Logic probe states in attract mode: U9 6800 and U12 555

Measured on a -35 MPU in attract mode. U9 (6800): pin 1 low, 2 high, 3 pulse, 4 high-pulse, 5 pulse, 6 high; pin 40 high, 39 low, 38 low, 37 pulse, 36 pulse, 35 low. U12 (555): pin 1 low, 8 high, 2 and 3 high-pulse, 6 and 7 pulse, 4 and 5 high. Inputs in the wrong state point to the chip feeding them; outputs point to the chip itself.

Systems

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

Related tips

• Six flashes only: 43 V, U10 or U14
• Keep spare 6800, 6821 and 5101 chips
• Seven flashes but no attract mode
• Garbled player 1 display and strange play from a bad 5101 RAM
• Five flashes only: U12 timer or open R113
• MPU board test point voltages
• MPU stops before the 7th LED flash (no 43 V)
• Add a ground jumper on every -17/-35 MPU

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