After adjusting a flipper EOS switch with the power on, the switch matrix (or the whole game) stops working.

Whole game dead after working on a flipper EOS switch

Cause: On games with lane change (Firepower onward, until System 11) the low-voltage lane change switch shares a stack with the high-voltage (28 V, later 50 V) EOS switch, insulated by a nylon actuator. Touching them together sends flipper voltage into the switch matrix and destroys IC15-IC18 and the IC11 PIA.

Checks

• Check IC15-IC18 and IC11 on the driver board.

Fix

• Replace the damaged chips. Only ever work on EOS switches with the game off.

Never adjust a flipper EOS switch with the game on.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Logic-probe the switch matrix
• Switch column or row fault on the driver board
• Switch matrix connector pins and their chips
• Driver board switch matrix resistors R204-R211
• Switch matrix column test at 2J2 with Leon's code
• Switch matrix row test at 2J3 with Leon's code
• Switches stuck on: board or playfield?
• A ball resting on one switch stops others in its column

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)