Switches stop working after a probe slipped, a coil was mis-wired, or a switch wire touched coil or GI power.

Switch matrix dead after coil or GI voltage touched it

Cause: 24 V DC coil or 6.3 V AC GI on a switch line has damaged Z8 (7404 strobes), Z9 (7405 returns 0-5) or Z28 (7405 returns 6-7), and possibly the U5 spider.

Checks

• Run the Z8/Z9/Z28 logic probe checks.

Fix

• Replace the 7404/7405 chips. If U5 is damaged the board cannot be repaired and must be replaced.

Systems

• Gottlieb System 1

Related tips

• System 1 switch matrix layout
• Logic-probing the switch matrix
• Slam and outhole switches sit outside the matrix
• Test PROM switch actions
• Testing switch return buffers Z9 and Z28
• Switch strobes missing at Z8
• CPU plug A1J6 (coin door) pins
• All coils and most CPU lamps lock on at power-up

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)