[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.

Thanks for the great advise. I'll pop it open when I get home and and
clean the ribbon connections.