Game does not boot, and LED D20 on the CPU board blinks a pattern.

Reading the CPU board LEDs at power-on

Cause: At power-on D19 and D21 light, then D19 goes out, D21 stays on and D20 blinks fast when all is well. A counted blink code points to a chip: 1 blink U6 game ROM, 2 blinks U8 RAM, 3 blinks U9 ASIC.

Checks

• Watch the three LEDs at the upper left of the CPU board as the game powers on.
• Count D20's blinks.

Fix

• Reseat or replace the chip the code points to, and check its socket for bent pins.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• WPC CPU board LEDs at power-on
• CPU diagnostic LED blinks a code instead of flashing steadily
• WPC CPU board LED meanings
• Reboots at start-up and when flipping: flaky power connector to the CPU
• Won't boot after a ribbon cable was fitted one row off: unplug boards to find the damage
• Factory settings restored at every power-up after new batteries: do a factory reset
• Resets: +5 V at the driver board test point is low
• Game keeps resetting during play: poor J101/J102 power connections

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part One, Clay Harrell (pinrepair.com)