Hey, that’s a nice amp !
Master model lead 100W Mk2
Filter caps to be replaced (this amp dated 1977), and parasitic noise on the Hi input …
Parasites were due to bad anode resistors value : they are marked 100k (as in the schematic, see below), but when measured, Ra for the first triode (Hi input) was > 1M and Ra for the triode corresponding to the Lo input (or second triode if you use the Hi input) was 150k. Once replaced with fresh ones (100k), no longer parasites !
100k anode replaced
Recapped !
Schematic JMP : 
Preamp
Amp and PSU
JCM 800 2203 : quite the same as the JMP 2203, and better readable !
IT11 Audio Web Site
I have the exact same amp, it’s getting an oscillation when the preamp control is above 7,
even with the master at 0. [with the guitar going straight into the amp]
When I connect my pedal board, It doesn’t oscillate and I can turn the preamp up all the way without a problem.
Any suggestions would be appreciated.
Oscillation is pain in the a* to debug (as it is for hum and other noise) …
Does it oscillate with nothing connected in the input ? (You use Hi input, right ?)
Does it oscillate with an other guitar connected ?
It will happen on the high input even with just a guitar cord plugged in. [again master volume can even be at zero]
symptom the same with other guitars tested. haven’t checked the 100k anode resistors mentioned above.
will not osc. when nothing is connected to the input
Maybe you could try C2 (100pF) see JCM800 schematic ?
Kind of funny that all Marshall did to make the JMP model you have is take the exact same PCB from the 1977 model 1959 MK2 Super Lead and Super Bass and just do the one wire mod that was already being done on them. The guy that made this video sent me a pic of the inside of the modded 1977 Super Bass he used and it looks exactly like your JMP.
Sorry, here’s the video:
https://www.youtube.com/watch?v=dSqGwEgEUgg
Thank you so much. I had the same amp with the same problem. You saved me from taking it to an expensive tech. My amp sounds amazing again, thank to this article.