A fuse keeps blowing, and with the game on the metal frame of the power supply (voltage regulator) board, and the screws holding the board to it, measure about 12 V DC to ground.

Gottlieb System 80: 12 V on the regulator frame, fuse blows: missing insulator

Cause: The insulator between the large power transistor and the metal frame was missing, so the transistor case and its mounting screws shorted the 12 V supply to the frame (and so to ground once the ground mods are done).

Checks

• With the power off, measure resistance between the metal case of the large transistor and a bare metal spot on the frame or heat sink: it should be open circuit.
• Remove the transistor and look for the insulator; the original one also has sleeves that go into the mounting holes.

Fix

• Fit the correct insulator under the transistor so it and its screws are fully isolated from the frame.
• If the original style is not to hand, sleeve the mounting screws with short pieces of wire insulation, insulate the transistor leads where they pass the frame, and recheck for an open circuit to the frame.

Unplug the game before working on the power supply; the transformer side carries line voltage.

Systems

• Gottlieb System 80
• Gottlieb System 80A
• Gottlieb System 80B

Related tips

• Ground mod for the System 80B power supply
• Which System 80 power supply boards are interchangeable
• How the 80/80A power supply makes its four voltages
• Typical voltages inside the 80/80A power supply
• Power supply output wrong
• Power supply MA-114 regulator and zener parts
• Power supply LEDs show voltage present, not correct
• Tie the 80/80A power supply logic ground to its heat sink plate

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