That was my thought too. If it's RFI/EMI from the surrounding electronics then a ferrite core will definitely work. I need to get my digital probe on the optos and see what signals they are registering. IIRC, the opto receiver just registers a logical "high" when it 'sees' light and a logical 'low' when the beam is interrupted. I am not quite sure how the logic board could misinterpret that. In the logic-days-of-old, there is either +5v (or -5V) as a logical high and 0v when a logical low. Presumably when the opto receiver sees light, it conducts electricity and when there's no light, it doesn't.

The twisted pair works because noise such as EMI/RFI would travel down both conductors equally. When you twist them together the 'positive' magnetic field generated on lead number 1 by the emi/rfi will be cancelled out by the 'negative' magnetic field on lead number 2. The noise like that tends to ride 180 degrees out of phase on the leads and therefore will net out to zero when twisted like that.

As an additional step, I'd probably throw ferrite beads on the leads from the opto receiver board back to the cpu.

On re-inspection of the pic a few posts above I think I may see what the problem is. Those leads run damn close to the magnet in the playfield AND the motor for the Vengeance. When that coil is energized (as it would be when the first ball is trapped), I'd gather there is all sorts of magnetic flux leaking from that coil. That magnetic energy would induce a voltage in those wires easily which might be enough to fool the cpu into thinking the opto is in a state it is not (eg., when the second ball breaks the beam, there is still sufficient voltage to make the cpu think it is in a 'high' state (beam not broken) when it actually is broken.

As a second suggestion, one could wrap the opto bundle in a foil-backed mylar, with the foil facing inwards (to not short anything out). Then just connect one end of the foil to ground.

Both Mouser and Jameco sell the foil-coated mylar wrap. I think I may have seen it at Parts Express too. If you're really daring and careful, you can even use aluminum foil and wrap. Just make sure that the foil does not touch (and will NEVER touch) anything else, especially connectors with any voltage on them whatsoever, because you would want to ground the foil and you will do significant damage (and blow fuses) if your connections get shorted to ground.

Alternatively, you could also buy shielded conduit (which is non-conductive on the outside) and run all the opto wires through that and just ground the shielding.

That being said, the twisting is easy to do. If you're nervous about it, just to be absolutely sure you put the right opto on the right pins, take a couple different colored sharpies and put a colored dot on the connectors and then as you remove them, put a colored dot on the plastic between the pins. then it's virtually impossible to plug them in wrong.

Prior to the way Stern labels the opto connectors, and with all my old Wms machines, I always put tags on every cable leading to a connector on the boards and under the playfield. I have a Brother labeler machine that prints the paper adhesive labels so I just make labels (J116, J111, etc) and wrap them around each cable. Two seconds of prevention is worth an hour of back-tracking trying figure out where sh*t goes!

try a ferrite bead on the wires. cheap, easy and won't harm anything. alternatively, you could buy a ferrite torroid (an "O") and wrap the opto wires round and around the "O". If you're going to go the ferrite bead route, put them close to the opto board.

small ferrite cores are like $0.30-$0.50 each. At ham radio swap 'n shops you can buy a bag of them for a couple bucks.

I'll bet you see the new machines have the wires twisted from the factory.

That's the correct way to put the ferrite bead on. The more times you loop it through, the more ac inductance you create because that's a choke. The more AC inductance you have the better the wires reject emi/rfi, since its equal to 2*Pi*F*L