At power-on the knocker bangs four or five times, then the game starts; a message about adjusting a switch appears with a switch number in a score display.

Knocker fires several times at power-on with a switch number shown

Cause: The game has seen that switch go unused for roughly 30 games and flags it as bad. A hard-to-make shot can trigger this falsely.

Checks

• Enter switch edge test and roll a ball over the reported switch.
• Under the playfield, look for a broken wire, dirty leaf contacts, a misadjusted actuator or a broken wire on a neighbouring switch upstream in the daisy chain.

Fix

• Clean leaf contacts with a business card and bend only the stationary blade to close the gap.
• Adjust or replace a microswitch.
• Activate the switch by hand in a game or in switch test to clear the report.

Systems

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

Related tips

• Displays show only 0s and Xs at power-on
• Instant tilt and scrambled switch test: burnt resistor networks on the CPU
• A whole switch row or column is dead
• "Check Switch ##" message
• Catapult sometimes throws the locked ball early: clean or replace its switch
• One switch triggers an unrelated switch (phantom closures)
• A switch does not register in the switch test
• How System 11 switch numbers map to rows and columns

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