Playfield coils energize on their own, overheat and burn, taking out their driver transistors.

Coils lock on and burn because of the Gottlieb ground fault

Cause: Cabinet ground and board ground meet at one point on the power supply. When that link gains resistance, driver board current lifts the board ground; at about 0.5 V above cabinet ground the solenoid drivers start to turn on. A second issue is a poor ground between the CPU and driver boards through one edge connector pin: 0.1 V difference or more can lock up or damage the boards.

Checks

• Compare CPU board C1 negative with CPU A1-J4 pin A, and with driver board Z12 pin 8 (the farthest ground).
• Compare CPU C1 positive with A1-J4 pin B and with driver Z12 pin 16 (the farthest +5 V). The smaller the difference the better.

Fix

• Do the full mandatory ground modification: extra ground wires from the CPU, driver, lamp and sound boards to the power supply frame and cabinet ground, and new connector pins.

Systems

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

Related tips

• Test the driver transistors while the board is out for the ground mod
• Test driver board outputs by grounding transistor tabs
• What the System 80 driver board transistors drive
• System 80 driver board transistor types
• Relay and coin counter transistors on the driver board
• Some 80B driver boards use the coin counter transistors
• Driver board transistors that look stuck on in a test fixture
• First power-up, step 5: add the driver board, then coil power

Source: Repairing & Upgrading Gottlieb System 80 Pinballs from 1980 to 1989, Part One, Clay Harrell (pinrepair.com)