[TowerTalk] TH7-DX 15M problem

[Radio K0HB]

Responding to Dave --- the amp works fine on a different antenna on 15M, and 
good as gold on all other bands on any antenna.  Hard to suspect the amp.

Responding to Matt --- the SWR meter is external, downstream from the amp.

The balun is the 4K version, not the BN86 with the bad reputation.  Hard to 
envision a balun weird on a single band.





-----Original Message----- 
From: David Robbins
Sent: Wednesday, August 26, 2015 7:43 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] TH7-DX 15M problem

a problem with the amp?


Aug 26, 2015 03:28:05 PM, kzerohb at gmail.com wrote:

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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