I might have found the cause of the problem. With the output transistors
removed, I was measuring a bias voltage of +/- 64V on their bases -
something obviously wasn't right. Putting a 1k resistor across the B-E of
the output transistor in the good stage made this drop to 0, but doing the
same on the bad stage instantly activated the protection mode. The bias
voltage problem seemed to go quite far back into the circuit. After some
very laborous voltage tracing, I discovered two transistors at the very
start of the preamp stage (one each for inverting and non-inverting halves
of that channel) with a small DC voltage - about 80mV, on their bases - the
manual said there should be none. Checking the same two transistors on the
good channel, I found they had about 60mV on their bases, again they should
have read zero. The only possible source for this small DC voltage was
either through the collectors or emitters of the transistors themselves.
With these two transistors removed, the bias voltage on both these preamp
transistors and all following transistors in the circuit returned to normal,
dead on to the manuals specs. The transistors in question test good with a
diode check function of a DMM, but at this point I'd have to say they are
leaky and are most likely the cause of the whole problem.
I still don't have 100% faith in my diagnosis, but that's the best theory
I can come up with. I'll replace all transistors in question and see if that
works, if not this thing goes in the junkpile. I normally wouldn't have gone
to this much trouble for an item worth $100 at most, but I'm a very stubborn
person and I often don't quit when I should.

"Chris F." wrote in message
...
Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them,
one channel worked but the other blew right out again. Having spent $45 on
this amp so far including a service manual, I don't want to gamble any
more money on it. So I was wondering if I can replace those transistors
with some non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.
Thanks for any advice.

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