None of the general illumination lights. All power supply fuses are good, the test points are right, and the GI wiring reads about 7 V AC at every point, meaning the circuit isn't being completed.

No GI at all: failed Q14 SCR on the main lamp driver board

Cause: Centaur's GI is switched for effects through a GI relay/triac controlled from the lamp boards. Here the controlling SCR, Q14 on the main lamp driver board, had failed.

Checks

• Bench test the triac on the rectifier board plate with a meter (it tested fine here and members say it rarely fails).
• Check the bulb mounted next to the auxiliary lamp board: it has to light for that circuit to work.
• Swap in a known-good main lamp driver board (another Bally game using the same board will do).

Fix

• Replace Q14 on the main lamp driver board with a new SCR (the poster used a 5064).

Systems

• Bally MPU AS-2518-35

Games

• Centaur

Related tips

• Coil sleeves by mechanism
• One controlled lamp dead even after a new SCR: lamp driver board fault
• Test lamp SCRs with the power off
• Substitute SCRs for the lamp driver board
• Lamp driver, auxiliary lamp driver and display driver test points
• Lamp driver board layout: SCRs, decoders and connectors
• Lamp driver board: which SCR drives which connector pin
• A controlled lamp that is always on means a shorted SCR

Source: Pinball Repair help group!! (Facebook group)