troubleshooting the circuit

[Grainger49]

Bugeyes,

Check that the ground on the bad side is good.  Just for yucks give it some heat and solder.  Can't hurt might help.

[JC]

I, too, would be suspicious of the ground for that section of your VC.  As I recall, the Volume Control is grounded at the center tags of the the two terminal strips just to the 'north' of it on the plate, with the ground connection for each section of the control going to its own terminal strip.

In turn, that point on each terminal strip is "grounded" by the ground bus wire on its side passing through the lower hole of the terminal.  I would check each end of that ground connection carefully on the channel you are having trouble with.  I can certainly see potential for a cold solder joint where the ground bus wire passes through the lower hole of the terminal strip.

[bugeyes]

Thanks for all the suggestions, I'm going to try them all. This may be a naive question, but would it be fair
to assume that the problem should be in the front end somewhere between input and the number 4 terminal on the tube socket?

[Grainger49]

If it is related to the volume control function it should be before the first tube stage.

[JC]

bugeyes, yes, that is the working theory, anyway.  Pin 4 on each tube is that channel's Voltage amplifier input, and the Volume Control simply attenuates the signal getting to Pin 4.  At least, that is what the VC is supposed to be doing!

Give those grounds a serious inspection; I cannot think of a likely mode of failure in a step attenuator that could cause the issue you describe without there being a problem with the ground somewhere.

So, you are going to want to check for continuity from the "ground" terminal of the appropriate VC section to the middle terminal of the nearby terminal strip, for starters; then, you'll want to make sure that middle terminal strip terminal has a good connection to the ground buss wire that runs through its bottom hole.