This may sound stupid, but I understand that if you engage the tuner switch, it mutes the signal for silent tuning. After all these technical suggestions, and as good a logic as they have, it might just be that the tuner switch is engaged. Just a thought...
Koy Carson
West Texas
Yes, check this out while you are at it. I bought a used Ampeg SVT-III that did the same thing. I plugged something into the effects return one day and then it started giving me problems. Turns out it was the jack not "closing" (Im no technical expert, so this might be a bad analogy) so it cut off the sound.