Marshall reverb 12 model 5205

[Diabolical Artificer]

Switching an amp on and off isn't a good idea, at least not without discharging caps. First thing I'd do is have the board out and check with a magnifying glass all solder joints, while your at it whip the big caps off and test. After that power it up with a bench PSU, current limited and check voltages on all op amps to see if they have power and are getting hot. If not bench PSU solder a resistor across the fuse, all initial tests at low volume/gain until it's running ok.

Andy.