[BC] Te st your wits on this

[Thomas G. Osenkowsky]

I was called upon by a transmitter manufacturer to perform field service on
a new 3500 Watt transmitter. Intermittent problem was the tx would go off
air. High plate voltage (single tube), no plate current. No IPA current with
high IPA voltage. Both higher voltages due to no loads. Exciter power
normal, exciter VSWR to IPA very low.

Client would open rear door, wiggle things around, normal operation would
return for a while. I modified the tx control to allow me to feed my Bird 43
and dummy load with all PA voltages off. Exciter power adjusted for about 75
Watts out of IPA. Using original jumper cable, simply moving the wattmeter
caused power to drop out. Cause was a sliver of brass shorting from the
center pin of the N male connector to the body. This was left over from the
Amphenol factory.

At another installation of the same model transmitter it would go into plate
current OL during antenna icing. Plate resistance was decreasing as antenna
iced. I was fortunate to have coiled up excess cable and removed 90
electrical degrees of line. Now, plate current decreases as antenna ices up.

Moral to the above: The longer jumper you used is transforming the impedance
the driver "sees" to a more acceptable value. A mismatched line lossy will
not have the same input and load impedance.

Tom Osenkowsky, CPBE

>AFAIK, my jumper is still in the FM-35...........