Lot's of Great help here. I have been doing a lot of reading and YouTube to solve this. What you folks are asking is related to some of that. Let me address your questions as best I can.
You should be careful adjusting the bias setting on the pot.
If you get it wrong, you will saturate the drivers Q17/Q20 and then rapidly the output stage Q212/Q22.
Before you even notice then, the weakest end of the pairs will burn out in a rapid thermal run-away.
I suspect this is what happened when I tried to adjust the bias the first time. The Positive side (Q17 base) was very close but the negative side (Q20 Base) was way off (1v). After I replaced 21/22 AGAIN they just blew right away, even with the pot all the way CCW like the drawing instructs.
I suspect that you have a problem in the bias stage Q3/Q18
Me Too. I found Q3 to be bad but Q18 I had no replacement for at the time. I never tested it but I have a replacement coming now anyways. I got adjustablity at Q17/20 back after replacing Q3.
Also if capacitor C12 is a dried out electrolytic one, it will no longer dampen rapid changes during adjustment.
Had replaced all caps with brand new ones just in case.
Notice the diagram voltages written at Q8/Q19 emitters.
These voltages should be spot on and replicated in the high end emitters Q1/Q16.
Will do. I am not 100% clear on the purpose of these transistors except that they seem to be somehow related to the feedback loop?
If these voltages are not spot on, you should investigate the differential amplifier Q2
I have recently become very suspicious of this section and doing a lot of research on "Long Tail Pairs". I actually only half understood it before but I think I have the whole Idea now. It's a phase inverter also and not just a diff amp. Seems like I have had this go bad in a few amps now. One was spitting out DC on the output jack. I did not check Q2 originally because it was a hard to get 2 in 1 package but I have GK recommended substitutes in the mail now.
have you done a basic check on the power supply ?
I checked the plus and minus 60 but I forgot the + and -15. Embarrassing. Will check it after all the transistors get replaced. I already paid for them. Going to put them in.
You can't just remove them in THIS amp. Then the whole amp has no DC feedback anymore.
You could remove them, and temp use a ~100-220ohm resistor between B and E.
I realized about the feedback issue after a while. Good tip about the resistors.
Just short the whole bias circuit. e.g with a jumper/solder blob over C12.
That way you can rule out the bias circuit while fixing the amp.
Without bias, the amp just has a lot of crossover distortion.
Are you sure this is ok??
Did you use branded transistors. There seem to be a lot of "fakes" around.
I got the first batch from Digikey. I'm getting this batch right from GK themselves.
Does the amp also blow without a load.
Yes
It also blew with an 8 ohm speaker but not for a much longer time and I think that time I turned it up to high for too long.
Thank You All.