Hmmm off the cuff id say an alignment issue for the first one and a bad/not powered emitter for the 2nd. Cleaned them with cuetip and alchohal already?

Did you replace the harness?

If you wiggle the gun harness under the playfield do the switches open/close?

actually the right doesnt sound like an alignment issue. Scratch that, maybe an opto board output flakey?

Broken down wires causing less than solid connection b/w switch & board.

Take your cell phone out. Put it in camera mode. See if you can "see" the optos through your cell camera. Cell phone cameras don't filter out IR light, so you should see it light on/off.

The harnesses were pretty pricy I found - I ended up using stranded cat5 cable and worked like a charm for years - Ethernet patch cables are stranded & easy to come by now days.

Did you reflow the solder connection to the opto? I am assuming that the harness for the opto has a Molex plug and that the original connection to the opto itself wasn't part of the replacement harness.

I guess I'm not understanding what the game is doing. I get what the optos are doing but what is it doing in game? Not loading to the cannon? Loading and not firing? Cannon not registering at all?

The optos do die over time. Have you tested the optos themselves? Usually the transmitter goes before the receiver.

RePlace Opts. Mine yellowed with age and were in need of replacement. The led lens side went from yellow or light blue to amber. Most of the IR light was being blocked and not getting to the photo transistor

On an off chance, check your VUK optos too. If the game is reading the VUK as empty after a ball rolled thru, I will think it misfired and loaded a ball to the cannon

Also, someone jump in here but don't all the optos go thru the 16 opto board? Couldn't a 339 be out?

Replace cap c2 (100 mfd) on the driver board. It's next to the small voltage regulator 7812. Probably needs done anyway...

This is an outstanding idea!

Most of the optos do go through the 16-Opto board, the ones that do not are the seven on the ball trough as they have their own driver circuits on the Ball Trough PCB; and yes, it's possible that there can be problems with the wiring to the opto board. A good thing to try is reseating the J3 and J4connectors on the 16-Opto PCB. Also, since you did replace both Optos I would check that they are wired correctly and that they are not blocked in any way.

I'm a little confused on the right cannon comments since ALL of the opto switches should read closed unless the light is blocked. A closed switch is represented by a square in the switch edge test menu T1. The switch changing from a square to a dot when a ball is in the gun signifies both right gun optos are working properly.

To aid in isolating the problem, here is how your Left Gun Shooter Switch works: All opto transmitters are constantly on whenever the game is turned on via the 16 Opto PCB. A current limited +12V is applied to the anode (A) of the optical transmitter via a grey-white wire from J3 pin 10 of the 16 Opto board, the return wire (black) connects the cathode (K) to J3-1 completing the circuit. The photo transistor has +12V on the collector (C) which comes from J4 pin 1 (Grey-Yellow wire), the emitter (E) of the photo transistor is connected to J4 pin 10 (orange-White); when the light coming from the transmitter hits the photo transistor, it turns the transistor on closing the circuit and applying the +_12 to the collector.

So how do you check them? ...
I would use a DVM, what you should measure on the transmitter (green PCB) is the operating voltage of the photo transmitter, roughly 1.8-2.1 V. If you measure +12V here it means the transmitter is NOT on because it has either failed or the return circuit is open.

The photo transistor will be off if the transmitter is not working so there is no need to check this unless you have a working transmitter. What you should see on the receiver when the photo transistor is on is nearly no voltage across the emitter and collector (because its almost the same voltage on both sides) and you should measure +12V to ground. When the receiver is off (IR missing or blocked) you will see +12V across the emitter and collector, 12V on the collector to ground, and no voltage on the emitter.

This is pretty much the same functionality for all of the optical circuits.

I had this exact same problem with my STTNG. It was driving me crazy. Turns out the white/green wire that comes from the 16 option board also attaches to another target. It had broken loose and was rubbing against it with every vibration of the table. The switch would show closed (correct) during switch test add table in down position. Once I soldered it back on, it worked fine. What a crazy issue!