I've been shunt-feeding my tower for awhile now using an L-network
and a motor-drive to adjust a vacuum variable. Until recently, all
has been just hunky-dory. However, last night it appeared that my
motor drive no longer worked as I couldn't adjust the match. What's
more, the match started to act flaky, with variations that appeared
to act randomly.
Today I opened up the box that holds the matching network and found
that there was some water inside and that the nuts holding the SO-239
connectors had corroded on the inside. Water had deposited some dirt
and detritus on the inside of the SO-239 and it was wet. So, I
cleaned everything out with alcohol (91% isopropyl).
Found the motor drive was a bit cranky, so I lubed it up and assured
my self that it worked as it was supposed to. But, no matter how I
adjust things, I no longer can get a match. I checked the vacuum
variable and it appears ok -- the mechanism works and all and it's
not shorted. All the wiring is intact. The shunt is still connected,
the grounds good, the antennas up top still work and are intact, etc.
The capacitor that's in there is a 5-500 pF, so as a sanity check I
substituted in a 7-1000 pF that I have laying around. Still no match
can be found. The transmission line checks good, so that's not it, either.
Anybody else ever seen this? What have I missed?
Kim Elmore, N5OP
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