diyAudio - Nakamichi CA-5 hum/buzz and does anyone have the schematics/service manual?

Hello everyone,

My Nak preamp has had a hum that goes up with volume control for quite some time. Frequency seems to be at 60Hz.

I checked the ±24V rails and no superimposed AC is left on them, nice flat DC, so I ruled out the smoothing caps as a problem.

What ticked me off was a couple of actual grounding issues that I fixed, but that did NOT fix my hum.

I found out that if I ground the selected input pin, the hum goes away. I check with my scope and my input pins seem to pick up a light 60Hz. I guess I could check for cracks and such in pcb traces as all components seem well connected and in good shape.

Any suggestions of where to look?

I always manage to dig up a copy of schematics or service manual of most gear I own or work on, but this one, nothing! I'd really appreciate if someone with a copy could help me out

Thanks in advance!

IG