With Leon's EPROM fitted, the CPU LEDs do not flash.

Leon code running but LEDs dark: bad CPU PIA IC18

Cause: The display PIA at IC18, which drives the LEDs, has failed even though the test code is running.

Checks

• Connect a tester LED with its plain lead to +5 V and touch the resistor lead to CPU IC1 pin 15 (use the LED, not a logic probe).
• If the tester LED flashes about once a second, the program is running.

Fix

• Replace IC18 (6821) and fit a socket.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Test the CPU board PIA IC18 with Leon's code
• Test the switch PIA IC11 with the Leon test EPROM
• CPU locks up as soon as the driver board is connected
• Damage after the black and white plugs were swapped
• Test EPROM flash codes: which chip failed
• A PIA output fails Leon's test but the PIA is good
• Test the driver board PIAs IC5, IC10 and IC11 with Leon's code
• CPU flickers then gives three flashes: bad PIA at U10

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)