I have one of these on my bench that I'm repairing:
Currently the only problem is a weak left channel when the "Warmth" knob is increased. When the "Warmth" control (VR200, bottom left of the schematic) for that channel is fully counterclockwise signal passes fine, however when it's brought up the channel goes almost completely dead, and the meter barely deflects.
I've traced the problem down to the circuitry associated with R4 and that tube section; with no signal applied the DC voltage across the anode resistors of all tubes sit at approximately half the 48V supply; however with the bad channel's "Warmth" control is brought up the average DC voltage across R4 collapses to around 200mV, while the others remain about the same.
In hopes of a quick fix I replaced the cathode bypass capacitor C38 and coupling capacitor C77, but no change in the symptoms. Tried several different sets of tubes and also no change. Suggestions?