The flipper relay won't pull in during a game. The driver board works in another machine, so the fault is on the MPU, and the MPU outputs look fine on the bench.

Bally -35: flipper relay never pulls in: hairline-cracked trace to a PIA pin

Cause: A hairline crack in the trace to pin 40 of a PIA. It still beeped for continuity but went open in the game.

Checks

• Use a logic probe on the MPU outputs with the board installed and running, not only on the bench.
• Look closely, with a magnifier, at the traces near the PIA and any old battery-damaged area.

Fix

• Run a jumper wire across the cracked trace.

Systems

• Bally MPU AS-2518-35

Related tips

• Bally: flipper relay won't fire in game: hairline trace break that still beeps
• Bally MPU loops through its 7 flashes and resets: check R134 and the interrupt traces
• One PIA output does not toggle in the test EPROM test
• Four switches on one return line dead with good wiring: failed 6821 PIA on the MPU
• No boot with a Pinball Lizard MPU and Rottendog driver: use the original driver
• Checking the PIA outputs with the test EPROM
• Four flashes only: U11 PIA, R134 or a bad display
• Spare MPU and driver chips with their locations

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