[TowerTalk] Cushcraft/MFJ Traps

[Don]

My in the air TH7DX was purchased in 1983 if I recall correctly, not to 
long after it was introduced in '82(?). I also have a second which 
purchased out of an estate type sale that was up for a period of time.

I had zero problems with the antenna or antenna traps until I moved to 
the coast and found myself looking down on the ocean some hundreds of 
feet away. Mucho salt spray and air. Common steel frame lawn 
chairs/tables and common plated steel hardware rust out in less than two 
seasons. I now know every nearby store who carries SS hardware now.

Two years ago I began having 15 and 20 M SWR problems. I was able to 
isolate the issues to the traps. I removed and cleaned up the15 M and 10 
M traps in DE 1 and the R which at the time was diagnosed as the problem 
area. Found the screws used to fasten the two ends of the coil to the 
element tubing were common cheap plated thread tapping screws and were 
corroded to the extent that little was left of the screws in a couple of 
cases. After removing the screws and cleaning up the surfaces, I 
replaced with SS screws using an antiseize compound my electrician uses 
for an interface between aluminum and SS. Rechecked after back up and 
this cleanup had solved the problem. Now I am seeing some SWR issues 
again on 15 and 20 so suspect I will have to do the same cleanup of the 
DE2 and also the D (yes, should have done them all at the same time, was 
in a hurry, bad decision). Will also check the matching assy. I have not 
seen any problem with the coil forms or with arcing in the coils and I 
do run the full legal limit when necessary.

Summary, I'd bet the most common problem with a TH7 trap is the screws 
used in the traps failing. Might take longer for them to corrode in non 
salt air areas but they will. I've got pictures of the trap interiors 
and the screws in the traps before rework I'd be glad to share, ugly! I 
did replace the plastic end caps with new ones from MFJ when 
reassembling, almost all the originals were split. These traps are easy 
to work on and I'd sure suggest any time someone has a TH7 down for any 
reason to do an examination of the traps. Check out the screw holding 
the aluminum trap cover too, also had to replace them.

Don T W7WLL

On 9/12/2019 3:07 PM, K9MA wrote:
> Since MFJ also owns Hy-Gain, it wouldn't surprise me if they were 
> using a common trap design. If so, I'd suspect the Hy-Gain traps would 
> have the same issues. Has anyone seen the insides of one?
>
> I can't tell what kind of material they're using for the coil form. 
> Two of the MFJ traps are currently on my A3, but I think I'll put the 
> originals back.
>
>
>
> 73,
> Scott K9MA
>
> On 9/12/2019 16:54, Glenn Pritchard wrote:
>> Scott,
>> That looks a lot like the Hy-Gain trap design they used years ago 
>> that MFJ is now using. They had that plastic trap form that cracked 
>> easily under pressure. I had an old 18AVT that used those, meh.
>> Why on earth they went cheap is beyond anyones guess.
>>
>> Glenn, VA7UO
>>
>> On Thu, Sep 12, 2019 at 2:44 PM K9MA <k9ma at sdellington.us 
>> <mailto:k9ma at sdellington.us>> wrote:
>>
>>     Here are links to photos of the A3 TC 15m trap, the original
>>     Cushcraft
>>     version circa 1992 and the recent MFJ one:
>>
>>     sdellington.us/hr/TC_CC1.jpg <http://sdellington.us/hr/TC_CC1.jpg>
>>
>>     sdellington.us/hr/TC_MFJ1.jpg <http://sdellington.us/hr/TC_MFJ1.jpg>
>>
>>     Note the two pop rivets in the CC trap, missing from the MFJ one.
>>     There
>>     are two more rivets on the other side. The extra screw in the CC
>>     trap is
>>     one I added.  Note also that the wire on one end of the MFJ coil is
>>     wrapped the wrong way. Originally, both terminations looked much
>>     worse.
>>     Later, I wrapped both the right way, and added SS flat washers. I
>>     also
>>     fixed that almost shorted turn.
>>
>>     The MFJ trap has 4 fewer turns than the CC trap, though the
>>     resonance is
>>     about the same. It looks like the MFJ coil form is slightly larger.
>>
>>     I don't like to bash MFJ, as I think they provide a useful 
>> service to
>>     the ham radio community, but I do wish they paid more attention to
>>     QC,
>>     especially for antennas, installation of which involves a lot of 
>> work
>>     and expense.
>>
>>     73,
>>
>>     Scott K9MA
>>
>>
>>     On 9/12/2019 13:22, K9MA wrote:
>>     > I recently ordered some replacement traps for my old A3S. One was
>>     > defective, so I took it apart. I was appalled at the poor quality.
>>     > While the original traps had several pop rivets, in addition to 
>> the
>>     > coil screws, connecting the tubing to the plastic insulator, the
>>     MFJ
>>     > version had only the two screws in the very end of the tubing. The
>>     > coil wires were sometimes wrapped the wrong way around the
>>     screws, and
>>     > some of the screws weren't even tight. Where the original coil was
>>     > coated with some kind of thick lacquer, the MFJ ones were bare.
>>     >
>>     > The moral of the story is don't even think about going to the
>>     trouble
>>     > of putting up an MFJ/Cushcraft beam without taking apart all the
>>     traps
>>     > and fixing the above problems.
>>     >
>>     > 73,
>>     >
>>     > Scott K9MA
>>     >
>>
>>     --     Scott  K9MA
>>
>>     k9ma at sdellington.us <mailto:k9ma at sdellington.us>
>>
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>>
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>>
>> -- 
>> /Never believe and Atom, they make up everything../
>
>