In studying the picture of the DMC20, I don't think the toggle switch has anything to do with your channel dropout issue. The switch on the front merely control relays that do the actual phase inversion of the output, so its my somewhat qualified guess that one of the relays, orange cubes located towards the rear of the PCB to the right behind the output terminals, or the two located in the center just behind these are at fault.
Best of Luck
Peter
Best of Luck
Peter