None of the CPU-controlled lamps light, but the game otherwise runs.

All matrix lamps out, everything else works

Cause: Blown lamp matrix fuse F3, or a failed backbox lamp bridge rectifier 6BR1.

Checks

• Check F3.
• Meter between 3J4 pin 1 (ground) and 3J4 pin 5 on the power supply board: expect 18 V DC.

Fix

• Replace F3, or replace the 6BR1 bridge if F3 is good and there is no voltage.

Line voltage is present in the cabinet and backbox with the game plugged in; unplug before working on wiring.

Systems

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

Related tips

• No lamp matrix voltage
• How the lamp matrix runs 6.3 V bulbs on 18 V
• Burnt GI plugs on the power supply board
• Burnt GI connectors on the power supply (System 7, Hot Tip, Lucky Seven)
• No general illumination: trace the 6.3 V AC
• GI in at the power supply but not out: check the GI relay
• Replacement GI relays
• All controlled lamps out: bad spade connections on the bridge rectifier

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