Noisy Fender Solid State Amp

Hello, I have a Fender Deluxe 90 solid state amp from the early 90's I believe. This amp works satisfactorily for what it is except for one big problem. I get a hissing crackling sound that starts off fairly quietly but over the course of about 15 seconds it rises to an intolerable crescendo to the point of ear shattering volume, whereby I switch it off. I have found
that hitting the amp hard on the top right hand side (as its facing me) stops it for a while until the next time. It happens constantly. It happens when no cord plugged in which eliminates any cord, pedals or guitar issues. It happens with the on board effects turned on or off. I happens on either channel with knobs all the way up or down and finally also does it at other locations ruling out household power issues. Taking the amp in to repair shop exceeds the worth of the amp. Any ideas folks such as capacitors or?