delayed signal input in tfm amps

I currently own 4 Carver tfm amplifiers. 3 of them have developed a problem over time where something is not switching on and the signal wont come in. Sometimes, playing around with the pots makes the sound kick in but it seems to be some kind of switch as opposed to just a burned out pot. One of the amps does not work at all anymore and the other two are getting worse.

it could just be dust, go to radio shack and get a can of air or "contack cleaner" and spray your inputs and outputs and anything else that looks dirty. I hope this info can hlep you I know what its like to lose a TFM amp