[TowerTalk] TH3MK4 15M SWR fluctuation

Gary Smith Gary at ka1j.com
Wed Apr 2 13:46:35 EDT 2014


I guess this is a case of look first and ask questions later. The 
affected trap was on the driven element & was obvious from the melted 
plastic at the far end of the outer trap. The other traps looked 
fine. [I would though like to know if there's an easy way without 
uncommon test equipment to ID what the affected trap would be if it 
weren't this obvious].

I found several things wrong and don't know which started the cascade 

- The coil had a small area where it had arced to the aluminum 
cylinder/capacitor surrounding the coil though I couldn't readily see 
where there was evidence of that on the inside of the sleeve.

- There appeared to have been arcing between the fastening screw at 
the coil at the proximal end of the trap and the aluminum element; 
perhaps there was a weak connection causing the arc.

- There are four plastic inserts that hold the sleeve in proper 
alignment over the coil and have slots to provide drainage. The 
distal one was partially melted on one side and the end cap of the 
trap was melted badly. I found what appeared to be a charred object 
blocking the one hole, one hole was well melted through on both sides 
and the 3rd hole was clear and not affected. maybe an insect/nest as 
suggested was in the hole and that started the arcing.

My impression is there was an arc which laid down a carbon trace and 
the high voltage present now had a persistent path to follow.

* I disassembled the trap and cleaned all the connections, 
resoldering the terminals on the coil with silver solder. and 
attaching them securely. 

* I was able to remove the plastic inserts. Thinking that even though 
I tried scraping and cleaning the damaged one, there was some some 
carboning impregnated in the plastic and in places hard to get to. I 
cleaned it the best I could and exchanged it with one from the other 
side of the trap, next to where the sleeve was attached to the 
element where there would be no HV issues.

* I wrapped the coil with teflon tape to discourage further arcing 
from the coil to the sleeve and ran some around at the end of the 
sleeve for the same reason. I used electricians tape to make a 
temporary trap cover & will order a new one from MFJ.

Just tried it out and its working fine now. Think I might raise the 
frequency as though I mostly only run CW, I've recently started using 
other modes as well and they're higher than the low end of the band.

Thanks for the reply.


> they are pretty easy to disassemble Use new ss hardware and clean 
them out. Insects love these traps and make nests in them.
> George NE2I
> On Apr 2, 2014, at 7:21 AM, Gary Smith <Gary at ka1j.com> wrote:
> > I rehabbed my father's old TH3MK4, renewing the electrical contacts, 
> > replacing the traps and installing a new DXE-BAL050-H10-A balun. It 
> > worked well on all three bands for the last week and its really a 
> > pleasure to have access to a beam again as I've only been able to use 
> > vertical wires since 1995.
> > 
> > But last night I noticed the SWR was wildly swinging up to 5:1 on 15M 
> > when using the amp and when running barefoot it dropped back to 1.2 
> > as it had been at before. More testing finds that anything over 
> > 70-80W raises the SWR and it remains stable at 1.2 below that power 
> > level. 10 & 20 are unaffected and have a 1.2 SWR. I ran the amp into 
> > a separate 15M antenna and there was no issue with SWR so it appears 
> > isolated to the TH3.
> > 
> > I'm suspecting a 15M trap has failed and I either need to replace it 
> > or rehab it. I don't know how to tell if it is the driven element 
> > trap or one of the parasitic traps that has failed. Deciding which 
> > trap is involved is the first problem, my guess is it would be the 
> > driven element trap. 
> > 
> > Once I have that sorted out, are these traps easily reparable & if 
> > so, any caveats? I've done a google search but haven't found that 
> > answer yet.
> > 
> > Suggestions on how to determine which trap & how to best rehab it?
> > 
> > 73,
> > 
> > Gary
> > KA1J
> > 
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