I own a MARSHALL JCM800 LEAD 50W 4210 (1X12", 2 CHANNELS, REVERB), with many years of use, I bought it new about 36-37 years ago!!!, and now the reverberation has started to fail, if I go up the REVERB potentiometer makes a lot of noise and when it goes past 1/4 of a turn feedback appears, I don't know if the problem is electronic, or if the reverberation module has broken, I think it has an ACCUTRONIX or a HAMOND (I haven't disassembled it) Can the module be repaired or does it have to be changed directly? What brand and models is it? Where can I get one? If the same model is no longer manufactured, what would be the equivalent model?
It could be an electronic issue, but the chances are the reverb tank is faulty. It's easy to check. If you are comfortable with doing the following then remove the reverb tank and check the continuity of the input connector (between the signal and the shield), there should be some resistance reading but not open circuit. Then do the same check on the output connector, likewise it should have a resistance. If either end is open circuit the tank is faulty and needs to be replaced.
There will be a code written on the reverb unit, something like 8BC2D1B this code defines all the parameters of the reverb unit, as long as you replace it with a reverb unit with the same code it will be fine. Just make sure the input and output connectors are the same as the old one. They are not particularly expensive and readily available.
Hope this helps