On power-up the CPU LED flickers and then flashes three times and stops. An aftermarket CPU board boots fine in the same game.

CPU flickers then gives three flashes: bad PIA at U10

Cause: A failed 6821 PIA at U10. The spare used PIAs tried first were bad too.

Checks

• Check the PIA socket for a broken-off leg and continuity from each socket pin to its traces.
• Swap U10 and U11: U11 drives the LED, so if the LED now stays solid, the chip that was in U10 is bad.
• Don't assume used spare chips are good.

Fix

• Fit a known good 6821 (a 6820 also works) at U10.

Systems

• Bally MPU AS-2518-35

Games

• Embryon

Related tips

• Three flashes only: U10 PIA
• Four flashes only: U11 PIA, R134 or a bad display
• One PIA output does not toggle in the test EPROM test
• Keep spare 6800, 6821 and 5101 chips
• Leon code running but LEDs dark: bad CPU PIA IC18
• CPU board PIA positions and jobs
• Test the CPU board PIA IC18 with Leon's code
• Segments missing or stuck on: CPU board PIAs 9B and 7B

Source: Pinball Repair help group!! (Facebook group)