Transistor Substitution Question

darlington

The stability issue will relate to the fT of the transistors.

Graham

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 news:i9Gbh.26927$ snipped-for-privacy@ursa-nb00s0.nbnet.nb.ca...

Aha ! See my reply above about problems back in the preamps of DC coupled amps ...

Arfa