'82 JCM 800 4104 Service and Mods

[physics]

Hmm.... Sufficient monkeying with the input cable seems to cause the same issues.... It seems like messing with the guitar-end gets the most results. I got some results while monkeying with the end plugged into the amp (pulling in and out repeatedly, moving around, etc.), but I'm not sure if I was shaking the guitar's connection, which was on the table at the same time whilst I was forcefully inserting and removing the cable. I have to move on to getting other stuff done today, but it seems like the search zone for next time is from the guitar up to the high-sensitivity input jack.