Hum - Marshall Valvestate VS265R

[Fast Edward]

Not filter caps. I'm having problems repairing the above. Hum in power amp, with volume down. Filter caps have been changed. I'm confused about the 2 diodes, cap and 10 ohm resister between chassis ground and power supply ground. I read 1/2 volt AC between chassis and power supply ground and feel this could be related. Any help would be appreciated.

[Enzo]

Those help isolate the mains ground from the circuit ground.

Hum in BOTH power amps?

Do ANY of the controls have ANY effect on the hum? That is do the tone controls make any difference in how the hum sounds?

Pull the tube - does the hum continue?

Plug a shorting plug into the FX return jack. Does this affect the hum at all?

[Fast Edward]

Thank you very much for your help. The hum is present with volume all the way down, tube, effect loop or tone controls do not affect it.
The unit came in with excessive hum, found one filter unsoldered. After resolder the hum was reduced greatly but still present. I figure it should be dead quiet. I replaced the filters and hum still present.

[Enzo]

Both speakers.

Have you checked the ripple level on both powr amp rails AND both 15v rails? Are both 15v rails setting at their proper voltage? Neither one is real low or real high?

[Fast Edward]

Thanks for your feedback.
My + 40 has 139mv, -40, 103 mv
That's not enough to cause hum is it?

The preamp board is slightly different than the schematic I have.
I have a + - 12v supply.
C19, 20 on my schematic is C 31, 32 on this board.

Yes, hum is in both channels.

Thanks again, Ed