[Elecraft] [K3] Sudden Hi-SWR on low end of 40M

[Don Wilhelm]

Terry,

I suggest you look for something arcing or otherwise breaking down at 
some power level in your antenna system.
The K3 is showing you the SWR achieved at the input to the internal tuner.
Your LP-100 is showing you the actual SWR of the antenna.

The only reason your MFJ-269 would indicate a very different SWR from 
the LP-100 is that while your antenna seems just fine with low power, 
something changes when you increase the power.  That is usually from a 
failed common mode choke, or an insulator arcing over or something similar.

Try reducing the power of the K3 to 10 watts - now what does the LP-100 
indicate for SWR?  I would wager that it will be similar to that 
obtained from your MFJ-269.

73,
Don W3FPR

On 2/18/2013 11:54 AM, Terry Schieler wrote:
> Yesterday, during the CW contest, I noticed that my K3-100 (#474) suddenly began to "stutter" during CW transmissions on the low end of 40 meters.  I hadn't noticed this before, including Saturday, the first day of the contest.  My CW transmission had a "chattering" sound something like the braaaaap of a machine gun (and no, my fist isn't *that* good).  My first thought was RF getting into the radio, but, I've never had RF issues before and, again, nothing in the system had changed:
>   
> K3 > LP-100A SWR/Power meter > 4 position Ameritron RCS-4 antenna switch > dipole.
>   
> I have never found a frequency on 40-10 meters that my K3's internal tuner could not handle with the only antenna I've ever used with the rig.  But the tuner could not eliminate this chattering yesterday.
>   
> After the contest I did some checking and noted the following:
>   
> 1.  At 100W, the chattering only occurred from 7.0 to almost exactly 7.1 then went normal again.  CW was normal on all other bands.
> 2.  If I backed the power down from 100W to 50W the chattering CW below 7.1 went away and all was normal again.
> 3.  I checked my DX-EE parallel dipole and it was without damage or change.
> 4.  All coax connections in the feed line chain were tight.
> 5.  The meter on the K3 indicated a *tuned* SWR of 1.2 at 7.033 during the chattering but the LP-100 meter showed 4.5 and up at the same time and its alarm sounded.  I'd never even heard the alarm before.
> 6.  I put my MFJ 269 analyzer on the PL259 that connects the 10-40 meter dipole to the K3 and swept the bands.  No band had an SWR greater than 2.4, which the K3's tuner should (and always has) handle well.
>   
> All in all, the rig performed extremely well in the contest except for this new chattering issue.  I logged many stations that I could barely hear.  Now all I can think is that the K3's internal tuner somehow is losing its memory.  So I went back to each band and did a few "manual" antenna tunes.  SWR on the lower end of 40 meters tuned to 1.2 in several places but the chattering CW continues there with power above 50 watts.  Are there any tests I could perform to determine if the memory function of the K3's internal tuner is intermittent or otherwise suffering.  Thanks.
>   
>