Left channel hum - rack + Speedball

[jj4001]

Ok. And should I leave the small board flipped, or put it back to normal?

 

[Paul Birkeland]

That won't really matter for this test.

 

[jj4001]

Ok. And for the bigger board do I exchange positions of the white wires with each other? And what about the black and red wires?

 

[Paul Birkeland]

The white wires switch, the red and black need to stay where they are. 

 

[jj4001]

Thanks again for your help!  After this switch the hum is still in the left channel.

 

[Paul Birkeland]

I think you may need to take the small PC board out and put the 22.1K resistors back and see if the issue goes away.  If it doesn't, tack in the 3K resistors and see if the noise goes away.  If it doesn't then something was disturbed while installing the Speedball that was present in the underlying build. 

 

[jj4001]

Anything that was taken out is long gone unfortunately. Are there any areas or solder joints i should look carefully at if you had to take a guess on what the issue might be?

 

[jj4001]

One interesting thing I just noticed. While I had the amp upside down, when I applied pressure (pressing down) in any area of the main chassis between the large tube and the transformer the hum greatly reduced.

When I flipped it upright and pushed down in the same area it didn't have the same result. It was only when pushing down on the under side while it was upside down.

 

[Paul Birkeland]

Tighten all your screws before doing anything else.

 

[jj4001]

Oddly, all screws were very tight from previous troubleshooting. Last night I tried experimenting with the tightness of the transformer screws while listening and loose or tight it didn't change the hum.

 

[Paul Birkeland]

If you just touch the chassis plate, does the nature of the hum change?  Often pressure will make slightly loose hardware appear to be tighter (electrically at least), while just touching the chassis plate can mess with what appears to be ground.  The key two screws are the one on the front 5 lug terminal strip and the ground lug by the power entry module.  It's also pretty critical that the volume pot nut is relatively tight, as the volume pot body needs to be making good contact with the back of the chassis plate.