It was just a fuse. When I bought this amp, the guy told me that it had a problem *one time where the internal fuse blew when someone plugged into the amp while it was on. It never gave me any problems and I made sure to plug in before turning it on just in case, but one day it stopped passing signal so I assumed that I must have triggered it. Tonight I took it apart, deoxed the pots, and put in a new fuse — plugging and unplugging in an attempt to confirm/deny the issue and I couldn’t make it happen. If it happens again I’ll dig deeper, in the meantime it’s a sweet little amp. (I’m a sucker for old Yamaha amps.)