A loud hum comes from the speakers and changes with the lamps. Fitting the few missing board screws made no real difference.

Loud hum from the speakers that follows the lights: clean every ground screw

Cause: Poor ground connections at the old, dirty ground screws throughout the game (boards and ground points), so the lamp current upset the audio ground.

Checks

• Check every board in the backbox has all its mounting screws.
• Look at the ground screws and the spots where they meet the boards and ground braid for dirt and oxidation.

Fix

• Take out all the ground screws in the game, clean them (a wire brush works), clean the contact areas on the boards with fine abrasive paper, and refit them all firmly.

Switch the game off and unplug it before removing board screws.

Systems

• Williams System 11A

Games

• Fire!

Related tips

• Loud constant hum that the volume control does not change
• Loud hum from the speakers in attract mode

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