Gallien Krueger 400RB Hum

[g1]

That's great you got rid of the hum.

All chassis panels and lids and all surfaces I cleaned with sand paper, sprayed with contact cleaner and tirelessly put together again without much of an effect.

Did that include cleaning all the star washers and panel surfaces where jacks and pots meet chassis?

[ddt]

So far no luck. Ideas? Enzo and others?

Tried these just now:
1. Connecting the ground lugs of the front input jack with the ground lug of the speaker jack at the rear with wire. No change.
2. Cleaned the back of the chassis where the ground connection from the poweramp board is mounted to. No change.

Everywhere I measure, the resistance is zero Ohms.

The only significant improvement is when the power amp board is connected with preamp board directly as in my post #14, but that introduces a ground loop, correct? And the amp is unstable, feedbacking.

[ddt]

That's great you got rid of the hum.


Did that include cleaning all the star washers and panel surfaces where jacks and pots meet chassis?

Yes, exactly, all of them.

[g1]

Any pictures of the guts and approximate age of the amp would be helpful.

[ddt]

The thick black wire from the poweramp board connects to the chassis at the back.
Amp is from 1986.

[ddt]

Regarding the DC voltages, they seem to be alright across all op-amps. Roughly +-14.5 V against the GND. Output offsets are in the order of mV.
I think I'll start tracing the hum with lifting the cap leads along the signal path starting from the master volume and going backwards.