Seems to me that most power supply problems usually occur in four areas: the
pass transistors, the regulator (usually an LM723 type), the transistor
controlling the pass transistors (usually a TIP41C or similar) and the
protection circuits. Sometimes the protection circuits work too well, and hide
the real symptoms of the problem.
I'd pull the pass transistors and check them first off with an ohmmeter. If
they are bad, I'd replace them as well as the ballast resistors. Do you know
if the 20 amp rating is peak or continuous? If it's peak, I'd be immediately
suspicious of the pass transistors...
Working backwards, I would then check the current amp driving the passes...and
then last I'd check the operation of the regulator itself to make sure it was
driving the current amp correctly.
That being said, the first thing I do with any supply I get is to socketize the
regulator IC if it isn't already...it's then easier to replace when it dies
(and trust me, it will).
As for expense, it really is worth your while to keep a couple of sets of pass
transistors, regulators, and current amps handy. It's hard to believe, but 4
30-amp pass transistors, a regulator, and a current amp runs a total of about
12 dollars. (Which raises another point--why are power supplies so darn
expensive??)
I don't consider myself an expert (and you shouldn't either)...sometimes I'm
lucky and fix the problem, and then sometimes I'm left scratching my head amid
a cloud of smoke. This is just the approach I would take.
Hope this helps...from a person who has spent quite a few late nights staring
at a schematic and wondering "WHY IN THE #$*% DOESN'T THIS THING WORK??!!!"
Steve, KE4MOB
Note: forwarded message attached.
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