The quintessential, Marshall JCM800 from 1983. Still has the original power supply caps and they are visibly bulgey. However, AC at the B+ node is only 5V at idle and 19 at clipping — seems pretty average. Amp still performs normal but the caps are probably towards the end of their days. It is year 32 after all. Layout is very similar to Marshall JMP. This thing appears to be untouched other than new Xicon 1k/5W screen grid resistors.
Broken connection on the ground buss wire for the preamp volume pot.
Bias kept creeping up and up for about 20 minutes with this amp before finally stabilizing. Gotta let this thing run for a bit (like most amps) before biasing otherwise if biased “normal” it will eventually turn into hot/way hot. Marshall JCM800 service manual with schematic and much more available at the amparchives page — JCM800