The diagnostic LED (D20/LED203) never blinks and the blanking LED (D19/LED201) stays on.

CPU won't boot, blanking LED stays on: check the game ROM

Cause: A bad or wrong game ROM at U6 (G11 on WPC-95): bent pins, fitted backwards, the wrong EPROM size for the code, lost data (unlabelled window), or a bad program.

Checks

• Check the ROM notch matches the socket notch and no pins are bent.
• Check the EPROM size is the one the code needs (for example 4 meg 27040).
• Verify the checksum in an EPROM programmer or try the ROM in a working game.

Fix

• Refit or replace the game ROM.

Systems

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

Related tips

• Dead CPU: no clock signal at the 6809
• Dead CPU: check the reset and IRQ lines
• Why coils stay off until the CPU boots
• CPU board completely dead
• What is on the WPC CPU board
• Which System 11 CPU boards fit which games
• The CPU must be a 68B09E
• Dead CPU: isolate it from the other boards

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