Ok, thanks so much! I did some further investigation and found
https://quadrevisie.eu/prijzen.html. Armand van Ommeren. In the Netherlands. Use Google translate to translate the pages. He specializes in repairing all Quads. I queried him, in English, via email and he was very helpful, sending two solutions and pictures that I could forward to my repair guy here in the Seattle area. Here is the text of his email:
"My solution is to drill a hole in the frame between the volume control
and the tilt and fit a rotating potmeter of 1K linear stereo in that hole
and lead some wires to the potmeter from the PCB. See picture.
Another solution is finding a slider potmeter of 1K, 5K 10K linear
stereo (not critical) that fits on the PCB. Any big parts shop (or
on-line Conrad.com of mouser.com) will have that. The only problem is
fitting the lever on the potmeter."
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My repair guy has a third suggestion that is based upon my own situation. That is that I seem to have a slight deficiency in my left ear, so, for me, I'm always adding more left than right. His idea is to use the one wiper I do have to control the balance in such a way that the right channel can be decreased with the control, leaving the left channel alone. He believes this will solve MY problem without having to drill holes/perform surgery. Although I'd rather it be 100 percent functional, I'm allowing him to explore this option. I will keep you posted. Hope this makes sense and doesn't duplicate something that's already been suggested.