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Re: [Amps] repair of older Command Tech VHF-2000 amp

To: "Mark Bitterlich" <markbitterlich@embarqmail.com>, <amps@contesting.com>, "Louis Parascondola" <gudguyham@aol.com>
Subject: Re: [Amps] repair of older Command Tech VHF-2000 amp
From: "Carl" <km1h@jeremy.mv.com>
Date: Mon, 14 Oct 2013 18:26:25 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
I didnt say it wasnt. Ive no idea what temperatures are encountered in those crammed USAF equipment racks. My 432 amp has a 2700W Pd tube in order to get 1000W as an AM linear; all that heat goes somewhere and the manual doesnt indicate what the external coax should be.

My relay, feed and antenna system cant handle close to what that tube can do on CW/SSB ! The most Ive pushed it is 1200W and normally run 800W which is 3dB above the AM-6155.

Carl


----- Original Message ----- From: "Mark Bitterlich" <markbitterlich@embarqmail.com> To: "Carl" <km1h@jeremy.mv.com>; <amps@contesting.com>; "Louis Parascondola" <gudguyham@aol.com>
Sent: Monday, October 14, 2013 4:46 PM
Subject: Re: [Amps] repair of older Command Tech VHF-2000 amp


When subjected to external heat, the best coax to use is a teflon dielectric.

Mark
wa3jpy



----- Original Message ----- From: "Carl" <km1h@jeremy.mv.com> To: "Mark Bitterlich" <markbitterlich@embarqmail.com>; <amps@contesting.com>; "Louis Parascondola" <gudguyham@aol.com>
Sent: Monday, October 14, 2013 2:40 PM
Subject: Re: [Amps] repair of older Command Tech VHF-2000 amp


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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