Curious hum in my Zenith 5k29 Amplifier (MK2608)

[bbs71364]

Hi there! Following up on my past thread here: https://audiokarma.org/forums/index...d-line-input-to-zenith-mk2068-console.975293/

My grandparent’s Zenith console is doing well! I ended up going with the switch between the Preamp output and 3.5mm connection for an AirPort Express as I spoke about and it works fantastic.

One thing that has bothered me for a few years since, but never got around to looking at it.. I seem to have a 60hz hum - but it appears to be directly tied to the volume pot, and it doesn’t actually move (get louder) as I increase the volume, and when I get to 10 (as high as it can go) the hum goes away. It’s also not there around 4-5. I’ve troubleshooted the entire signal chain thinking I was injecting noise in somewhere (happens when no source is connected) I’ve also connected a transformer isolator from my wall outlet thinking it might be my power, but I can’t seem to correct it externally. I’ve done everything except for wiring a ground to the chassis.

Here’s a video demonstrating the issue- it isn’t consistent through the volume range:

Any thoughts or ideas would be greatly appreciated!

 

[gadget73]

is the pot itself well connected to chassis? Sounds like the sort of hum that happens with open inputs, which is pretty typical. If the pot itself isn't grounded it picks up extra noise. Same if there is a missing or bad grid to ground resistor at the tube the volume pot feeds into.