Here's a video of the L2 mini playfield opto flickering in test:

More info:

The opto switch wasn't working at all. All other row and column switches work fine.

I soldered in a new receiver and it still didn't work.

I replaced the Orange-Yellow signal wire from the switch receiver board to the opto board under the playfield. Then it started flickering.

Then I soldered in modern opto emitter and transmitter boards (replacing all 5 switches with new components)--no change.

Then I replaced the opto board under the playfield with a modern remake--no change.

Finally replaced the MPU board with a modern board--again no change. I acknowledge this couldn't be the problem, but did it anyway.

All the IDC connectors SEEM fine, and my multimeter shows continuity on all the lines from the switches to the opto board, and from the opto board to the MPU board.

Still, I'm missing something. It has to be the wires or IDC connectors, right?

You're right about the game link; thanks for doing that and for your reply.

I had the same thought about the emitters and did do the camera check, several times. I also tried offsetting the receivers to either side so the 4th receiver would "see" the emitter to the left or right. In both cases the problem stays with the 4th receiver, so we can rule out the emitters as a problem. It's something downstream from the 4th receiver.

The only thing I haven't tried yet is completely replacing the wire from the receiver to the opto board again, just in case something is wrong with the first one I put in. And possibly doing the same with the corresponding switch row wire from the opto board to the MPU board.

That weird flutter is why I'm skeptical it's the wire or IDC connectors. But it does the same thing with the emitters to either side, and the receivers to either side work with the #4 emitter. So it can't be the emitter.

I wonder if one of the other switches on the same row could somehow cause this flutter, while not manifesting it?

This is a bit deeper than I've had to work before, too.

But it makes sense that it has to be an issue within how the switch matrix works. If one of the adjacent switch diodes is missing or reversed it might leak a signal to this switch while not affecting the switch itself. That makes sense to me, so I'll investigate the adjacent switches. But I welcome any other thoughts.

Not goofy at all. Since the optos are all new I don't think they're dirty. But I will double-check that this one isn't blocked somehow. It's conceivable that the old one was bad, and the new one in the same spot coincidentally got blocked or dirty somehow. That would definitely explain my confusion.

Agreed. It feels like I've checked them thoroughly, but it has to either be something so obvious I can't see it or so obtuse I'm looking in the wrong places.

Do you mean short across the Common and the #4 wire? If so, shorting across these doesn't change anything.

The unregulated 12V is reading as 11.2V. Not sure what that means?

Shorting the legs of the phototransistor has no effect. That pretty much confirms the optos aren't the issue, doesn't it?

I had a spare Rottendog power driver board, so I put it in. The unregulated 12v line is now a normal 13.7v. It did not change the switch flutter behavior.

No other mods. I'm wondering about hacks by previous owners, though.

Is that the opto board under the playfield? I already tried replacing that. Both the original and the modern version show the flicker.

The optos are normally closed, then open when the emitter is blocked. I think it has to be something else on one of the rows or columns that's generating interference. I'm going to disconnect the whole row and then connect just this switch via a separate wire to see if it fixed it, then do the same with the column. If it is something else, that should tell me where to look.

Well, I fixed it. I was correct that one of the adjacent switches had to be interfering.

The problem: One of the poles of the Left Outlane switch was bent and touching a pole of the adjacent extra ball light. Electrically the Left Outlane switch is adjacent to the 4th opto in the bank. So it was feeding current into the switch matrix, and I assume the switch diodes were preventing it from leaking out into the rest of the row, leaving only this one switch affected.

Ultimately I found the problem by just going through all the switches looking for problems. I didn't do the harder row and column diagnostic I was considering.

I guess the lesson here is: Check for all the easy and obvious problems before going deep.

Thanks for your help guys. Talking through it got me thinking in the right direction, so this really helped.