Sounds like either switch issues or RAM issue, can you verify all your saucer switches are working in addition to right and flipper buttons that are used for passing?

Maybe try a factory reset?

Maybe roll back to old ROMs and see if they work better?

You can have ROM issues without getting an error, but if you are noticing the same issues with different ROM sets, it's probably not the ROM. You can also have RAM issues without getting an error. My guess is it's something more simple (like switches), but RAM is a possiblity.

Defender optos should be NC, as is typical of all opto switches. Are you seeing any other optos as being NO besides the defenders? If you have the original printed manual, the switch matrix table in the back shows you which are NC, but this is hard to see in a scanned version.

This supports a switch problem. Do you have a switch matrix tester? If you disconnect all switch connectors from CPU board what happens in switch test?

I’d also try pulling all the balls from the trough so no optos are blocked and see what shows up in switch test, and move the defender (defender test?) so it’s not blocking an opto.

FWIW - flipper opto switches are not NC, they are NO, an exception to the NC opto rule - and apparently a typo in the manual. Press a flipper button and it goes from a dot to a square, not the other way around.

Fingers crossed!

Can you go into switch test, unplug the switch connectors, and see if you get any random triggers? That could help to rule out CPU. Also get a switch matrix tester from Siegecraft and test the switch matrix without the playfield connected to totally rule out CPU board having switch matrix issues.

My guess is the random coil firings are related to the switch issues.

One thing that is different is the gameplay code is running and not the diagnostic menus. Not sure if they occupy different memory space or not. You tried a factory reset right? Latest version of code?

So it sounds like it was confirmed as a switch matrix issue if replacing U20 solved it. Nice work!