Marshall JCM 800 50W Split channel rev. Combo sounds like crap!!

[Steelwitch]

In general it’s been hard to diagnose, there seems to be things done to the amp from some previous repairs, including one of these 330uf caps mounted under the board. There are some lifted solder pads that I reflowed the connection for after scraping away to get the bare trace to solder to

[Helmholtz]

0 ohms

O.K. I assume you measured resistance between chassis and your wall outlet safety ground (the mains cable's ground wire could have a bad contact which would be a safety issue). Your video seems to indicate a difference in potential between the amp chassis and the scope's ground.

I suggest the following: Cover a cardboard large enough to cover the complete chassis with aluminum foil and use it as as shielding bottom for the chassis. Make sure that the foil has good electrical contact to the chassis. Noise gone?

[Steelwitch]

Ah sorry for bumping the thread y’all... it was a bad ground on the reverb transformer of all things. Just needed some flow— everything’s cool with this amp now... for now.

[Steelwitch]

It seems like the grounding for the reverb runs directly to the preamp ground on the board, so no wonder I was getting all this wonky grounding nonsense