At power-on neither CPU LED lights (System 3-6) or the System 7 digit stays dark.

CPU LEDs (or System 7 digit) never light at power-up

Cause: No +5 V, a short on the board (often battery corrosion or solder splashes), both LEDs dead, or a failed LED driver chip.

Checks

• On System 6/7, check test point TP9 for +5 V.
• On all boards, check +5 V at CPU IC1 pin 8 (ground at pin 1) and at interboard connector pin 1 at the far right.
• If +5 V is missing, trace back for an open path or a short to ground. Check the 0.01 uF bypass capacitors at each chip for shorts and look for solder splashes.
• System 3 to 6: check +5 V at IC2 (8T28 LED driver) pins 2, 5 and 16. With good power but nothing on pins 2 and 5, check that the CPU is running; if it is, replace IC2.
• Remember both LEDs can be bad.

Fix

• Clear the short or restore power, then replace the failed LED driver or LEDs.

Systems

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

Related tips

• CPU LEDs come on and stay on with no flash
• CPU LEDs flash once, then come back on and stay on
• Add two test LEDs to a System 7 CPU board
• How a good System 3 to 7 CPU behaves at power-on
• Power a System 3 to 7 CPU board on the bench with a PC power supply
• System 7 CPU LEDs are swapped versus System 6
• Late System 7 CPU boards lack LEDs and DIP switches
• Is it the CPU board or the driver board? Boot the CPU alone

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