This 24 ch mixer has been "repaired" twice for a very rare random fault.
Recurs
months after being "repaired". Loss of L channel output at the XLR. Previous
person had obviously looked at the most likely suspect of the 1/4 in sockets
for insert , with bypass contacts and XLRs thmselves.

Both Main/Sub push-push switches, 4 pole C/O are very suspect in the FX
sections, half a mm pushing when out will loose the effects component of L
or R channel and a bit more, full loss of one or both channels. Any other
suspects to check?

I'm surprised the schema are out there but minus smps so some details taken,
just in case.
switcher driver ? (reverse of pcb not explored) maybe designated
04N68, TO220 , usual 0/O possibility (obstructed viewing) other marking
maybe 1M360