The Twilight Zone Switch 26 Eddy Board
This board sits near the ball trough. It uses a coil on a PCB to sense the ball in the #1 position in the trough. The system suffers from a number of problems...
First, let's take a look at what we are talking about. These pictures show two versions of the board. One is from an original machine; the other is a reproduction set sold by Marco. Near as I can tell, this product was sourced in China and manufactured some time in 2006 or early 2007 - it is not an original part. It is hand-assembled. Unfortunately, it appears to be causing trouble to buyers. There's no obvious reason for this problem.
Here's the controller board:
And this is the sensor board.
How Does it Work?
On the bench, The eddy board is a bit different to most other systems, in that the LED should be off. If the LED stays on, the system is not working correctly.
The device works a little bit like a metal detector. It oscillates at about 3MHz; when a piece of metal gets near to the coil, it absorbs energy. This energy loss stops the circuit from oscillating. When the oscillation stops, the LED comes on. Of course, if the circuit doesn't oscillate, the LED will always be on.
And Why Doesn't Mine Work?
Unfortunately, even if there is nothing wrong the sensor may still not work. I've not yet figured this out, but the Marco replacements seem to be particularly unreliable. These boards are close to the edge of operation. I've not found a material difference in the component values, but I think that a component change may fix the board. Both C1 and C2 could do with a boost.
What Can I Do About It?
First, you can test it by holding the connectors from the two boards against each other. Hold it so that the outside pins on the coil board contact the two pins on the sensor board. If the LED goes out, your electronics are working. It is a bit fiddly, but achievable
Then, there are a couple of things to try. One is to put about a dozen twists in the connecting cable. This can help enough to move a board from unreliable to reliable.
Another is to solder the wires directly to the boards. This is not a problem for working on the machine, as the double assembly is easily removed.
If you are into electronics and have some resistors about, try tacking something across R1 (next to C1). Try a 10k, then a 5k, then a 2k. One of those should do it.
Ray Johnson of Action Pinball has a mod that adds a potentiometer to the coil side of the circuit, along with a couple of other suggestions. This mod should make the circuit less sensitive, as it absorbs energy. I don't like this fix.