Unstable KXPA100 output on 40 Meters.

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Unstable KXPA100 output on 40 Meters.

Hans J Rasmusen OZ7BQ
Someone on this list may have an answer.
The output and SWR green LED status indicators on my KX3 (sn 18xx) KXPA100/KXAT100 (sn 08xx) combo exhibit some unusual behavour on 40 Meters CW.
The problem (if it is a problem) is present only on 40 Meters when running 80 Watts or more, at which point the output power indicator begins to jump between 80 and 110 watts and the SWR indicator between 1.2 and 2.
The indications does not appear to follow the keying strictly but happens randomly starting a few seconds after start of keying. The indications appears mostly at keying speeds below approx 22 WPM using a paddle. There appears to be few or no indications at slow speed hand keying.I do not experience any jumping output indications when the KXPA is put in Tx mode by grounding the KXPA PA Key line.The problem disappear, if I disconnect the connection from the KX3 ACC1 to the KXPA.I use as a KXPACBL adaptor Cable Set.

The above problem appears to be RFI related, as the jumping output/SWR indications are present only when feeding an antenna. No indications are present when feeding a dummy load.Liberal use of ferrite on feedlines and cables does not appear to make any difference on this issue. I have build a G4HUP type RF current meter, and can not trace any common-mode RF on Feedlines,on power lines or on wires between KX3 and KXPA.My K2-100 does not exhibit any jumping output power or SWR indications on the same antennas and at the same power levels.

My questions are:Has anyone experienced anything similar?Is it a problem?and if so, where does the RF enter?
Is it likely that the RF get into the KXPA through the RDX or TDX lines on the RJ45 connector?

Thank you in advance, good DXOZ7BQ,
Hans Jørgen (Joe).
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Re: Unstable KXPA100 output on 40 Meters.

AC6SL
On Sat, Jan 16, 2016 at 7:58 AM, Hans J Rasmusen OZ7BQ
<[hidden email]> wrote:...
> The output and SWR green LED status indicators on my KX3 (sn 18xx) KXPA100/KXAT100 (sn 08xx) combo exhibit some unusual behavour on 40 Meters CW...
> The above problem appears to be RFI related, as the jumping output/SWR indications are present only when feeding an antenna. ...
> My questions are:Has anyone experienced anything similar?....

I experienced similar problems on 40m with my K3+KPA500+KAT500, i.e.
the SWR would suddenly jump, the amplifier would fault, and the
automatic band selector antenna switch would chatter. Because the
problem only seemed to occur on 40m, I investigated the antenna, found
a loose connection, but it did not fix the problem. Eventually,
similar fault occurred on 20m.

Finally, I found that the short coaxial cable between the KAT500 and
the antenna switch was intermittent. I could observe change in receive
signal strength when I twisted the cable, near the PL-259 connector.
Substituting a different cable has completely fixed the problem. I
suspect a bad soldered shield connection in the bad cable.

At one point, while investigating this problem, receive sensitivity on
40m only, was 30 dB worse, although transmit was normal. The RX_ANT
had been activated, and there was no receive antenna connected!

-John  AC6SL
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Tex
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Re: Unstable KXPA100 output on 40 Meters.

Tex
In reply to this post by Hans J Rasmusen OZ7BQ
I have/had the same problem with the KXPA100/KAT100 except it would go into fault.  I checked everything I could and couldn't find any bad connections.  My antenna is a cobra folded dipole and the twin lead is very close to my operating position and there is a balun at the twin lead.  I added more coax between the balun and entry point and put a couple of snap on torroids on the coax and the problem has gone.

Tex
ka5y