Bally or Williams EM has power but the lights won't come on.

Bally or Williams EM won't light up: game over relay switch

Cause: These makers use a latching game over relay, and a single switch on it feeds all the lamp power. A dirty or misadjusted contact there leaves the game dark.

Checks

• Find the game over (latch) relay and inspect its switches.

Fix

• Clean and adjust all switches on the game over relay.

Systems

• Electro-mechanical

Related tips

• Make the GI come on at power-up by locking out the hold relay
• Player 2 score lights always on, even in a 1-player game: wired into the GI
• Bally lamp sockets are the weakest

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)