Check the U9 address and data lines, and suspect U11

U9 address pins 9-20 and 22-24 and data pins 26-33 should all pulse (pin 24 is used only with 2732 EPROMs; pin 21 is ground). Even with every pulse correct, a bad U11 PIA or socket can keep the LED on, and U11 sits in the corrosion zone.

Systems

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

Related tips

• Checking U6 and the jumpers with the U9 VMA line
• Proving U6, U9 and U11 in a known-good MPU
• One PIA output does not toggle in the test EPROM test
• Four flashes only: U11 PIA, R134 or a bad display
• MPU board test point voltages
• MPU stops before the 7th LED flash (no 43 V)
• Add a ground jumper on every -17/-35 MPU
• Testing an MPU in the game with only J4 connected

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