Switches stop registering (whole columns or rows, or all of them) after someone adjusted switches or coils under the playfield with the game running.

Switch matrix dead after an under-playfield adjustment with power on

Cause: A coil lead (+25 V or +50 V) touched a switch lead and drove coil voltage into the matrix. Damage usually starts at the column transistors and can reach the 74LS244 at U40, the 4011 row chips and the 6821 PIA at U38.

Checks

• With the game off, test the eight 2N3904 column transistors Q42-Q49 with a meter on diode test; they are first in line and often the only casualties.
• If columns test good but the matrix still fails, follow the row path from 1J10 to the 4011 chips U30/U39 (U51/U52 on System 9) and then U38.
• After replacing parts, unplug 1J8 and 1J10, enter switch test, and confirm no switches show closed. A whole row showing closed means the row chain is still bad.

Fix

• Replace the failed 2N3904s first, then U40 (74LS244), the 4011 row chip and finally the 6821 PIA at U38 as testing shows.
• Always make switch and coil adjustments with the game turned off.

Systems

• Williams System 9
• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Fixing a dead switch column on the CPU board
• Fixing a dead switch row on the CPU board
• System 11 switch matrix: connectors and chips
• Whole switch column dead after working under the playfield
• One switch column dead: parts to check in order
• One switch row dead: parts to check in order
• Whole switch column dead and board fails the jumper test: column transistor
• Switch column (drive) circuit on the CPU board

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell