20 Nov
2022
20 Nov
'22
10:49 a.m.
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
