Assymetrical correction (?) waveform

[g1]

I haven't paid to much attention to feedback waveforms before, but I assume that is what is happening here. But I would expect it to be the same on top and bottom.
The non-symmetrical waveform appears at the output pin of the op-amp, IC101.
The output of the amp is a pretty good looking sinewave (second photo).
I believe the power amp part should have more gain as the limiter is kicking in at about half power, and gets it's signal off the op-amp.
What I'm wondering is what could be causing the flattening of the lower half of the waveform. It has that same shape for all output levels.
Power amp schematic is on 2nd last page of attached service manual.

[Chuck H]

Without having it on the bench it's hard to give a confident answer, but...

The waveform in the beginning of the feedback loop is likely distorted to some degree. The feedback waveform would be opposite and analogous to that. The amps actual output is "corrected" by the interaction between them. This is the principal of correcting distortion with feedback.

[The Dude]

What does the signal look like at the input to the op amp? Is there any DC on the amp out?

[loudthud]

It would seem like the gain of the discrete transistors in the power amp is different on plus and minus sides. Looks like the output is in phase with the opamp output. Since the waveform looks bigger on the plus side, that would be a fault that lowers the gain on the plus side, Q103, Q105 or Q107 and the associated resistors.

[Helmholtz]

Yes, looks like a problem in the forward path of the power amp and the opamp is trying to correct it.

[g1]

What does the signal look like at the input to the op amp? Is there any DC on the amp out?

No substantial DC at the output. From what I could tell, the signal at input of op-amp was fine, and the distorted waveform was only at the output pin. But it was quite small at the input so I will try it at full tilt and see if there is anything noticeable at input pin. It has to be coming from somewhere.

[g1]

The waveform in the beginning of the feedback loop is likely distorted to some degree. The feedback waveform would be opposite and analogous to that.

Not sure what you mean by 'beginning' here. R104 connects the amp output to the input pin of the op-amp. The 'good' image (second) is the amp output.

[Helmholtz]

No substantial DC at the output.

The opamp provides lots of DC NFB.
So within its limits it keeps the output at 0V.

[g1]

Another question, Q101 is mounted on the heatsink. Is it just for thermal compensation? (schematic is unclear on the labeling, Q101 is to the left, Q102 shown to the right is for standby)

[g1]

The opamp provides lots of DC NFB.
So within its limits it keeps the output at 0V.

This is as I thought. Is there something I can do with the feedback resistor R104 to check for DC issue?
I didn't mention how difficult this thing is to work on. Tough to reach anything and the flimsiest traces I've ever seen. Pushing a component leg with the probe can crack the trace.
Any shortcuts I can use are very useful.
The outputs and drivers have been replaced (Q105 to Q108).

[Helmholtz]

What are opamp input and output DC voltages?

It looks as if the standby mode shorts together the bases of Q103 and Q104.
What is the amp output DCV in standby (no load) ?

What makes things tricky here is that the amp has an inner and an outer NFB loop (the outer one includes the opamp).

Did you check transistor base-to-emitter voltages?

[g1]

The DC on the op amp is 0V at inputs, and -50mV at the output.
I pulled the board again to inspect the traces and my jumpers. In testing the caps that go across the driver emitter resistors, I found C129 to be in the nF range with around 80R ESR. C130 was better, but only around 80uF.
I hope I have some, or at least anything close to test with.

[Chuck H]

Not sure what you mean by 'beginning' here. R104 connects the amp output to the input pin of the op-amp. The 'good' image (second) is the amp output.

Sorry for that. The beginning of any distortion within the loop would be at the output of the first amplifier in the loop. In this case that would be the output of U101 at the junction of the two 3.3k resistors. I'll bet the waveform there doesn't look like a typical sine wave. Though it's possible (probable?) the signal at the input of that stage looks better. I know this is a low tech observation but I'm not an engineer. I have learned, however, not to expect "normal" looking waveforms within feedback loops. I'm sure this doesn't help

EDIT: Sorry gain. The above answer is also incomplete for what I intended to clarify. What I'm saying is that abnormal waveforms within a feedback loop are what I expect though I don't always know how to interpret them. Since you have a good looking sine wave on the output side of the loop it's possible that whatever problem you're having isn't because of the loop function. That is to say possible it isn't. I honestly don't have the chops to say one way or the other. But if this helps to keep from chasing a red herring I wanted to mention it.

[g1]

The beginning of any distortion within the loop would be at the output of the first amplifier in the loop. In this case that would be the output of U101 at the junction of the two 3.3k resistors. I'll bet the waveform there doesn't look like a typical sine wave.

That is where the first image is taken from.

So replacing those caps (C129, C130) got the gain back in order and the amp is now getting to full power when the limiter kicks in. Sinewave looks normal at speaker out.
The waveform at the U101 output is much less lopsided now though there is still stuff going on which I think is normal. I'm going to call it done unless someone thinks these new waveforms are problematic. They are at U101 output, first a lower power then at higher output levels.
(P.S., sorry about the image quality, and thanks for everyone's input)

[Chuck H]

If you've followed any of my design ideas in our long history you know that I've used "virtual earth" mix stages for reverb circuits. You wouldn't believe how strange waveforms within the loop look for this circuit. But all good on the other end In your case it seems there must have been something going on for a stage within the loop. You found it quick. I might have missed that for a frustrating amount of time.