Twilight Zone: adding an adjustment pot to the trough proximity sensor

Steps

• Get a small PC-mount trimmer pot with a low value, ideally around 100 ohm (1k ohm is the common low value but very touchy). The working setting is about 20 to 30 ohm, so meter the pot first.
• Power down and take the sensor board off the underside of the playfield (it is held by two hex-head screws).
• On the component side, cut the only trace, the one between the connector pin and the sensor, and confirm with a meter that it is open.
• Scrape the coating off the sensor end of the cut trace so a wire can be soldered to it.
• Mark and drill three small holes (1/16 inch or less) for the pot legs, fit the pot and bend its legs to hold it.
• Run one jumper from the back of the connector pin and another from the scraped trace to the pot: one to the centre leg, the other to either outer leg.
• Refit the board and cable and power up with no balls in the trough. Turn the pot until the driver board LED just comes on, then back until it just goes off.
• Test: one ball in the trough should light the driver board LED, and moving it away should turn the LED off.

Systems

• WPC Fliptronics

Games

• Twilight Zone

Related tips

• Twilight Zone: "clock is broken" error
• Twilight Zone: LED lighting mod for the clock
• Twilight Zone clock uses eight sensitive U optos
• Twilight Zone: diode mod to cool the clock with stock bulbs
• Twilight Zone ninth switch column (clock switches)
• Test report lists switches the game doesn't have
• Ninth switch column on Twilight Zone, Indiana Jones and STNG
• Random display lines and all flippers energising at power-on

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)