RG50TC stuck in clean channel

[bondmatt]

I've got an RG50TC that switched itself from the overdrive channel(s) to clean while playing. It seems to behave normally while in the clean channel. When I try to switch to the overdrive channel, both the overdrive and clean channel LEDs are lit but neither Gain 1/2 LED is on. There is no change in tone. Other than the LEDs, it stays in the clean channel. Tried the footswitch and the pushbutton on the front panel, both without effect.

Schematic is here:
http://www.echostar.pl/~zelman/RG50TCSchemat.pdf

Any ideas would be very much appreciated.

I tried to test Q16 with the power off. I shorted the drain to source to eliminate any stored charge. The source to drain resistance (short removed - at no point was it shorted with power applied) is ~ 200 Ohms. Gate to drain and gate to source resistance is 1000k Ohms regardless of polarity. I'm not familiar with FETs but a quick search online seems to indicate the 1000k Ohms readings are only correct for one Ohmeter polarity. Checking Q16 is a shot in the dark after reading about a somewhat similar issue.

 

[bondmatt]

I think my FET test was not right. Haven't revisited it yet.

A little more info: reverb seems not to be working. Wish I'd tried it before disassembly. Can't be sure if I did that or if it is related. I hadn't used this amp in a very long time, can't say I know reverb was working before channel issue...

Disconnected the internal wiring harness (inside the amp) for the channel switch on the footpedal. Stays in clean channel, no LED or other noticeable changes when front panel switch is toggled.

Reconnected and previous symptoms reappear. Noted that clean channel LED dims (hardly noticeable) when I try to switch channels and there is a slight buzzing noise.