I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

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.

http://imgur.com/a/JMosx

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?