The game reports switch rows or columns shorted to ground, often several.

Row or column "ground short" errors

Cause: Seldom a real ground short. Usually CPU U20 (ULN2803), then U14 (74LS374; U23 on WPC-S/WPC-95), then LM339s U18/U19, or LM339s on opto boards (including the IJ-to-Demolition Man trough board).

Checks

• Game off, unplug J205, J212, J206/J207 and J208/J209, and meter each row and column wire to ground; usually there is no short.
• With J206-J209 unplugged, T.1 switch test should show nothing except switch 24. A whole row on means that row's LM339; a whole column means U20.
• If only one or two switches show, reconnect the switch plugs and unplug the CPU-to-driver ribbon; if it clears, suspect the 4N25s U7/U8 on a pre-WPC-95 driver board.
• Unplug the opto boards and see if the error changes.

Fix

• Replace U20, then U14/U23, then U18/U19; replace opto board LM339s as indicated; trace any genuine ground short to the last switch on its chain.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Switch matrix chips on the CPU board
• One switch does not work
• "Slam tilt stuck closed" at power-on
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Lamp matrix faults with all lamp transistors good
• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test
• Testing CPU switch rows with the switch levels test

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)