You might try, for instance, switching the input pods for channel #1 and channel #2. You want to see if it is the pod or the board(s) behind the pod that
are faulty. In other words, does the fault follow the pod change or not.
If I remember, the channel #1 and channel #2 pods and circuitry are the same, so switching is permissible. I'm not sure you can play the same game
with the two triggers, however.
Also, try doing a two-key reset (power on holding the top and bottom function keys down) and then run the extended self-tests. My experience has
been with the 54112D, not your 54111D, but the two are quite similar. And mt unit's self-tests pointed to D/A problems on two of my channels (that was
fixed with new D/A chips). I expect the 54111D diagnostic tests to be similarly thorough.
ArtekManuals has the complete service manual set including the Component Level Information Package (CLIP) containing the schematics. You might
eventually need them if something obvious (e.g., are all the internal cables properly connected?) doesn't present itself.