Pioneer SA-6500 channel distorted

[tdlunsfo]

It feels like this issue is staring me in the face, but for the life of me I can't figure this out. Around 3-5 mins after power up, one of the channels begins to distort. All the transistors, diodes, and resistors check good. One major clue is there is no bias regulation on the bad channel. The collector of the VAS on the good channel (Q11) never makes it past 1.2vdc and the bad channel (Q12) it will go above 10vdc. I have tried swapping the VAS, a driver and a diff transistor but no change. Any ideas or tricks to find the circuit imbalance? Note that it hasn't been recapped but the majority of the caps I have tested for ESR have passed.

[The Dude]

It seems likely heat related. I'd try some freeze spray and see if you can find a component that's failing when it warms up.

[tdlunsfo]

Looks like I got it. I swapped both output transistors and the problem followed, although they test fine out of circuit they are not too evenly matched on HFE. I'm going to chalk this one up to wounded chip syndrome.