Vox Berkeley II

[Tom Phillips]

I tried redoing the grounding with alligator clips. No change. Ended up reassembling the thing and all the hum went away. Guess the front panel of the chassis needed to be screwed on to maximize noise rejection. I had had it clipped to ground while working on it but that was allowing it to hum. Amp is VERY quiet now...

Also consider that there could be more than one independent ground connection to the front panel. That is, the board traces don't carry connect all the ground points when the board is removed from the front panel. If so, then you might have had a floating or high resistance ground which caused the hum.

[lowell]

Good call Tom.

[R.G.]

Also consider that there could be more than one independent ground connection to the front panel. That is, the board traces don't carry connect all the ground points when the board is removed from the front panel. If so, then you might have had a floating or high resistance ground which caused the hum.

Good point.
Thomas Organ used independent grounding for each section of the PCB, and did not interconnect all the grounds on the PCB. Rather, separate black wires went from each section to the main filter cap negative. It's a great way to minimize interaction, a form of lumped star grounding, but it's a real PITA to deal with if one of your grounding wires breaks.

It's been too long since I was inside a Cambridge/Berk II to remember the details of those two for me.

That was an issue I worried about on the repair boards. I did a clumped/star grounding system on the PCB, but it was complete on the PCB. All the wires to controls went from PCB to controls and did not rely on the chassis grounding for signal path return at all. It seems to have worked, as the very brave first adopters didn't run into hum issues with the panel off.