I've got a WPC95 MPU board that is having issues with the switch matrix. With the playfield disconnected and testing the switches with a jumper, the switch I'm testing works but then the rest of the switches in the same row will going on and off rapidly. It does this for all 8 rows.
I've swapped out/tested U20 along with U18 & U19. U23 has also been replaced but no change. 12v is good as well.
Any ideas?
http://arcarc.xmission.com/Pinball/PDF%20Pinball%20Misc/Williams%20WPC-95%20Schematics%20Manual.pdf
By the looks of the schematics maybe u13 feeds u18/u19 but I’m no professional. I know pinsider zaza knows his shit he would be able to help you I think maybe hit him up on a pm.
Also found this. Seems to have a good section on testing switch matrix rows.
http://homepinballrepair.com/index.php/digital-probe-part-2/
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Frankfort, NY