The fact that no knob tweakage was altering the sound indicates for 99% sure that the preamp is not at fault.
So that leaves us with anything after the master volume, starting with the effect send connector, which turns off the internal bypass when something is plugged in. Hence the jumper test.
The mp3 test would be a good idea also, because it would indicate if the problem lies in anything after the return effect plug (but as the amp now works fine, it is difficult to test that until the next occurence of your problem).
Was this amp stored and not played for a long period of time?
Because even if I may not be able to explain exactly why, but I suspect dry dust to be a electric isolation material for weak current, whereas it might be burnt and turned conductive (more or less permanently) by a stronger signal, which is consistant whith what I observed on amps that where not played for a long time and with your symptoms.
So what are your options now?
Play this amp often, and when you rehearse with your band, always bring with you a spare patch cord to shortcut the effect loop if this happens again. Anyway some carefull visual inspection might be a good idea but I fear the faulty contact is tiny enough for not being noticed this way. Maybe ohm testing every connexion past the master volume knob would help, but I'm not even sure you'll find anything and that can prove an harassment to do so.
So that leaves us with anything after the master volume, starting with the effect send connector, which turns off the internal bypass when something is plugged in. Hence the jumper test.
The mp3 test would be a good idea also, because it would indicate if the problem lies in anything after the return effect plug (but as the amp now works fine, it is difficult to test that until the next occurence of your problem).
Was this amp stored and not played for a long period of time?
Because even if I may not be able to explain exactly why, but I suspect dry dust to be a electric isolation material for weak current, whereas it might be burnt and turned conductive (more or less permanently) by a stronger signal, which is consistant whith what I observed on amps that where not played for a long time and with your symptoms.
So what are your options now?
Play this amp often, and when you rehearse with your band, always bring with you a spare patch cord to shortcut the effect loop if this happens again. Anyway some carefull visual inspection might be a good idea but I fear the faulty contact is tiny enough for not being noticed this way. Maybe ohm testing every connexion past the master volume knob would help, but I'm not even sure you'll find anything and that can prove an harassment to do so.