Twilight Zone Status

Trough proximity switch #26 error

This fault can be one of the most frustrating faults because it often re-appears.

The trough proximity switch is basically a metal detector which along with a normal switch is used to detect the type of ball, metal or ceramic, which is about to be fed from the trough.

The whole proximity switch assembly consists of a small Trough Proximity Board A-16528 (TPB) containing an inductor (coil) which is placed close to the last ball in the trough. This is connected by two wires to the larger Proximity Sensor Board A-16534 (PSB). The PSB has an LED on it to show its state of detection. Unfortunately the LED works in a manner which is not as useful for fault finding as it might be.

Condition	LED State
Metal ball in last trough position	On
Ceramic ball in last trough position	Off
Disconnected lead between the two boards	On

It is the final position which causes the problem as a faulty lead give the appearance of a metal ball being detected. We therefore come to our first conclusion.

1) Always test the board with a ceramic ball in the last trough position.

I've measured the resistance of the inductor and it is less than 2 ohms. Therefore detecting a broken inductor should be straight forward. I've attempted to wind my own inductor but have not as yet been successful. Because of the very low resistance of the inductor, only a slightly bad contact will be enough to cause the LED to come on. Because of this and because we have all suffered from these bad connections we can make our second conclusion.

2) Assuming the inductor is okay, if there is a ceramic ball in the last trough position and the LED is on, the connector on the PSB is bad.

I don't believe that the Molex connectors were ever designed for such low voltages/currents and it doesn't matter how often I clean or bend or tin the pins within a few weeks the problem appears again. So I tentatively come to the third conclusion, tentatively because I have myself refrained from doing this one (as yet).

3) Stop wasting your time and just solder the wires onto the PSB!

I hope this has been of some use to you. If I even manage to wind a test/new inductor I'll post the details here.

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