Interlock circuit on WPC pinball machines

Sometime during the production run of the Twilight Zone pinball machine, Bally/Williams introduced an 'interlock circuit' to their WPC pinball machines. This circuit includes extra wires that connect to the playfield wiring and a switch located near the coin door. When the coin door is open, solenoids and flashers no longer receive power.

You’ll know this feature is installed on your WPC game when you see the message: “COIN DOOR IS OPEN COILS AND FLASHERS ARE DISABLED” on the display.

You can also visually confirm it—there are now two switches inside the cabinet that the coin door closes, whereas older games only had one.

So what does it do? Exactly what the message says: it physically disables power to the flashers and coils. This safety feature helps prevent accidents. For example, someone might try to adjust a switch and accidentally short 60 volts into the switch matrix. It happens to all of us.. Or you might reach under the playfield without looking and touch coil lugs, sending 60 volts into your hand. Now you understand why disabling this power is a smart idea.

The downside is that coils and flashers won’t work during self-tests. To bypass this, you can either press the top button manually or use a special tool:

Interlock lock.

This yellow, oddly shaped tool was shipped with new pinball machines.

Its purpose is to slide over the top power switch, keeping it pressed in so you can run self-tests with power enabled for coils and flashers.

Important: this circuit must be plugged in! Many operators simply unplug it. If it’s loose, like in the picture, and not connected between the transformer and backbox wiring, the power won’t be cut when you open the coin door, even though the display says it is. Always double-check whether it’s plugged in. Don’t rely solely on the display message.

Note that older games, such as Stern’s Orbitor One and various Gottlieb machines, use a different type of cutoff switch. These will cut power to the entire game, including PCBs, when the coin door is opened. To run tests or make adjustments with the coin door open, you’ll need to pull the switch out to restore power.

Note:Around 2020 Stern Pinball has also started to implement an 48v cutoff switch. Similar to WPC games as described here, these machines will also cut the power to the coils when you open the coindoor.

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