The 68 V output dies after repeated quick power cycling; the two DZ2 zeners are burnt, and the U1 LM317 regulator often with them.

Aftermarket "C" supply: 68 V zeners burn at power-on

Cause: Inrush current at switch-on overloads the pair of 1N4748A zeners (22 V, 1 W) at DZ2. It can also kill the U16 PAL on the display controller.

Checks

• Look for burn marks on the DZ2 zeners and the U1 regulator.

Fix

• Replace the pair with a single 1N4748 whose banded end goes to a 10 ohm 1/2 W resistor (the maker's revised layout), and replace U1 if needed.

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 3
• Data East/Sega CPU Version 3b

Related tips

• Resets: +5 V at the driver board test point is low
• Power supply gives no +5 V
• Zener diodes D10 and D11 by DMD power supply revision
• Power supply board versions are not interchangeable
• Z1 on the power supply is a varistor, not a diac
• Power supply board versions and their +5 V filter capacitors
• Power supply CN1 pinout, pins 1-6
• Power supply CN1 pinout, pins 7-12

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