Teflon is very prone to cold flow as are some foam cables.
Neither should be subjected to sharp bends and the manufacturers specs
should be followed.
RG-213/214 type dielectrics are much less prone to cold flow but are very
susceptible to heat flow from the center conductor.
Use of the correct cable and installation procedure for the job usually
works.
Several VHF amps use a coax stub filter to meet FCC requirements. If the end
isnt properly finished corona can develop and eat away the dielectric, even
Teflon.
My 432 amp is ex military and was used as a 1000W output AM linear and the
internal interconnects to the Type N output connector have no sharp bends,
just a few gentle swoops.
Carl
KM1H
----- Original Message -----
From: "Mark Bitterlich" <markbitterlich@embarqmail.com>
To: <amps@contesting.com>; "Louis Parascondola" <gudguyham@aol.com>
Sent: Monday, October 14, 2013 12:44 PM
Subject: Re: [Amps] repair of older Command Tech VHF-2000 amp
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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