should behave though, but it seems to make logical sense. 
> During all that time the duty cycle of Q1 does not change.
> 
> I am not too sure where to go from here. I hope the above makes sense. I 
> would appreciate any further thoughts.


Switching power supplies are, to coin a phrase, voltage/current-ratio power 
translators.
They will attempt to adjust the (cycle-averaged) input-current demand in 
inverse proportion to the input voltage, to meet the power demand of the load.

When you load a switching supply, and run it with a low input voltage, it will 
attempt to increase the input-current demand, either with increased peak 
current or increased duty-cycle (ON-time of primary switching transistor(s)).

Suppose you have a load demand of 100W. At 100V input the input current needed 
is 1A.
At 10V input, the input current needed is 10A.

If a supply is not explicitly designed for low supply voltages, it can lead to 
excessive primary-side currents.
This is why it is a bad idea to 'run up' switching supplies from a variac or 
otherwise run them outside their specced input voltage range.

You don't say what the observed duty-cycle (ON-time) is. What would be expected 
is it's running 'wide-open' because it's trying to get enough energy through 
the transformer to meet the load demand while gasping for resources from the 
input because the input voltage is so low.

So from the scenario you've set up, it's difficult to discern whether the 
behaviour is normal or faulty (the scenario masks the otherwise-observed faulty 
behaviour).

All this is also dependant on how large your dummy load is (as a % of the rated 
max power output of the supply).

If you want to run at a low input voltage, remove or very lightly load the 
output.
From your schematic, there is a small load presented internally from various 
voltage dividers around the outputs, although not all the R values are in the 

RE: VAXmate PSU

[Rob Jarratt via cctalk]

I found time to follow Mattis’s suggestion today and I got some interesting 
results.

 

I powered the UC3842 with about 16V from a bench power supply. I lifted R32 so 
that the transformer would not supply it. I then used an isolating transformer 
to power a variac and applied the variac to the AC inlet. I also used a load 
board from a MicroVAX 2000 and an old RD53 disk as the load, so there should be 
enough load.

 

I found that I can vary the AC input up to a maximum of about 40VAC before the 
SCR triggers, the  5V output reaches about 400mV. If I raise the AC input more 
slowly, it will usually cut out before that, around 30VAC. I noticed that the 
inrush thermistors also get quite hot at these low AC voltages, I don’t know if 
this is because of the relatively low AC supply voltage, or if this indicates a 
problem of some kind.

The voltage coming out of L3 into the T1 “bounces” somewhat. I guess this is 
because the AC input is only 20V or so, or it may be expected ripple from the 
smoothing capacitors? In the description below, the peaks of the bounces are 
used. Throughout the variation from 0VAC to 40VAC the duty cycle of the 
oscillation of the UC3842 output does not change, I guess because the output 
voltage has not reached its target value.

 

With the AC input at about 25VAC the circuit seems to be stable (apart from the 
bounces mentioned above). At this supply voltage, the voltage at the source of 
Q1 reaches 2V. The current sense resistor is 1 Ohm, which means 2A must be 
flowing through it at that time.

 

When the Q1 source is at 2V, the other end of R14 is at about 0.5V, which is 
just below the trigger voltage for the SCR. This makes sense because R14 and 
R15 form a voltage divider that looks to be nominally 25% of the Q1 source. 
Given the SCR nominally triggers at about 0.8V, this means that the current 
sense resistor is set to trigger the SCR at about 2.5A, I think. This would 
suggest that the duty cycle on Q1 is too high and causing too much current to 
be drawn. So presumably the feedback to the UC3842 is not working correctly.

 

I tried setting the AC input at 120V and using a one-shot sample. Q1 is 
switched for about 30ms and then there is a spike on the SCR gate to 2V and it 
triggers. The gate voltage then remains at 1V. However, there is no spike 
across the current sense resistor (R13), so I don’t know if the spike is 
because the SCR is being turned for some other reason. There is nothing unusual 
on the anode of D19 to cause it to trigger due to avalanche breakdown. I got 
the same result when the AC input was 220V. I wonder if the SCR is behaving 
slightly differently because I have lifted R32?

 

Since there might be a feedback problem, I looked at the VFB input to the 
UC3842 when doing a one-shot test at 240VAC. I can see VFB steadily rise over 
the period when Q1 switched, up to a maximum of 4V. I don’t really know if this 
is how it should behave though, but it seems to make logical sense. During all 
that time the duty cycle of Q1 does not change.

 

I am not too sure where to go from here. I hope the above makes sense. I would 
appreciate any further thoughts.

 

Regards

 

Rob

 

 

From: Rob Jarratt  
Sent: 29 March 2020 11:40
To: 'Mattis Lind' ; r...@jarratt.me.uk
Cc: 'General Discussion: On-Topic and Off-Topic Posts' 
Subject: RE: VAXmate PSU

 

Thanks, I do have an isolating transformer as well, so I will use that. I have 
done previous tests with a bench PSU on the UC3842 and found it needs 16V to 
get going. I just checked the Technical Description and it says the minimum 
current on the 5V output is 6.4A and on 12V it is 0.17A. I checked my load 
board and it is only going to sink 3A, so I need more load as I think the IDE 
disk is not going to be much. Would insufficient load really cause it to 
shutdown so quickly though?

 

Not sure I understand your comment about “designators for networks”, is there 
an example you can point me at? One of the things I have tried to do, but 
clearly not very successfully, is to minimise the lengths of the wires.

 

Regards

 

Rob

 

From: Mattis Lind mailto:mattisl...@gmail.com> > 
Sent: 29 March 2020 08:51
To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> 
Cc: General Discussion: On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> >
Subject: Re: VAXmate PSU

 

Hello Rob,

söndag 29 mars 2020 skrev Rob Jarratt mailto:robert.jarr...@ntlworld.com> >:

Thanks Mattis. I do test it with a load, although to be honest I forgot to do 
this when I took the measurements yesterday. I use a modern-ish IDE disk and a 
load board from a MicroVAX 2000 as the dummy load. I don’t know if that is 
sufficient.

 

Do you know the nominal output rating for the supply?

It might be the case that even with load board the load is uneven. But it 
sounds less likely.  

 

I don’t know enough about PSUs to make the secondary side drawing more logical 
unfortunately.

 

Can be hard

RE: VAXmate PSU

[Fred Cisin via cctalk]

On Sun, 29 Mar 2020, Rob Jarratt via cctalk wrote:
Thanks Mattis. I do test it with a load, although to be honest I forgot 
to do this when I took the measurements yesterday. I use a modern-ish 
IDE disk and a load board from a MicroVAX 2000 as the dummy load. I 
don’t know if that is sufficient.


I have NO specific knowledge of that PSU.

However, it seems likely that a "modern-sh IDE disk" in unlikely to be 
enough load.



OB_useless_nostalgia:
In the old days, we used an automotive headlight bulb.  That also provided 
a lot, even excess, light on the workbench.  Since my microcomputer days 
overlapped the period that I ran a Honda car repair shop (until 1980), I 
used to save any dual filament (High/low) bulbs that we replaced due to 
ONE of the filaments burning out.  I made an aimable mount for my 
workbench load out of an early VW headlight mount.

RE: VAXmate PSU

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Matt Burke via
> cctalk
> Sent: 29 March 2020 10:41
> To: cctalk@classiccmp.org
> Subject: Re: VAXmate PSU
> 
> On 29/03/2020 08:50, Mattis Lind via cctalk wrote:
> > Hello Rob,
> >
> > söndag 29 mars 2020 skrev Rob Jarratt :
> >
> >> Thanks Mattis. I do test it with a load, although to be honest I
> >> forgot to do this when I took the measurements yesterday. I use a
> >> modern-ish IDE disk and a load board from a MicroVAX 2000 as the
> >> dummy load. I don’t know if that is sufficient.
> >>
> >>
> > Do you know the nominal output rating for the supply?
> > It might be the case that even with load board the load is uneven. But
> > it sounds less likely.
> 
> The output specifications for the H7270 power supply are on page 9-13 of the
> VAXmate Technical Description (EK-PC500-TD-001).
> 
> +5.1V - Min 6.40A, Max 10.24A
> +12.1 - Min 0.17A, Max 1.37A
> -12.0 - Min 0.12A, Max 0.33A
> +28.0 - Min 0.45A, Max 0.55A
> -9.0 - Min 0.18A, Max 0.20A
> 300 - Min 0.00A, Max 0.46A (for the expansion box)
> 
> I have one of those load boards from a MicroVAX 2000 but the 5V load was not
> connected by default. I had to install some zero ohm resistors. The board I 
> have
> draws 1A at 12V and 3A at 5V so you will need more load on the 5V rail at
> least.


Thanks Matt, I just found that bit in the documentation and realised I am not 
generating enough load. I will have to add more. I did connect the 5V load 
because as you say it wasn't connected.


> 
> As Mattis has mentioned there looks to be a crowbar circuit on the secondary
> side (Q2) to provide overvoltage protection. The 5V and 12V output are shorted
> by Q2, which causes the primary side to sense an overcurrent condition and
> shut the supply down.


I can't test it now until I get another fuse, but I am sure I didn't see this 
happen, although it seems to be quite hard to tell.


> 
> Matt

RE: VAXmate PSU

[Rob Jarratt via cctalk]

Thanks, I do have an isolating transformer as well, so I will use that. I have 
done previous tests with a bench PSU on the UC3842 and found it needs 16V to 
get going. I just checked the Technical Description and it says the minimum 
current on the 5V output is 6.4A and on 12V it is 0.17A. I checked my load 
board and it is only going to sink 3A, so I need more load as I think the IDE 
disk is not going to be much. Would insufficient load really cause it to 
shutdown so quickly though?

 

Not sure I understand your comment about “designators for networks”, is there 
an example you can point me at? One of the things I have tried to do, but 
clearly not very successfully, is to minimise the lengths of the wires.

 

Regards

 

Rob

 

From: Mattis Lind  
Sent: 29 March 2020 08:51
To: r...@jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts 
Subject: Re: VAXmate PSU

 

Hello Rob,

söndag 29 mars 2020 skrev Rob Jarratt mailto:robert.jarr...@ntlworld.com> >:

Thanks Mattis. I do test it with a load, although to be honest I forgot to do 
this when I took the measurements yesterday. I use a modern-ish IDE disk and a 
load board from a MicroVAX 2000 as the dummy load. I don’t know if that is 
sufficient.

 

Do you know the nominal output rating for the supply?

It might be the case that even with load board the load is uneven. But it 
sounds less likely.  

 

I don’t know enough about PSUs to make the secondary side drawing more logical 
unfortunately.

 

Can be hard. But a good idea is try to minimise the length of wires. Maybe use 
designators for networks instead of wires to make the schematic more readable. 
On the other hand I was reading the post on the iPad. 

 

I have a variac and a bench power supply, so I could do what you suggest. 

To stay safe you need a protection transformer as well. Otherwise the the bench 
supply will end up at line potential. With the protection transformer in place 
the circuit will be left floating in relation to earth. With a variac you can 
then vary the input and keep it within safe limits. If you don’t have a 
protection transformer and variac then another bench DC supply that can give up 
to 100 V can be used instead. 

 

Could you be a bit more specific about where to apply what, so I don’t do it 
wrong or damage something? Would you put the bench PSU across the UC3842 Vcc 
and Gnd pins? I am not sure what would happen if the normal supply to the 
UC3842 was still in place with the bench power supply also trying to supply 
power. Would it be wise to lift R32 so nothing conflicts with the bench power 
supply? 

 

Absolutely right. I should have been more explicit about where to introduce 
external supply. But as you say, lift R32 to isolate the switching controller 
and feed in the bench supply current at VCC. Check the data sheet for what is 
approriate voltage. 

 

Good luck!

 

/Mattis

 

Thanks

 

Rob

 

From: Mattis Lind < <mailto:mattisl...@gmail.com> mattisl...@gmail.com> 
Sent: 29 March 2020 06:39
To:  <mailto:r...@jarratt.me.uk> r...@jarratt.me.uk; Rob Jarratt < 
<mailto:robert.jarr...@ntlworld.com> robert.jarr...@ntlworld.com>; General 
Discussion: On-Topic and Off-Topic Posts < <mailto:cctalk@classiccmp.org> 
cctalk@classiccmp.org>
Subject: Re: VAXmate PSU

 

Hello Rob,

lördag 28 mars 2020 skrev Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> >:

I have posted here a couple of times because I have a failed VAXmate PSU. I
have just posted a little bit more information here:
https://robs-old-computers.com/2020/03/28/further-analysis-of-the-vaxmate-h7 
<https://robs-old-computers.com/2020/03/28/further-analysis-of-the-vaxmate-h7270-psu-failure/>
 
270-psu-failure/ with some scope traces and a greatly improved schematic.
Although the schematic is likely to have errors still. Unfortunately, a
stray scope probe ground lead blew the fuse so now I have to wait for a new
fuse to arrive before I can continue work.



I would really like to know if all the spiking I am seeing is to be
expected, and any suggestions why it appears to be detecting an overcurrent?
There do not appear to be any shorts on the secondary side, but that could
be wrong of course. I don't know if a genuine short anywhere would cause it
to trip the SCR quite so quickly (within 20ms of the switching transistor
starting to switch).

 

This is fly back design and I would expect some spiking when the transistor 
shuts off.  

 

Then for over-current. It might be so that there are over-voltage protection on 
the outputs that kicks in. A crowbar that short circuits the output. It looks 
like there is such a circuit on 5 and 12 V. But to be honest the output circuit 
schematic is hard to read. 

 

If you have no load or little load or un-even load the PSU might hae problems 
to regulate. I know for fact that the PSU in the MicroVAX 2000 need to have a 
dummy load when no hard drive is installed otherwise

Re: VAXmate PSU

[Matt Burke via cctalk]

On 29/03/2020 08:50, Mattis Lind via cctalk wrote:
> Hello Rob,
>
> söndag 29 mars 2020 skrev Rob Jarratt :
>
>> Thanks Mattis. I do test it with a load, although to be honest I forgot to
>> do this when I took the measurements yesterday. I use a modern-ish IDE disk
>> and a load board from a MicroVAX 2000 as the dummy load. I don’t know if
>> that is sufficient.
>>
>>
> Do you know the nominal output rating for the supply?
> It might be the case that even with load board the load is uneven. But it
> sounds less likely.

The output specifications for the H7270 power supply are on page 9-13 of
the VAXmate Technical Description (EK-PC500-TD-001).

+5.1V - Min 6.40A, Max 10.24A
+12.1 - Min 0.17A, Max 1.37A
-12.0 - Min 0.12A, Max 0.33A
+28.0 - Min 0.45A, Max 0.55A
-9.0 - Min 0.18A, Max 0.20A
300 - Min 0.00A, Max 0.46A (for the expansion box)

I have one of those load boards from a MicroVAX 2000 but the 5V load was
not connected by default. I had to install some zero ohm resistors. The
board I have draws 1A at 12V and 3A at 5V so you will need more load on
the 5V rail at least.

As Mattis has mentioned there looks to be a crowbar circuit on the
secondary side (Q2) to provide overvoltage protection. The 5V and 12V
output are shorted by Q2, which causes the primary side to sense an
overcurrent condition and shut the supply down.

Matt

Re: VAXmate PSU

[Mattis Lind via cctalk]

Hello Rob,

söndag 29 mars 2020 skrev Rob Jarratt :

> Thanks Mattis. I do test it with a load, although to be honest I forgot to
> do this when I took the measurements yesterday. I use a modern-ish IDE disk
> and a load board from a MicroVAX 2000 as the dummy load. I don’t know if
> that is sufficient.
>
>
Do you know the nominal output rating for the supply?
It might be the case that even with load board the load is uneven. But it
sounds less likely.

>
>
> I don’t know enough about PSUs to make the secondary side drawing more
> logical unfortunately.
>
>
Can be hard. But a good idea is try to minimise the length of wires. Maybe
use designators for networks instead of wires to make the schematic more
readable. On the other hand I was reading the post on the iPad.

>
>
> I have a variac and a bench power supply, so I could do what you suggest.
>
> To stay safe you need a protection transformer as well. Otherwise the the
bench supply will end up at line potential. With the protection transformer
in place the circuit will be left floating in relation to earth. With a
variac you can then vary the input and keep it within safe limits. If you
don’t have a protection transformer and variac then another bench DC supply
that can give up to 100 V can be used instead.

>
> Could you be a bit more specific about where to apply what, so I don’t do
> it wrong or damage something? Would you put the bench PSU across the UC3842
> Vcc and Gnd pins? I am not sure what would happen if the normal supply to
> the UC3842 was still in place with the bench power supply also trying to
> supply power. Would it be wise to lift R32 so nothing conflicts with the
> bench power supply?
>
>
Absolutely right. I should have been more explicit about where to introduce
external supply. But as you say, lift R32 to isolate the switching
controller and feed in the bench supply current at VCC. Check the data
sheet for what is approriate voltage.

Good luck!

/Mattis

>
>
> Thanks
>
>
>
> Rob
>
>
>
> *From:* Mattis Lind 
> *Sent:* 29 March 2020 06:39
> *To:* r...@jarratt.me.uk; Rob Jarratt ;
> General Discussion: On-Topic and Off-Topic Posts 
> *Subject:* Re: VAXmate PSU
>
>
>
> Hello Rob,
>
> lördag 28 mars 2020 skrev Rob Jarratt via cctalk :
>
> I have posted here a couple of times because I have a failed VAXmate PSU. I
> have just posted a little bit more information here:
> https://robs-old-computers.com/2020/03/28/further-
> analysis-of-the-vaxmate-h7
> 270-psu-failure/ with some scope traces and a greatly improved schematic.
> Although the schematic is likely to have errors still. Unfortunately, a
> stray scope probe ground lead blew the fuse so now I have to wait for a new
> fuse to arrive before I can continue work.
>
>
>
> I would really like to know if all the spiking I am seeing is to be
> expected, and any suggestions why it appears to be detecting an
> overcurrent?
> There do not appear to be any shorts on the secondary side, but that could
> be wrong of course. I don't know if a genuine short anywhere would cause it
> to trip the SCR quite so quickly (within 20ms of the switching transistor
> starting to switch).
>
>
>
> This is fly back design and I would expect some spiking when the
> transistor shuts off.
>
>
>
> Then for over-current. It might be so that there are over-voltage
> protection on the outputs that kicks in. A crowbar that short circuits the
> output. It looks like there is such a circuit on 5 and 12 V. But to be
> honest the output circuit schematic is hard to read.
>
>
>
> If you have no load or little load or un-even load the PSU might hae
> problems to regulate. I know for fact that the PSU in the MicroVAX 2000
> need to have a dummy load when no hard drive is installed otherwise there
> will be uneven load which it has hard time to handle sonce the output
> regulation is based on the sum of the outputs somehow. It will trip the
> crowbar on over voltage on one of the outputs otherwise.
>
>
>
> What if you supply the control circuitry on the primary side using a bench
> lab supply and then connect a protection transformer and a variac in series
> to the normal AC inlet.
>
>
>
> Slowly increase input AC voltage while monitoring source voltage and
> output voltages.  At what AC input voltages does it trip? What is the
> output voltages at this point?
>
>
>
> If both voltages exceed normal and the crowbar trips I would think that
> the feedback network somehow reports to low output voltage to the control
> circuitry. Maybe the opto coupler is bad?
>
>
>
> Sorry. A lot of guessing here. But it is hard to tell withour more
> measurements.
>
>
>
> /Mattis
>
>
>
>
>
> Any thoughts gratefully received.
>
>
>
> Thanks
>
>
>
> Rob
>
>

RE: VAXmate PSU

[Rob Jarratt via cctalk]

Thanks Mattis. I do test it with a load, although to be honest I forgot to do 
this when I took the measurements yesterday. I use a modern-ish IDE disk and a 
load board from a MicroVAX 2000 as the dummy load. I don’t know if that is 
sufficient.

 

I don’t know enough about PSUs to make the secondary side drawing more logical 
unfortunately.

 

I have a variac and a bench power supply, so I could do what you suggest. Could 
you be a bit more specific about where to apply what, so I don’t do it wrong or 
damage something? Would you put the bench PSU across the UC3842 Vcc and Gnd 
pins? I am not sure what would happen if the normal supply to the UC3842 was 
still in place with the bench power supply also trying to supply power. Would 
it be wise to lift R32 so nothing conflicts with the bench power supply? 

 

Thanks

 

Rob

 

From: Mattis Lind  
Sent: 29 March 2020 06:39
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: Re: VAXmate PSU

 

Hello Rob,

lördag 28 mars 2020 skrev Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> >:

I have posted here a couple of times because I have a failed VAXmate PSU. I
have just posted a little bit more information here:
https://robs-old-computers.com/2020/03/28/further-analysis-of-the-vaxmate-h7 
<https://robs-old-computers.com/2020/03/28/further-analysis-of-the-vaxmate-h7270-psu-failure/>
 
270-psu-failure/ with some scope traces and a greatly improved schematic.
Although the schematic is likely to have errors still. Unfortunately, a
stray scope probe ground lead blew the fuse so now I have to wait for a new
fuse to arrive before I can continue work.



I would really like to know if all the spiking I am seeing is to be
expected, and any suggestions why it appears to be detecting an overcurrent?
There do not appear to be any shorts on the secondary side, but that could
be wrong of course. I don't know if a genuine short anywhere would cause it
to trip the SCR quite so quickly (within 20ms of the switching transistor
starting to switch).

 

This is fly back design and I would expect some spiking when the transistor 
shuts off.  

 

Then for over-current. It might be so that there are over-voltage protection on 
the outputs that kicks in. A crowbar that short circuits the output. It looks 
like there is such a circuit on 5 and 12 V. But to be honest the output circuit 
schematic is hard to read. 

 

If you have no load or little load or un-even load the PSU might hae problems 
to regulate. I know for fact that the PSU in the MicroVAX 2000 need to have a 
dummy load when no hard drive is installed otherwise there will be uneven load 
which it has hard time to handle sonce the output regulation is based on the 
sum of the outputs somehow. It will trip the crowbar on over voltage on one of 
the outputs otherwise. 

 

What if you supply the control circuitry on the primary side using a bench lab 
supply and then connect a protection transformer and a variac in series to the 
normal AC inlet. 

 

Slowly increase input AC voltage while monitoring source voltage and output 
voltages.  At what AC input voltages does it trip? What is the output voltages 
at this point? 

 

If both voltages exceed normal and the crowbar trips I would think that the 
feedback network somehow reports to low output voltage to the control 
circuitry. Maybe the opto coupler is bad?

 

Sorry. A lot of guessing here. But it is hard to tell withour more 
measurements. 

 

/Mattis

 



Any thoughts gratefully received.



Thanks



Rob

Re: VAXmate PSU

[Mattis Lind via cctalk]

Hello Rob,

lördag 28 mars 2020 skrev Rob Jarratt via cctalk :

> I have posted here a couple of times because I have a failed VAXmate PSU. I
> have just posted a little bit more information here:
> https://robs-old-computers.com/2020/03/28/further-
> analysis-of-the-vaxmate-h7
> 270-psu-failure/ with some scope traces and a greatly improved schematic.
> Although the schematic is likely to have errors still. Unfortunately, a
> stray scope probe ground lead blew the fuse so now I have to wait for a new
> fuse to arrive before I can continue work.
>
>
>
> I would really like to know if all the spiking I am seeing is to be
> expected, and any suggestions why it appears to be detecting an
> overcurrent?
> There do not appear to be any shorts on the secondary side, but that could
> be wrong of course. I don't know if a genuine short anywhere would cause it
> to trip the SCR quite so quickly (within 20ms of the switching transistor
> starting to switch).
>
>
This is fly back design and I would expect some spiking when the transistor
shuts off.

Then for over-current. It might be so that there are over-voltage
protection on the outputs that kicks in. A crowbar that short circuits the
output. It looks like there is such a circuit on 5 and 12 V. But to be
honest the output circuit schematic is hard to read.

If you have no load or little load or un-even load the PSU might hae
problems to regulate. I know for fact that the PSU in the MicroVAX 2000
need to have a dummy load when no hard drive is installed otherwise there
will be uneven load which it has hard time to handle sonce the output
regulation is based on the sum of the outputs somehow. It will trip the
crowbar on over voltage on one of the outputs otherwise.

What if you supply the control circuitry on the primary side using a bench
lab supply and then connect a protection transformer and a variac in series
to the normal AC inlet.

Slowly increase input AC voltage while monitoring source voltage and output
voltages.  At what AC input voltages does it trip? What is the output
voltages at this point?

If both voltages exceed normal and the crowbar trips I would think that the
feedback network somehow reports to low output voltage to the control
circuitry. Maybe the opto coupler is bad?

Sorry. A lot of guessing here. But it is hard to tell withour more
measurements.

/Mattis


>
> Any thoughts gratefully received.
>
>
>
> Thanks
>
>
>
> Rob
>
>

RE: VAXmate PSU Failure

[Rob Jarratt via cctalk]

> The UC3842 datasheet does show you can draw up to 20 mA from the Vref pin,
> which should certainly power up a 555.  But, this is a 5 V regulator, and
may
> not be working properly.  it may be that driving several mA to the 555, E2
and
> other components has cooked it over time.  With the 555 removed, apply
> resistors between pins 1 and 8 of the 555, and see how much load the
UC3842
> can supply.  I'm guessing that at 1 - 2 mA the Vref output is going to
collapse,
> indicating the UC3842 is "weak". If the Vref does droop, make sure the Vcc
is
> still well above 5 V.  If so, it has to be the UC3842. If the Vcc is also
drooping,
> then follow the chain back to the power input to find the defective
component.
> It could be Q1, R27 R28,
> R32 or for some reason the crowbar D19 is being fired.


I just tried this with a 1K resistor I had to hand, so 5ma not 1-2ma, and
the Vref droops. Vcc does not droop though, so I think the UC3842 needs to
be replaced. I will try that.

Incidentally, the UC3842 datasheet says that Vcc must have a short-path
bypass capacitor to ground. If I socket the UC3842 is that going to be an
issue?

Thanks

Rob

Re: VAXmate PSU Failure

[Jon Elson via cctalk]

On 02/15/2020 12:17 PM, Rob Jarratt via cctalk wrote:

A little while back I posted here because I needed help with analysing the
failure of the PSU from my VAXmate. Since then I have had some comments on
the reverse engineered schematic which I have now improved and which is
here:
https://rjarratt.files.wordpress.com/2020/02/h7270-control-pulse-width-modul
ator.png

  






The UC3842 datasheet does show you can draw
up to 20 mA from the Vref pin, which should certainly power 
up a 555.  But, this is a 5 V regulator,
and may not be working properly.  it may be that driving 
several mA to the 555, E2 and other
components has cooked it over time.  With the 555 removed, 
apply resistors between pins 1 and 8 of
the 555, and see how much load the UC3842 can supply.  I'm 
guessing that at 1 - 2 mA the
Vref output is going to collapse, indicating the UC3842 is 
"weak". If the Vref does droop, make sure
the Vcc is still well above 5 V.  If so, it has to be the 
UC3842. If the Vcc is also drooping, then
follow the chain back to the power input to find the 
defective component.  It could be Q1, R27 R28,

R32 or for some reason the crowbar D19 is being fired.

Jon