Quoted from jimy_speedt:
the CPU is brand new, and if I remove the SM 20x connectors, no square is lit anymore. Same if I remove j3 from the opto switch 10 PCB. I do not see any wrong contact between lugs nor contact with the ground. I guess I could have continuity with the ground braid in that case. The only switches I have not tested are the 2 optos. As It requires the removal of the mini pf, I would like to check first if they can be faulty
Look under the playfield, look at the coin door, look at the interconnect boards, look at the glass in place switch; check that every diode is installed the correct way with the NON-banded end toward the row wire (white with red stripe).
The row wire daisy chains from one switch to another, and is NOT wired in the order of the matrix, but is SHORTEST distance to next shortest distance. So the white-red wire may go from the MPU to the mini-playfield home opto, to the S switch in escape, to the E switch in repair, to the right jet, to the 5 bank right opto, to the coin door, to the glass switch. (only a guess, order may be different). Somewhere that wire is SHORTED to something else, or a diode is backwards.
If I were to guess, I'd try for the glass switch or the coin door closed switch as the location of the issue first. But it can be at any location even in between switches where the white red wire is pinched and now shorting to something else...
There is no easy way other than to take time and examine the entire sets of the white-red wire, and everywhere it splits and jumps from one switch to the next. In this case there are a number of connectors it will go through too...
You can try disconnecting it from each switch and see if the problem goes away, then you can figure out that that switch, or a downstream one, or downstream wire is bad. But examination of the wiring is the only way to find the issue.
If you've tried to repair something, or installed some modification or toy, check or remove that first.
The under playfield pic below shows SOME of the places the white-red wire lands and splits off to the next switch....Inkedimage-5 (1)_LI (resized).jpgdrwho (resized).JPG