Heres some info:Adjusting Eddy Sensor Boards.
Often eddy sensors can go out of adjustment and become less sensitive. This can cause the eddy sensor to not activate when a ball passes above it on the playfield. To adjust an eddy sensor do this:
* On the under the playfield eddy sensor control board, turn the potentiometer counter-clockwise until the LED just turns on.
* Now turn the potentiometer back clockwise until the LED just turns off.
That is all that is required to adjust the STNG/ToM/RS eddy sensor. To test the sensor, put the game into WPC diagnostic's first switch test. Then move a pinball over the playfield area where the eddy sensor is located. The switch should activate on display. Also from the bottom of the playfield, the eddy board LED should go ON as a ball passes in front of the eddy board's senssor (this can be seen anytime, the game does not need to be in switch test.)
Different R1/C1 Eddy Sensor Values (Fine Tuning).
Because the ball sensors are different on some games, the value for R1 on the Eddy sensor boards can be different. For example, on Star Trek Next Generation and (two of the eddys on) Theatre of Magic), R1 is 4.7k ohms (these games uses the small ball sensor). But on roadshow and the ToM trunk, which uses a much larger ball sensor, R1 is 2k ohms. So if you switch an Eddy board between these games, the Eddy R1 resistor may need to be changed to the correct value.
The purpose of the R1 resistor is to make the adjustment pot "centered" for the particular ball sensor. For example, if you use a 2k ohm R1 eddy board in STNG, the adjustment pot will be turned almost all the way up (with very little adjustment range). It still works most of the time, just the adjustment range is not centered.
With this in mind, I once had a roadshow where I could not get the eddy board's LED to turn off, no matter where the adjustment pot was moved. Normally roadshow uses 2k ohm R1 resistors for all three eddy boards - but in this case I had to replace the R1 resistor with a jumper wire (0 ohms). This put the adjustment pot about dead center, and the eddy boards worked great (with the 2k ohm R1 resistor, the eddy boards would not adjust, and hence would not work.) Another trick is to change the C1 cap to 3300pF, which widens the field of the eddy sensor a bit