The A8 board's LED is lit solid instead of flashing (it normally flashes steadily, a little slower than the CPU LED), often with garbage on the dot matrix.

DMD controller LED stays on

Cause: The custom GAL logic chip at U8 on A8 has failed.

Checks

• Confirm the CPU board LED flashes normally.

Fix

• Replace U8 with a programmed GAL for this board.

The A8 controller makes roughly +62 V, -100 V and -112 V for the display; keep hands off it with power on.

Systems

• Gottlieb System 3

Related tips

• Garbage on the dot matrix at power-on
• The A8 dot matrix controller board
• Dot matrix dark: +62 V missing
• Display voltages the A8 board should supply
• Never hot-plug the A8 board or the display
• Rebuilding the A8 high-voltage section
• Nonsense message at boot, game stuck in a loop
• A5, A6, A8 and A20 plug identification

Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)