Playfield lights dim more than slightly whenever a pop bumper or other coil fires.

Gottlieb: lamps dim noticeably each time a coil fires

Cause: A slight dip is normal, but a big one usually means the tilt relay's switches (which carry lamp and solenoid power when not tilted) have poor contact or weak tension, and open slightly as the game vibrates.

Checks

• Inspect the tilt relay contacts and the tension of its blades.

Fix

• Clean the tilt relay contacts and tension the blades so they press firmly when the relay is not energized.

Systems

• Electro-mechanical

Related tips

• Playfield lamp fuse blows when one pop bumper fires: lamp socket shorting
• Change pop bumper lamp sockets from wedge to #47 bayonet
• 6 V lamp fuse keeps blowing
• Repair a loose playfield lamp socket
• Dim or flickering lamp sockets
• Wedge sockets in older wide-body pop bumpers: the bulb may not clear the cap

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