Hello, I am trying to figure out an issue with a new to me rumble 500..I bought it as is hoping it is something semi simple.  It fires up and thumps powerfully when shutting it down.  No sound from speakers...faint sound from external jack into another cab.  The effects loop seems ok as i can plug into that and get sound...low volume unless I put a pedal to it.  It seemed to get a little better after I plugged headphones in and out of it.....im leaning toward input jack but any help is greatly appreciated.

Jim