20 Nov
2022
20 Nov
'22
8:37 p.m.
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 <cctalk(a)classiccmp.org>
Sent: 20 November 2022 18:50
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
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):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-
supp
> ly-fai
> lure/
>
>
>
> Thanks
>
>
>
> Rob
> 