I'm having the exact same problem - the left cannon GunShooter opto does not register on the switch edge test. Initially I thought it was the receiver opto because shining a light did not trigger the switch. I replaced the receiver opto (new) and that did not solve the problem. Next I checked the CPU board by shorting the matrix pins and that worked fine - so CPU is ok. That leaves the 16 Opto PCB. Interestingly, someone previously replaced the U4 LM339 (it's been socketed).

I also noticed on the opto PCB that some of the J5 wires (running to the CPU) passed close to the large C6 capacitor and when they touched the cap the switches would close. No sure if this caused damage to any of the LM339s?

Here's my question, would a bad LM339 cause a single opto to fail?

Also, is it possible to test the opto receiver by jumping the two leads - would that simulate the switch?

Get a flashlight and shine it into the receiver and see if it registers.

I had tried that but nothing.

I actually figured out my problem (mostly). The original issue was that the left cannon would randomly rotate. I assumed this was an issue with a short in the harness so I ordered a new one. After wiring in the new harness the opto stopped working completely. Next I replaced the opto but still nothing. Turned out the harness I purchase had been wired incorrectly - gray and black opto wires (12v and gnd I think) were reversed! After swapping those wires the opto started working again. But now I'm back to the intermittent cannon rotation. Grrrrr.