A U10 or U11 output pin stays fixed while the others toggle each second.

One PIA output does not toggle in the test EPROM test

Cause: A dead PIA output, or a short on the board line it drives.

Checks

• Briefly link the silent pin to its toggling neighbour: if both now toggle, the first output is dead.
• If neither toggles, bend that PIA pin out of the socket and probe the pin itself: if it toggles, the short is on the board; if not, the PIA is dead.

Fix

• Replace the 6821, or find and clear the shorted line.

Systems

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

Related tips

• Three flashes only: U10 PIA
• Four flashes only: U11 PIA, R134 or a bad display
• Keep spare 6800, 6821 and 5101 chips
• One PIA output does not toggle under the test EPROM
• CPU flickers then gives three flashes: bad PIA at U10
• Test the switch PIA IC11 with the Leon test EPROM
• Leon's test EPROM fitted but the LEDs do not start flashing
• Leon code running but LEDs dark: bad CPU PIA IC18

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