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
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:
DEC used a lot of A114x diodes in their PSUs. They looked exactly like that
one. Those are fast recovery diodes.
I would replace it with a UF4007 or something similar.
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?
Also check all other semiconductors. Also on the outputs. If there is a 1
ohm fusible resistor in the base drive circuit check that one as well. In
the VT100 PSUs it happens that it blows.
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):