How to Fix Nuisance Resets on WPC Games

by Spike Tsasmali

Do you have a WPC game that resets when you hit both flippers at the same time? Have you spent hours tracking down this problem? Changed out boards, replaced capacitors, ribbon cables, tightened screws? Well there is a hidden "component" lurking in the dark that causes this problem -- especially on IMPORTED WPC games.

Remove the glass and raise the playfield. Locate the POWER SWITCH BOX just to the RIGHT of the coin door on the inside of the cabinet. Remove the four 1/4" screws holding this assembly to the cabinet bottom. Be sure the game is unplugged and remove the two hex nuts holding the two case halves together. Look inside this assembly and find the THERMISTOR (small round ceramic component) or SAND RESISTOR (large square-rectangular ceramic component) connected to the FUSE. REMOVE THIS COMPONENT and BYPASS IT with a piece of #18 gauge wire.

You will now find that the flippers won't reset your game anymore!

Why is this the fix and what purpose does this part serve?

This part is Williams answer to "soft start". On US (DOMESTIC) machines, the THERMISTOR is used. The thermistor has a NEGATIVE temperature coefficient, meaning as it gets hot it conducts more. When the game is first switched ON, the current load drawn by the transformer at start up is "cushioned" by this thermistor. As the game draws power, the thermistor warms up and allows more power to make it to the transformer, thus creating a "soft start" condition.

On European games which run on 220-240v, this thermistor is replaced with a SAND RESISTOR which acts as a "current limiter". Since the current drawn is 1/2 that of the current used when operated at 110v (ohm's law), the "soft start" is natural and does not need any help.

What happens is one of two things:

(1) On domestic games, this thermistor breaks down due to age and begins to "not conduct" as well. It eventually gets so that when it gets hot it no longer increases conductivity and begins limiting the current the transformer can draw. When both flippers are flipped simultaneously, this current draw is hampered by the bad thermistor, causing the voltage across the entire transformer to drop below RESET threshold.

(2) On imported, former European games, which are equipped with a SAND RESISTOR, this sand resistor limits the available current making it to the transformer and a similar effect to that of a failing thermistor occurs.

Williams felt that it was necessary to include a "soft start" to protect certain circuits however this is not necessary since the game's power supply is well regulated.

The thermistor or sand resistor can be safely removed. Once removed you will not notice a thing different about your game except for the fact that it no longer resets when both flippers are flipped at the same time.

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