A coil or flashlamp energizes as soon as the game is switched on and stays on.

One or a few coils lock on at power-up

Cause: A shorted TIP120/TIP102 driver transistor, sometimes along with its 2N4401 pre-driver, 7408/7402 gate or the IC5 solenoid PIA. (If almost all coils come on, see the blanking signal instead.) Pops and slings can also lock on from a stuck trigger switch.

Checks

• Switch off at once to save the coil.
• Check whether it is a special solenoid (pop or sling) with a stuck trigger switch.
• Diode-test the driver transistor and its pre-driver, then the gate chip.

Fix

• Replace the TIP120 with a TIP102 and replace its 2N4401 as well.
• Replace the 7408/7402 if it tests bad; replace the PIA if the outputs are wrong.
• Check the coil diode and coil resistance before powering up again.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Diode-test the TIP120/TIP102 and 2N4401
• Solenoid driver test at 2J9 and 2J11 with Leon's code
• A coil or flasher comes on and stays on at power-up
• Flash lamp stuck on and melting plastics on L/R reversed games
• Quick check of all TIP122/102 drivers before first power-on
• Quick test for shorted TIP122/TIP102 drivers
• CPU and driver board transistors by position
• Test a TIP120/TIP102 solenoid driver with a meter

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)