One of the things that was bugging me - I've been working with mixers a long time and never seen this before - these kinds of problems are almost always mechanical- a pot or fader that's dirty. But wagging the pots and faders doesn't affect it. Or a jack, but I tried different channels ...
... And then it hit me (and though I haven't got the mixer open yet to try the fix, I'm sure because the symptoms make sense now)- the master insert jack is dirty!! Logical cause that explains the issue, unsurprising in a unit stored ten years, and the last physical interrupt in the right channel signal chain I hadn't thought of. Just in case someone else finds similar problems. I'll pop back to verify my repair outcome.
