Arrrrggghhhh!!!! This is driving me up the wall!
I've got a Crest CA9 here which I've been working on for ages now, probably well over a year. It came in needing a complete rebuild of one power amp stage, which I did, then it decided it didn't like the new power transistors, blew most of them again and I had to rebuild it for a second time! It had various other faults since like burnt out relays which took me a while to find too.
Anyway, it's now basically up and running but I can't quite get it to bias properly. There is a slight imbalance between the NPN and PNP sides such that one side tries to bias on harder than the other. Which means I get crossover distortion but only on the positive going side of the amp (I think it was positive, one or the other). There is no DC offset.
I've been right through the driver and bias stage, checked every resistor value and even tried swapping driver transistors round, nothing makes any difference, there is still this imbalance of 0.3V in the bias chain which won't go away!
I'm sure it's something silly but I sure as hell can't find it!
Thought I'd found a dodgy driver transistor earlier and got all excited. Replaced it but that wasn't the fault.....
After the time and money I've spent on this amp, I'd really like to fix this final fault and get it out of my workshop and into an amprack!
