20 Nov
2022
20 Nov
'22
5:40 p.m.
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
