The 100 V outputs stay low even with the displays disconnected.

Displays weak with low 100 V even unplugged

Cause: Zener diodes D6 and D8 (1N5228) have started to leak.

Checks

• Replace R8 and R9 first and recheck.

Fix

• Replace D6 and D8.

The display power supply carries about 100 V or more; work on it with the game unplugged and let the capacitors discharge.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3

Related tips

• Run alphanumeric displays at 91 V for longer life
• Alphanumeric displays dim or dead
• Alphanumeric displays show only a few digits at a time
• Rebuild the alphanumeric ±100 V section
• Alphanumeric supply: parts for the +100 V side
• Alphanumeric supply: parts for the -100 V side
• DMD dark after work in the backbox: transformer wire pulled out of its plug
• Score display boards by game range

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)