I just put insert jacks on my ‘things to check first’ – list. A couple of days ago I repaired a Soundcraft mixer, after disassembling it and figuring out the signal path (didn’t have schematics) it turned out that the contacts on the insert jacks were corroded disabling the signal flow.

The very same problem just occurred in an audio amplifier whose headphone jacket had the same problem leading to a defect left channel.