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

[Peter Coghlan via cctalk]

Hi Rob,

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

I probably do.  I guess the base drive for the main switching transistor
comes from another smaller transistor via the inductor and the diode which
failed? Or maybe the diode is in parallel across the drive voltage?  Anyway,
it is this smaller transistor providing the drive that I'm suggesting
checking next as it may have got a belt of anything up to three hundred and
something volts DC when the main switching transistor shorted out an instant
before it exploded.  If that one has failed too, check further back to
whatever is driving it.

>
> I will check on the output side as you suggest.
>

The rectifiers on the output side may be tedious to figure out and test
due in part to the very low resistances of the transformer windings
feeding them unfortunately, especially if there are double rectifiers
(maybe in packages which look suspiciously like transistors) connected
to either side of centre tapped transformer windings.

>
> The circuit breaker did pop out when it failed. The onboard fuse is intact.
>

If you mean the house breaker as opposed to a breaker in the power supply,
I would guess it was probably rated at 25A or more and given the
destruction of the transistor, I would think the instantaneous current
was way higher than that.  Perhaps the fuse is a time delay one.  It
seems likely things could have been worse if the breaker wasn't able
to cut the power as soon as it did and you had to wait for the fuse
to blow :-(

>
> The house RCD triggered and cut the power to the whole house when the
> transistor exploded!
>

I guess that the transistor exploding produced a momentary cloud of
ionised gas which allowed arcing/tracking from the live transistor
terminals to something earthed (power supply case maybe?) which
triggered the RCD.  Or maybe the heatsink the switching transistor
is mounted on is earthed and the insulating washer between the
heatsink and the transistor failed causing it's destruction and
the heatsink then became the path to earth that triggered the RCD?

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

Sorry, I have no idea what the diode is.  Hopefully Antonio will be able
to help with that.  Or if your spare power supply is the same design as
the failed one, can you look at the diode in that one?

It has dawned on me that another mechanism for the switching transistor
to have failed so spectacularly is for it to have been switched on hard
and held on for more than an instant by something driving it resulting
in it effectively shorting out the rectified mains supply through the
primary of the chopper transformer.

When you next try powering it up, it might be good to use the old
(filament) light bulb in series with the mains supply trick in case
whatever caused the initial problem is still lurking in there.  This
can be useful for locating shorted output rectifiers too.  You might
see a slight voltage rise from the outputs with working rectifiers when
power is applied through the light bulb and a lesser or no voltage rise
on an output which has a shorted rectifier or other issue that needs
closer inspection.  It also means there is no need to hide behind the
sofa when you switch it on :-)

Regards,
Peter.

> 
> Thanks
> 
> Rob 
> 
> > -Original Message-
> > From: Peter Coghlan via cctalk 
> > Sent: 20 November 2022 18:50
> > To: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Cc: Peter Coghlan 
> > Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> > Supply
> > 
> > Hi Rob,
> > 
> > I'm only guessing here.  I think the sequence may have been that the main
> > switching transistor failed first as it would be under more stress than a
> diode
> > in the base circuit.  If the transistor shorted E-B-C then the HT would
> become
> > connected to the circuitry at it's base which would be compelely unable to
> > cope with voltages and currents involved.  This probably resulted in the
> > failure of the diode.  I think it may be worth looking at the components
> > further back the drive chain from the diode.
> > The inductor could be ok unless it is a very frail little thing but small
> signal
> > semiconductor components and/or resistors further back may not have
> > fared as well as it.
> > 
> > It might also be worthwhile checking for shorted rectifiers on the output
> side
> > in case this was the cause of the stress on the switching transistor.
> > However, the power supply might have an overcurrent trip to reduce the
> > possibility of this sort of damage.  If there is an overcurrent trip or
> thermal
> > trip, this may have been reset after the power supply was powered off for
> a
> > while and when it was powered on 

[cctalk] digital group's Richard Bemis

[Whitney Scharer via cctalk]

Hi Brad,
I just came across your post from last year about my father, Richard C.
Bemis. He passed away in June. I wish I knew more about the digital group
and would love to see the video or article you are working on. I can also
fill you in on what my dad did after dg.

Thanks,
Whitney
___

Whitney Scharer
*The Age of Light*
Order today from Bookshop.org  or
Amazon


whitneyscharer.com    |  @wscharer
  |  instagram.com/wscharer

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

[Rob Jarratt via cctalk]

Thanks Antonio,

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

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

Regards

Rob

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

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

[Antonio Carlini via cctalk]

On 20/11/2022 17:40, Rob Jarratt via cctalk wrote:

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

  




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



I have Rainbow PSUs H7842A, H78420 (which I suspect I may have misread! 
...) and H7842D available. I can look tomorrow; if you can supply an 
overview picture and maybe circle the location of the offending parts 
that might help me identify them more quickly (always assuming that they 
are marked at all, of course).



Antonio


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

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

[Rob Jarratt via cctalk]

Hello Peter,

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

I will check on the output side as you suggest.

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

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

Thanks

Rob 

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

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

[Peter Coghlan via cctalk]

Hi Rob,

I'm only guessing here.  I think the sequence may have been that the
main switching transistor failed first as it would be under more stress
than a diode in the base circuit.  If the transistor shorted E-B-C then
the HT would become connected to the circuitry at it's base which would
be compelely unable to cope with voltages and currents involved.  This
probably resulted in the failure of the diode.  I think it may be worth
looking at the components further back the drive chain from the diode.
The inductor could be ok unless it is a very frail little thing but small
signal semiconductor components and/or resistors further back may not
have fared as well as it.

It might also be worthwhile checking for shorted rectifiers on the output
side in case this was the cause of the stress on the switching transistor.
However, the power supply might have an overcurrent trip to reduce the
possibility of this sort of damage.  If there is an overcurrent trip or
thermal trip, this may have been reset after the power supply was powered
off for a while and when it was powered on again, the already damaged
transistor could have been teed up to fail more spectacularly?  Like I
said, just guessing here.

Were there no fuses failed or cutouts cut out?  Does it look like there
should have been? I would think a shorted switching transistor should
have caused some safety device to operate.  Or is it the case of the
old adage that the faster acting transistor managed to sacrifice itself
in time to protect the quick-blow fuse from blowing?

Regards,
Peter Coghlan.

> 
> The H7842 PSU in my Rainbow failed yesterday. At first the machine just
> powered down and there was a slight burning smell, I wasn't next to the
> machine when this happened, so I didn't see or hear anything to tell me
> where the problem might be. Not being sure if there was a short in the
> machine or a problem in the PSU, I disconnected the fans, FDD and HDD and,
> probably foolishly, I applied power again to see if the machine would work.
> At this point there was a bang and a flash in the PSU.
> 
> 
> 
> On opening up the H7842 power supply I found that one of the transistors had
> completely disintegrated. It looks to be the main switching transistor, here
> is a picture of it:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg.  I
> have identified a source for this transistor, but if anyone can suggest a
> modern replacement that would be useful too. However, that is not my main
> problem.
> 
>  
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
>  
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
>  
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
> 
>  
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
>  
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
> lure/
> 
>  
> 
> Thanks
> 
>  
>
> Rob
> 

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

[Rob Jarratt via cctalk]

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

 

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

 

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

 

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

 

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

 

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

 

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

 

Thanks

 

Rob

[cctalk] Re: RIP Fred Brooks (1931-2022)

[ED SHARPE via cctalk]

Alas another one gone.



Sent from the all new AOL app for Android 
 
  On Fri, Nov 18, 2022 at 7:34 PM, Tomasz Rola via 
cctalk wrote:   As the subject (and wikipedia) say:

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

-- 
Regards,
Tomasz Rola

--
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** As the answer, master did "rm -rif" on the programmer's home    **
** directory. And then the C programmer became enlightened...      **
**                                                                **
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