[Bluesbreaker]

Sounds like a problem with the main outputs. You may have a loose or
dirty ribbon cable connector. It's a common problem with these mixers. If
you can take it apart far enough to get to the ribbon cables, you can
disconnect them, clean them with DEoxit, and put it back together. Remove
one at a time so you'll get them back in the right place.

Well, I've cracked open the mixer. There are only a few ribbon
cables. All are soldered rather than being attached via a plug. I'm
not sure that DeOxit will help in this case.