Marantz 1180DC bias issue? One channel sounds slightly muddier

[Raccoon1400]

I restored/recapped one of these amps for a client. It came back with him saying one channel didn't quite sound as clear as the other. I can barely even hear the difference but I think I do. We both listened to it and we think we narrowed it down to a power amp issue.

This model doesn't have a bias adjustement. Or even a procedure for checking bias. I measured across an emitter resistance (0.33ohm) and got 3mV for the good channel and 0mV for the lesser channel. Could this be the issue? What can I do about it? Anything else I should check?

I don't notice anything when I do power tests but I don't have fancy equipment, just an old analog scope. I tested it to full power with a 1kHz sine wave and I tried some 10kHz squarewaves as well.

 

[huggygood]

You have all the voltages displayed on the schematic , you only have to compare the good side with the bad side.

 

[Raccoon1400]

How does the bias circuit work in this design? Am I likely to find something off enough to be measurable? Since I didn't find any other measurable issues when doing power tests.

 

[BSST]

The bias spreader is analogous to the Vbe spreader you've seen before, but thermistors Q746 Q748 contribute to the temperature dependance of the spreader voltage. It features mirror image symmetry but the principle is the same. It doesn't seem to have any trimmer. It should present similar voltages in the other channel.

Judging from +/- 0.6V at the emitters of Q725 and Q727, I surmise there must be off-board power transistors. You should probably post details.

 

[Raccoon1400]

@BSST Here's the clipping showing the output transistors.
There's no trimmer. Another thread said the predriver circuit is precise enough they didn't need one. I'm not sure how that works though.