Suddenly, channel switching started to switch on its own, and this happens usually when I play at mid output or more, with guitar strumming... could be a faulty capacitor, or switching Jfets went bad? Any help will be grateful.
My first thought is the footswitch jack itself. If you are NOT using a footswitch, there is a cutout contact in the jack that completes a circuit through the panel switches. WITH a footswitch, it cuts those switches out and ignores them. If that cutout contact gets iffy or the solder cracks under the jack, you can get similar symptoms.
If you ARE using the FS, the jack still can cause these problems.
Without the FS, check the panel channel switch with an ohm meter.
The Fender one wire FS system puts an AC voltage on the FS jack, then various diodes and zeners switch in and out and then op amps decode the result to control the amp.
Isolate the problem. Random channel changes could mean the control circuit is directing the signal circuits to do it, OR the control circuit is OK, but the signal switching is screwed up. The question to answer is this: when the channel switches itself, do the indicator lights on the panel shift or stay steady? If they shift, that means it is in the control circuits - like my FS jack notion - and the signal switching is just following orders from it. If the lights stay where they should be, then you have a signal switching element at fault.
And another question: is it really actually changing channels? Or is it more a matter of the channel stays put but the gain makes major changes?
Well, first of all, thanks for these tips! The 16V zener zone on the PCB is a bit burnt, so, I'lll change them, along with filter caps (1000 uf for old 47u, why such a poor filter?). Channel switches but not completly, lights change too (from clean or red overdrive always to green) but, its satyats in the middle. Oh, now you've mentioned it... I think FS jack is damaged. I'm gonna solve this for sure!
Thanks again, you've showed me the way!
Well, in fact was simplier than the matter itself! Just one zener bad solder... hehehehehe