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

A whole switch row dead or all closed

Cause: The row's 1N4148 diode or LM339 comparator on the CPU board, or U13 or U15.

Checks

• Test the row's 1N4148 diode.
• Check the LM339 for that row, then U13 and U15.

Fix

• Replace the failed part.

Systems

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

Related tips

• 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
• Testing CPU switch rows with the switch levels test
• Reading WPC switch numbers and the switch test display
• Tracing a column of dead switches from the manual chart

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