Confirmed it’s the unit first by swapping L&R monitor outputs (no change) and by swapping another identical unit in to the same setup and retaining the same settings (headphone works fine but had bad slider on this one). Any ideas? Bad pot or solder joint between master and headphone out?
Why have you made a new topic in a MC4000 section?
Guess I got impatient, and hoping there’s a better chance of someone repair-savvy that’s familiar with the internal workings of these units coming across it in the current, much more active forum.