Twilight Zone Eddy Boards

1 How does Twilight Zone keep track of where the powerball is?

Twilight Zone has two eddy sensors to sense magnetic balls. One is at the ball eject position in the ball trough. The other is located on the under playfield tunnel that feeds balls from the piano, gumball, camera and deadend holes back to the slot machine eject.

The under playfield sensor is rarely a problem but the trough eddy sensor is one of the worst problems in the game, second only to the clock. If the machine is having trouble keeping track of where the powerball is, it's probably due to a problem with the trough eddy sensor.

The machine discovers the location of the powerball by using a combination of four different methods:

• By using the eddy sensor in the under playfield tunnel. If a ball triggers the piano, camera or dead end switches and then the slot switch triggers *but* the eddy wasn't triggered in between, then we know the powerball must have gone through.

• By using the eddy sensor in the ball trough

• By failing to grab a ball successfully on the loop magnets.

• By counting the powerball as it goes in and out of containers during single ball play, such as the lock, gumball and trough.

So for example if the machine knows there's two balls in the trough and you drained the powerball during single ball play, it'll be out the trough again in 2 balls time. By doing this it's possible for the software to work around broken/failing eddy sensors.

2 Typical problems

• Machine says "Powerball" when the powerball is not in play or vice versa
• A credit dot with a "Switch 26" error.

3 Causes

3.1 Connector Spring Failure

The trough eddy board and its sensor are joined by a small cable. With the vibration from the trough, the brass connector pins lose spring and the connection becomes unreliable. There are two solutions. You can solder the wires directly to the boards, or you can buy or make a new cable.

3.2 Cracked / Bad Solder Joints on Sensor

The following applies if trough detection problems of the poweball persist, even after the leads of the trough sensor board (WMS / BLY technically refers to it as an A-16528 "Trough Coil Assembly"), have been directly soldered or replaced. Carefully inspect the four solder leads where the sensor is soldered to the board. Due to vibration, the solder joints can become cracked, and cause the sensor to sporadically detect the powerball.

3.3 Damaged Sensor

The sensor is adjustable, and needs to be within 1/16 of the ball to work. If it is too close, the ball hits it and destroys the plastic. If it gets to the wire, the sensor will permanently fail. Although the sensor may look beaten up, the problem is still usually the cable.

3.4 Out of Tune Circuit

The design doesn't accommodate all variations of parts and conditions, and may end up being too robust and never respond to a close ball - or it may never start up. One fix to try for a stuck red light is to add some twists to the cable, although be careful as the cables/connectors are known to fail easily.

There are two modifications suggested to allow adjustment to the circuit allowing it to be "tuned in" with more accuracy. The first, suggested on http://www.actionpinball.com/tech/tz_prox.htm is bogus and does not work except by accident as suggested by Kenny McCormick. Undertaking this modification will not help the circuit to function. He further suggests a modification replacing the 2.2K ohm R1 resistor with a 20K ohm potentiometer. https://groups.google.com/forum/#!topic/rec.games.pinball/1cvD4hZO9E0

Recommended is removing R1 and replacing with a 10K ohm pot in series with a 1K ohm resistor (1/4w). The 1K ohm resistor acts to prevent the pot from being adjusted to a dead short. One of the end leads of the pot is left unused and clipped off, the other is placed in one of the vacated R1 PCB holes The middle connection of the pot goes to one end of the 1K ohm resistor the other end of the 1K ohm resistor is placed in the other vacated hole of R1. This completes the modification, the board is then reinstalled and the pot adjusted so the LED just goes out with no metal ball in proximity. The LED ought to then light when a ball is placed in the trough.

3.5 Failed Optocoupler Output

The output signal is driven by an optocoupler across the switch matrix. A short to the switch matrix can destroy the optocoupler transistor. Replace the optocoupler if the LED works correctly but there is no switch signal.

4 If All Else Fails

A replacement board is available from pinbits.com that uses the original sensor and new, adjustable board.