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@sdellington.us
<mailto:k9ma@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@sdellington.us <mailto:k9ma@sdellington.us>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com <mailto:TowerTalk@contesting.com>
http://lists.contesting.com/mailman/listinfo/towertalk
--
/Never believe and Atom, they make up everything../
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
