We often get 70’s Marshall JMP amps in for repair; these are excellent rock amps and are still very good value for money.
One the major issues with these amps are the voltage and impedance selectors which are prone to falling out.
Someone had done a really good job of wiring the mains selector in this amp!
Not only is this a total hazard, the amp is actually wired to the wrong voltage! Suffice to say I hard wired the amp to 240 V, and disconnected the unused voltage taps from the voltage selector.
The input jacks also had the wrong value resistor soldered to them; the resistor in the photo is 4k7, but should be 470k. It looks very much like a mistake at the factory.
Additionally one piece preventative maintenance I always do with this era of Marshall amps is to change the caps in the bias supply. If these go leaky then the amp can go into thermal runaway.