System 11A-11C diagnostic LED flash codes
Count the middle LED flashes at power-on or after pressing SW2. 1: U25 RAM failed (stops here). 2: memory protect failure: coin door shut, memory protect switch stuck, protect logic active, or U25 bad (the test assumes the door is open). 3: U51 PIA (display). 4: U38 PIA (switch matrix, special solenoids). 5: U41 PIA (display data, special solenoids). 6: U42 PIA (sound board). 7: U54 PIA (lamp matrix, special solenoids). 8: U10 PIA (sound outputs, solenoids, flipper relay). 9: IRQ fault. 10: U27 ROM checksum. 11: U26 ROM checksum. A PIA code can also mean a fault in that PIA's circuits rather than the chip.
Systems
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Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)