Every switch in one column reads open, or all read closed at once.

A whole switch column dead or all closed

Cause: A failed column driver (U20) or U14 on the CPU board, or the column wire shorted to ground.

Checks

• Check the column wire for a short to ground.
• Check U20 and U14.

Fix

• Repair the wiring or replace the failed chip.

Systems

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

Related tips

• Tracing a column of dead switches from the manual chart
• A whole switch row or column is dead
• How the switch matrix reads a switch
• Switch 24, the always-closed switch
• How the always-closed switch 24 is wired
• After a new U20, work every flagged switch once
• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test

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