A single switch never registers in switch test.

One switch does not work

Cause: A broken upstream wire, a misadjusted or broken microswitch actuator, loose solder, dirty leaf contacts, a bad switch diode, or a failed gate in U20 (column) or U18/U19 (row).

Checks

• Microswitch: listen for the click; no click usually means misadjusted or broken.
• Buzz the wire from this switch to a working switch in the same column (white) or row (green).
• Meter on continuity, one lead on the common lug (banded diode end): the normally open (green) lug should beep only when pressed, the normally closed (white) lug only when released.
• Test the switch diode.
• Check other switches in the same row and column.

Fix

• Clean leaf contacts by drawing a business card through them; never file the gold-plated contacts.
• Replace a bad switch.
• If a whole column is dead replace U20 (ULN2803); a whole row, U18 or U19 (LM339).

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
• Row or column "ground short" errors
• "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)