[TowerTalk] TH7-DX 15M problem

Don W7WLL w7wll at arrl.net
Wed Aug 26 19:35:11 EDT 2015


Some questions, some probably obvious!

1. Full set - you mean all 6 15M traps??  If a one band issue and the SWR is 
high (5-10)  I'd be looking for the problem in the DE's. If lower (assuming 
it was working below 3:1 before) in the 3-5 range and the DE traps check out 
OK, I'd be looking at the reflector traps. Examine the screw between the 
inner and outer trap tube inside a trap (none are SS and none have any 
factory applied non-corrosive compound, at least that I've found).

2. Are you using the original BN-86 excuse for a balun?  I have burned out 2 
of these using an honest amp.

3. You might also check the two crossover straps used on the beta match 
phasing tubes. Being aluminum they are subject under vibration to cracking. 
I just experienced this, one totally cracked and clearly disconnected and 
the other cracked but not totally and not obvious without close examination. 
I've had two of these TH7DX's up and now keep one for spare parts for the 
one in the air.

Don W7WLL

-----Original Message----- 
From: Radio K0HB
Sent: Wednesday, August 26, 2015 12:21 PM
To: towertalk at contesting.com
Subject: [TowerTalk] TH7-DX 15M problem

I have an odd issue with a TH7-DXS.

A while back I started to see a higher-than normal SWR, just on 15M, and 
just when running some smoke.

While barefoot the problem never arises.

The problem never happens on 20M or 10M under any circumstances.  I can put 
a brick on the key at full legal limit on those bands, and the SWR is 1:1 
forever.

If I key down full power (actually any power over about 500W) the SWR will 
be fine for about 20-30 seconds, then will rise (not instantly, but “ramps” 
up) to about 2.5:1 across the 15M band.  If I unkey, kill the amp, then key 
up barefoot the SWR drops back to 1:1 and stays there forever.

It is as if something heats up under power, then changes value.  BUT ONLY ON 
15M.

I suspected one of the 15 traps, and hunted down a full new set for a very 
nice price.

This morning I replaced all of the 15M traps, and freshened up all the slip 
joints in the antenna.  It looks like new!

The problem persists.

Suggestions?

73, de Hans, K0HB
--
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