Every switch on one row or column fails, or one switch shows as several, or a wrong number appears.

A whole switch row or column is dead

Cause: Faulty CPU switch input/strobe circuit, or a broken or shorted row/column wire.

Checks

• Test the other switches on the same row and column first.
• Run the CPU board connector test (disconnect the matrix and jumper pins).
• If the CPU passes, check continuity of the row or column wire from the switch back to the board.

Fix

• Repair the wiring, or repair the CPU board if it fails the test.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Whole switch rows read closed and U20 keeps failing: replace U23 on the CPU
• A whole switch column dead or all closed
• A whole switch row dead or all closed
• Fixing a dead switch column on the CPU board
• Fixing a dead switch row on the CPU board
• Row or column "ground short" errors
• Tracing a column of dead switches from the manual chart
• Testing CPU switch rows and columns with a diode and clip lead

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer