The MPU completes its 7 LED flashes, then resets and starts the flashes again in an endless loop. Another MPU boots fine in the same game, and the socketed chips from the bad board work in a good one.

Bally MPU loops through its 7 flashes and resets: check R134 and the interrupt traces

Cause: A break in the interrupt circuit: here resistor R134 or the traces to it were open. The software crashes if the zero-cross and display interrupts are missing or far out of timing.

Checks

• Check R134 and the traces leading to it for continuity.
• Check continuity of the IRQ lines between the processor and the two PIAs (U9, U10, U11), including at the sockets.
• Check the display interrupt timing capacitor (C16) is firmly attached.

Fix

• Repair the open trace or replace R134.

Systems

• Bally MPU AS-2518-35

Related tips

• Only three players allowed on an aftermarket MPU: reseat J1, clear memory, reset DIPs
• Four switches on one return line dead with good wiring: failed 6821 PIA on the MPU
• MPU stops before the 7th flash until sound board power is unplugged: replace CR3
• Only GI lights and MPU LED never flashes: loose wiring at J3 on the solenoid driver board
• Controlled lamps stuck or not flashing: reflow MPU and lamp driver header pins
• Seven flashes, then it starts over endlessly
• Display digits flicker then vanish after the game warms up: poor board connector contact
• CPU flickers then gives three flashes: bad PIA at U10

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