:Help: Are these transistors interchangeable?

[tarnationsauce]

It's not desirable to mix different transistor models in differential pairs since that would degrade performance. It would be a good idea to replace the other KTA in case you haven't done it yet.

Thanks for mentioning that. But I already thought of that. I figured since they are all >$1 I'd replace them all and make the pairs matched. I even did the other channel so the channels would still be matched.

 

[grahamprie]

Zener Diodes?

Hi, I hope you've given the wrong part number - the 1N4148 is a small signal diode - NOT A ZENER.

I've just fixed a power amp with similar symptoms, but the problem turned out to be the output stage bias adjuster - it went open-circuit once it warmed up from the current flowing through it and turned all 8 output devices hard on.

Unfortunately, the power fuses are the wrong side of the smoothing caps, so the caps discharged through the output devices and usually at least half of them would go dead short and the resulting voltage on the output would trip the protection circuit.

I remember when the idea of using a vbe multiplier for bias was first around ( the late '60's ) and the advice was to put the pot in the voltage divider chain so the bias would turn off if it failed. Seems like someone forgot...

Graham.

 

[unclejed613]

ramping up DC? sounds like:
a) bad DC feedback capacitor (or DC servo)(after reading the text in the picture above, dc servo)
b) bad diff amp (LTP) transistor