See
this recent thread. Had to fix one of these with very low output on one channel.
Incidentally I m now working on a somewhat bruised series 15 and the treble pot has had a knock, fortunately the wiper, track pcb, knob, thrust washer and shaft are there. Just the silver rear plate is missing so it falls to bits! I mention this as you may be able to reconstruct yours if all the bits are inside the machine.