Louis,
This phenonamom used to be seen quite regularly in certain military aircraft
where coax types such as RG-214, RG-213, RG-217 were used. Any kind of
medium to sharp bend at all, co-located with a heat source caused it as you
mentioned. Typically around turbine compartments or bleed air lines.
All of these coax runs were eventually replaced with Teflon dielectric
types, such as RG-142 & RG-393. That lesson learned, all of my amps have
been re-plumbed with the same teflon type coax. :-)
Mark Bitterich
wa3jpy
----- Original Message -----
From: "Louis Parascondola" <gudguyham@aol.com>
To: <amps@contesting.com>
Sent: Monday, October 14, 2013 12:17 PM
Subject: [Amps] repair of older Command Tech VHF-2000 amp
I performed what seemed to be a rather odd repair on an older Command Tech
VHF-2000. I thought I would share this since it could possibly affect the
HF models. This particular amp like most has a piece of coax that comes
off the output of the tank circuit and goes over to the antenna relay. In
this case a harmonic filter. The coax was affixed to the side of the wall
and dove down to the bottom and over to the relay. Filter in this case.
There was what seemed to be a soft arc to the coax as it bent over the edge
and down. The amp exhibited no output whatsoever and acted like there was
not antenna connected. I tracked down the problem to that coax being
internally shorted. An autopsy of the piece of coax was amazing.
Apparently the owner of this amp operated digital modes like JT65 so I
would think there were many periods of heavy duty cycles. Apparently over
time the dielectric center conductor heated up and little by little the
wire migrated to the outer edge and e
ventually broke through at the "arc" strain point and shorted to the
braid. Several inches of the center wire was way far away from the center
of the dielectric. Go figure.
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