googling around turned up

"Incidentally, on more than one occasion I have repaired a Behringer with
this exact same symptom (no L ch), and it turned out that one of the main
bus tracks
runs right at the far right edge of the PCB from fader to output jack socket
section. When the PCB was installed into the chassis, something sharp
caught on the PCB and partially cut through the track, which eventually
failed some time later."

for when I get in there proper