info on the K9AY loop
Listening test and other observations
How to make your own K9AY
The importance of good ground
How to place wires best
of the KPAY
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
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
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.