I removed Q303 and replaced with 6.8k and voltage on C404 rose to 28V. Replaced Q 305/310 with usual alternatives:2N3440 and 2N 4033 . Now voltage at C404 is at ps rail 84 V. Also had removed all other transistor in output. The amp is connected to a light bulb limiter which is not showing any excessive current draw. I'm stumped. I should be looking at complementary pa configuration with a single ended supply rail. It's as if Q305/314 are not connected?

The bases of the long tail pair are both at 10.5V. The voltages around Q310 are VB=10.1 VE=10.7 and VC=0 Q310 is off..

What does the zener do on the base of Q305 - shouldn't it keep Q305 at 4.1V? (1N4731A)

need sanity check here. The amp now shows 12.6 V on collector of Q 304 and 10.2V on Q304 emitter(assume Q304 okay by the 2v difference and it was replaced with drop_in alternative) . That explains why there is only 12v at C404 which should be VCC/2. But why? Base of Q305 is sitting at 12.6v though DMM doesn't show anything wrong with Q305. Before I remove Q305 what am I missing?

I get 180mv across R 341 the voltage across R340/339 is 0v though. CR310 is two diodes in series both check out okay. All other diodes are good. All low value resistors all test good. I have been testing the amp with no load. frustrating....

I'm assuming I should see the undistorted signal at the collector of Q301 (long tail pair) - even if the feedback loop was having issues?

I removed the JFET (Q315) and input limiting diodes (CR301-4) and now have a good signal to the base of Q301. Unfortunately, the output waveform is still distorted. All caps were previously replaced and I've checked out resistors in signal path. I'm missing something???

Thanks for the explanation. It looks like there was a SPST switch in the limiter circuit but this amp has it depopulated. It looks like the switch was there to pull the base of Q316 to ground? Still wonder why the collector voltage on Q316 is so low and there isn't a .6V difference between VC and the voltage at node R333/4? Looks like no current through R330 so FET stays near IDSS (i.e. on) ??

I'm looking at removing the JFET and seeing if that isolates my problem.

I usually use the same MPN for replacement of semiconductors and did so with both Q315/6.

What does Q316 do? I noticed that the schematic shows about a .6V difference between the collector and node between R333/334. This amp has the collector of Q316 at 8.55V and the voltage at node R333/4 is 8.48V? The base of Q316 is .259V. The resistors in the limiter appear to test okay. More head scratching..

The waveform shown is on the line input side of R301 if that makes sense. The waveform on the other side of R301 is almost a square wave (if I remember). How does the limiter work on this amp?

I'm stumped why the output of this amp is limiting or at least I think that's the case.
DC tests:
The bases of Q301,302 are both at 12v and Q304, the bias transistor, has VC=48.3 and VE=46.1. The limiter circuit has the collector of Q316 at 8.55V and the voltage at node R333/4 is 8.48V? The base of Q316 is .259V
I replaced Q315,Q316 and Q303 and the tantalums.
CR301-304 are neither open or short.
Attached waveforms are voltage at R301 (sinewave) and the voltage waveform at the base of Q310

What am I missing?

I noticed with a signal generator (with chassis ground connection) plugged into the input the noise drops 20dB but is still noticeable at idle. I plugged the sig gen into the amplifier input and I get a good clean output with no noise at all. The problem is in the preamp. Not sure if this is another issue but the amp was in pieces when I received it and I had to wire up the preamp board connections from some images someone sent me of their amp. If anyone has any experience with this model I sure could another pair of eyes or brain cells.

Need some help understanding the grounding strategy with this design.
Input/output jack and volume jack for footswitch are all isolated from ground with fiber washers. Power amp input and preamp out are NOT isolated.

Power supply consists of center tapped secondary with a bridge rectifier and two 4700uF off of center tap which is physically connected to chassis (ground).

Attached preamp schematic (not complete?) doesn't show how the rails are filtered or decoupled.

The amp passes a signal just fine but there is a very loud 120 Hz buss present. So power supply ripple (chassis ground) is mixing into audio path but where?

Amp has a 120 Hz buzz - power amp seems to be clean so it looks like the preamp section is the issue. Also not sure this is the correct schematic for the preamp.

Opps missed the -48V rail thanks for the help as usual.

Okay just noticed that L1 and R25 and the rest of the Zobel(?) network and there must be a misprint on the schematic since there should be a fairly large capacitor in the output. Still not sure about the isolated output jacks? Why does the ext. jack have small hole in chassis and no fiber washer???