Solenoid selection: four MPU lines decoded by a 74154

The MPU U11 PIA sends four select lines plus a strobe out on MPU J4 (pins 1-4 and 10) to SDB J4 pins 3-7. A 74154 decoder on the SDB turns the four-bit pattern into one of 16 outputs, which drives up to 15 momentary solenoids plus the flipper relay. This applies to the AS-2518-22 and AS-2518-17 solenoid driver boards.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• Solenoid select lines: MPU PIA U11 to SDB 74154 pin map
• Baby Pac-Man solenoid board select lines
• Test the coils behind the solenoid expander
• No coils work at all
• Solenoids fire out of order in solenoid test
• Wrong coil fires or a coil never fires
• Coil locks on at power-up or stays on
• Flippers and some coils work, others are dead

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