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

[Mark Bitterlich]

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

Mark
wa3jpy



----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: "Mark Bitterlich" <markbitterlich at embarqmail.com>; 
<amps at contesting.com>; "Louis Parascondola" <gudguyham at 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 at embarqmail.com>
> To: <amps at contesting.com>; "Louis Parascondola" <gudguyham at 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 at aol.com>
>> To: <amps at 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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>>> http://lists.contesting.com/mailman/listinfo/amps
>>
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>