I have had several issues in the past with Switch 31 Opto popper, so these are my uneducated ramblings. First I had a wire that goes to the Tardis light that would block the Opto when the playfield was down but not when it was propped up. A ty-wrap fixed that. Second, the plastic cup that pushes the ball up was old cracked plastic and got caught returning, also blocking Opto. Replaced it with new part. If lived closer, I could loan you my DumbAss Opto tester that I built from his kit. As a shameless plug, I am including a picture of the 10 Opto board, that I just built from him as well. J6 runs directly from the 10 Opto board to the Opto Popper with 4 wires. I replaced the IDC's with a molex 5 pin plugs. I also included a picture of the Opto popper that I rebuilt.

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topkat - looking at the connectors on the board. If you have voltmeter, set it to 20 volts. Locate Pin 9 on connector J1 and put black lead (GND/ground) into it. Then according to pin wiki. Put the red lead into any of the 4 Popper Opto wires on J6. They are paired (Green-White) and (Grey-Black). They should all read 12volts with the game on. Then use a piece of paper to block either opto to prevent the LED beam from being received by the transmitter. The transmitter will still read 12volts on either wire, but the with the receiver blocked one of the wires on that side will not have 12 volts. That is if they are working and the 10 Opto board is not fried. Then for giggles and grins. Measure the voltage on the other J4 and J5 connectors, which should all be getting 12 volts or what ever voltage your Power board is sending out.

I will try this theory out myself when I have personal time.

OK - I opened my working game up, lifted the Playfield. Put the black lead on Multimeter on 20 DC Volts onto pin 9 on J1 - Ground. The following voltages showed up with game on. They did fluctuate up and down by .2 volts while game was in attract mode flashing GI bulbs.
J6 - Green (E) 12.6V, White (C) 12.38V, Grey (R) -0.0, Black (K) 1.27
J4 - Green (E) 12.2V, White (C) 12.5V, Grey (R) -0.0, Black (K) 1.27

I played a game immediately after and no errors, with VUK working perfectly. Hope that helps.