On the bench, a board known to work keeps its LED lit and never starts flashing.

Good MPU stays LED-on with a PC power supply

Cause: Some PC switching supplies take longer than about 50 ms to bring +5 V up to a steady level, so the reset circuit releases the 6800 CPU before power is stable.

Checks

• Try the board with a different (faster) supply or in the game.

Fix

• Touch a small screwdriver across U9 (6800) pins 39 and 40 for a moment; this pulls the reset pin low by hand and should start the flash sequence.
• The same short restarts the flash count at any time without switching off.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Keep spare 6800, 6821 and 5101 chips
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• MPU stops before the seventh flash: check the 43 V at R113
• What each MPU LED flash proves
• MPU boots only when U9 pins 39 and 40 are shorted
• MPU boots only some of the time
• MPU reboots right after the seventh flash

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)