Marshall JCM800 2203KK Hum issue

[SoulFetish]

I just worked one one of these today with noise issues. Turned out to be a faulty 100k plate resistors. Check out R126, which is the 100k plate resistor for V105a. and also the 100k cathode follower load resistor R142 (Which is usually connected across the 2 triodes right at the tube socket)

[ct88]

yep already tried that but had no luck (also replaced ceramic caps with quality micas)

Next thing i'm going to try is to replace those sh***y ribbon cables, maybe using all quality shielded cables...
Btw, why are the shielding on the ribbons grounded on both sides?? Isn't that a bad thing for ground loops??

Other than that, i doublechecked almost everything! starting to think that the background noise is part of that pcb design...

[Enzo]

Sometimes the shield carries ground from point to point. Also sometimes on a pc board, there will be traces that go nowhere and are there to act as shields between two other traces. A shield ground could end at one of those.

As to ground loops, the context always matters.

[ct88]

I'm aware of that, thank you for pointing out
just worried about grounds and ground loops Other than that, everything else looks healthy

[cartman_gb]

Hi! issue solved?

I'm aware of that, thank you for pointing out
just worried about grounds and ground loops Other than that, everything else looks healthy