Reading the MPU flash code

Steps

• Switch the game on and watch the MPU LED next to the battery.
• A healthy board flickers faintly once, then flashes seven times (six on Baby Pac-Man and Granny and the Gators).
• Note how many flashes you get before it stops; the next test in the sequence is the one that failed.
• For anything short of a full count, take the board out and work on it with a bench supply.

Systems

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

Related tips

• What each MPU LED flash proves
• What a good MPU does straight after the last flash
• MPU LED locked on with no flicker: likely causes
• MPU stops before the seventh flash: check the 43 V at R113
• No seventh MPU flash with a good solenoid fuse
• Good MPU stays LED-on with a PC power supply
• MPU boots only when U9 pins 39 and 40 are shorted
• MPU reboots right after the seventh flash

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