Tracing the VMA signal through U19, U14 and U15

Steps

• With the board in attract mode (or booted on the bench), probe each point below; each should pulse unless noted.
• U9 pin 5 (VMA out of the CPU).
• U19 pin 8 (pulse) and U19 pin 9 (steady high).
• U19 pin 10 (pulse out).
• U14 pin 11 (pulse, about 1.3 V on a meter), then U14 pin 12 (high pulse out).
• U15 pin 4, U15 pin 5 (from U16 pin 10; only on a fully booted board) and U15 pin 6.
• U15 pins 1 and 2 (tied together), then U15 pin 3 (about 1.8 V on a meter).
• Where the pulse stops, replace the chip. U15 (MC3459L or 74S37) fails most, then U14, then U19.

Systems

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

Related tips

• No clock at the CPU: U15, U16, C14/C15
• Two flashes only: U8 5101 RAM
• Switch row (return) circuit on the CPU board
• Whole switch column dead after working under the playfield
• One switch row dead: parts to check in order
• System 11 switch matrix: connectors and chips
• Switch matrix dead after an under-playfield adjustment with power on
• Fixing a dead switch row on the CPU board

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