Ampeg VT-120 channel-switching fade-in, ways to address?

[jamesmafyew]

I've been rehabbing an Ampeg VT-120 combo. It was awful, had a botched filter cap replacement job, poor soldering everywhere, weird noises, microphonic tubes, the works. I've got it cleaned up, re-did every solder joint in the amp, new tubes, new proper filter caps, replaced intermittent power supply resistors... probably spent 12 hours in there.

Now that it's up and running, one thing is bugging me -- all the channel switching is seamless EXCEPT going from either drive channel back to the clean channel. When I do this, the volume drops out and then fades up over about 2 seconds. I'm trying to understand from the preamp schematic why this is happening. The only thing I can figure is that Q6 is some kind of muting FET. I don't see any reason for it to be there except to either lower gain for one channel, or as a muting piece. It's also the only FET in the amp with a cap from gate to source. The gate resistor, R40, connects to the collector of Q18 in the switching circuit.

I'm trying to figure out if Q18 is responsible for the fade-in, without removing the board from the amp again and getting experimental. If I can understand the circumstances of Q18 shunting the gate voltage of Q6 -- a momentary thing on a channel switch? Or a steady-state thing that changes the gain structure for a particular channel?

ALSO: Does anyone have the service bulletin on these amps about re-routing certain PCB traces that can cause arcs/fires/other problems?

[jamesmafyew]

Just to follow up on this -- Q6 lowers gain for the clean channel. That's it. I found a solder bridge in the switching circuit -- fixing that bridge helped with the fade-in, but it's still there to some degree. Still looking for other possibilities. Probably it's a slow optocoupler (OC-1).