Reset and clock are good, but the CPU LEDs stay locked on or off.

Good reset and clock but LEDs locked: check IC17 and its bus lines

Cause: Bad EPROM at IC17 or its socket, or a broken or shorted address or data trace.

Checks

• With a logic probe on IC17 check address A0-A7 (pins 8 to 1), A8-A10 (pins 23, 22, 19), chip select (pin 20) and data D0-D7 (pins 9-11, 13-17). Most should pulse; A8-A10 may not.
• At CPU IC1 check A0-A7 (pins 9-16), D0-D7 (pins 33-26) and A8-A10 (pins 17-19).
• Any line not pulsing: check continuity from the buffer chips to every ROM and RAM chip.

Fix

• Replace the EPROM or socket, or repair the trace.

Systems

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

Related tips

• CPU locked: check the R/W and VMA lines
• CPU locked: check the EPROM chip select
• System 3/4 CPU: no clock at the 6800 or MC6875
• Add two test LEDs to a System 7 CPU board
• Why the CPU LEDs stay on when the program does not run
• Leon's test EPROM fitted but the LEDs do not start flashing
• Check the CPU address and data bus for shorts
• System 7 CPU LEDs are swapped versus System 6

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