A coil locks on even though drivers test good and the ground mods are done.

Coils lock on from a playfield ground pin

Cause: Each playfield ground runs separately through a Molex plug to the copper strip; one high-resistance pin lifts that ground.

Checks

• Inspect and test the playfield ground pins at the plug.

Fix

• Tie all the ground wires together on the playfield side of the plug so one bad pin cannot break the path, and insulate the splice.

Systems

• Gottlieb System 1

Related tips

• Why System 1 coils lock on: the ground shift
• Games with under-playfield 2N5875 driver transistors
• A shorted coil means replacing its whole driver path
• Coils or driver transistors run very hot
• Kings drop bank never resets: under-playfield transistor wired wrong

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)