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Check for worn out Vactrols

By George Maroti
October 14, 2000

When moving my loops closer to the house, I had a good chance to see how the relay/transformer box held up, and also to test everything.
Much to my disbelief, my Vactrol, type VTL5C2, was significantly out-of-spec. The resistance range was now on the order of 2,000 to 15,000 ohms, as opposed to 200 to infinite ohms.
I checked my controller box, dropping resistor, connections etc. before I un-soldered the defective Vactrol. Sure enough, for whatever reason, the Vactrol's characteristics had changed. I replaced it with a new one, and the expected range of 200 to infinite ohms was restored.

You can check your Vactrol resistance without removing the Vactrol from the circuit board.
I use a digital mulitmeter with the probes applied to one of the loop pair terminals, say the North-South terminals.
Then, I adjust the potentiometer in the control unit to the two extremes.
Normally, with the Vactrol I'm using, this range is between 200 ohms and an open circuit.

I don't know if this technique will work with the Wellbrook Units. I run separate control wires for the relays and Vactrol, and my coax just carries the RF energy from the loops.
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