First, if the amp has an effects loop, plug a spare cord directly from send to return. If that helps, replace the return jack or service it. Likewise test if ther is a pair of jacks marked preamp out and power amp in.
If that doesn;t help. pull the board and resolder all the controls. And jacks.
Is there a headphones jack with a speaker cutout? Check that.
And if that doesn;t work, apply a signal, connect the chassis to a speaker and start poking with a chopstick, looking for anything that responds to motion.