Fender Super 210 Red Knob - Channel Switching Problem

[moegreen77]

Recently picked up a Fender Super 210 (Red knob, late 80's/early 90's). The clean channel (vol 1) sounds and operates fine but when switched to the drive channel (vol 2) the gain control has no effect, yet the clean volume and the drive volume both function at the same time. I does get loud but does does not produce much overdrive.

After studying the schematic and reading of similar problems in forums I decided to replace the three opto-isolators involved in the (rather complicated) switching circuit - no change. Also, I can see that someone has been in there and maybe done some repair or mod, so that might complicate things. Is it really even a switching problem? So, I was wondering if I post the schematic, would anyone have a suggestion for what to try next?
Thanks!
Fender Super112 Schematic.pdf

[Enzo]

The optocouplers turn on and off by lighting an LED inside, whwich shines on a photoresistor. Don't just replace them out of hope, test them. Use your volt meter to see if the LEDs are passing current or not when they should. You can also usually test the photocell part for changing resistance.