Testing the power supply with its outputs unplugged, the meter reads over 100 V DC on the J3 pins that should carry about 4 V and 8 V, while other pins read correctly.

System 1 power supply shows about 110 V where 4 V and 8 V belong: wrong ground

Cause: A measuring error, not a fault: the 4 V and 8 V supplies are referenced to the normal logic ground, while the display high voltages (about 60 V and 42 V) use their own separate common return. Measuring the low voltages against the high-voltage common gives a false high reading.

Checks

• Put the meter's black lead on the logic ground for the 4 V and 8 V pins, and on the separate high-voltage common for the display supplies.
• Correct readings should then be close to 4 V and 8 V.

Fix

• No repair needed if the readings are right against the correct ground. Do not tie the display high-voltage return to the logic ground when doing the usual ground mods; it is meant to stay separate.

The display supply on this board is high voltage. Keep fingers and probes clear of those pins with the game on.

Systems

• Gottlieb System 1

Related tips

• No +5 V or only 2.4 V from the power supply
• System 1 power supply outputs
• Power supply output connector pins
• Power supply voltage drifts or overvoltage trips
• Quick checks on the similar System 1 power supply
• Replacing power supply filter capacitor C1
• Acceptable System 1 supply readings
• The mandatory System 1 ground modifications

Source: Pinball Repair help group!! (Facebook group)