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I am deaf in one ear and cannot locate the direction from which a sound
originates so I was relying on the balance knob and not the speakers, to
tell me that the left channel was dropping out, even when I switched the
preamp right and left outputs into the power amp inputs. That was an
entirely erroneous episode of troubleshooting. The balance knob
continued to to tell me that the left channel was dropping out, but in
reality the output at the speaker jacks was switching from the left to
the right channel. I only discovered this when measuring the AC voltage
at various points in the power amp wiring. I was puzzled at the readings
I was getting and went to the speaker jacks where the speaker wires were
connected.

Long story short, the dead channel at the speaker jacks switched from
left to right when I switched the preamp outputs going to the power amp
inputs.

I have to put this aside for a minute and re-read all the relies I have
gotten dealing with possible cause on the preamp side of the unit.

My apologies to all who followed me down the wrong path with this unit.

Time to start fresh... mañana.

Jack