Re: K3-100/KPA500 High SWR on 80 Meters Only

Bob McGraw - K4TAX


Do you have any Serial Number Range of units affected?   I've been through 2 KPA3A assemblies, with the 3rd now in the radio since May of this year.  The radio was purchased new from Elecraft in August 2015.  The serial number is 10163.

Bob, K4TAX

On 10/28/2018 8:36 PM, Randy Farmer wrote:
Hi, Jeff. I encountered the same problem. My early production K3S PA was going into parasitic oscillation at low powers on 80 meters. The out-of-band spurious causes the high SWR indication. According to Elecraft Service this is a known problem with the first run of KPA3A boards. They sent me a new KPA3A module to swap in and that cured the problem. It's possible your K3 may be doing the same thing.

Randy, W8FN

On 10/28/2018 7:52 PM, Richard Williams via Groups.Io wrote:

Did you try transmitting into a dummy load?   If you have no issue with a dummy load, start looking at  RF feedback into the radio.

Dick, K8ZTT 

On Sun, Oct 28, 2018 at 17:59, Jeff Clarke
<ku8e@...> wrote:
I'm running a K3-100/KPA500 on 80 meters thru a Denton antenna tuner feeding a 135 center fed zepp with 450 ohm feeder. I'm having a strange problem.

With the KPA500 powered off and slowly increasing the drive on the K3 up to 13 watts the SWR is OK. It goes sky high (above 13 watts) when the internal 100 watt amp in the radio kicks in. It starts to go back down (normal SWR) as I continue to increase the drive on the radio. The SWR seems to return to normal when I get it up to about 50 watts or so. Unfortunately this is too much power to drive the amp and it faults. 

When I turn on the power for KPA500 and put it into standby I get the same symptoms I described above. When I take it out of standby it looks like everything is working normal... but I'm not sure if having the amp on is really solving the issue because there still is a SWR issue somewhere?  I also removed the KPA500 completely out of the circuit ( to see if it was the problem) and plugged the antenna directly into the K3-100 and the symptom comes back. I also hooked up my spare IC756Pro2 to the KPA500 and tuner and it appears everything works OK. So I don't think the problem is with the amp.

The K3-100/ KPA500 combination seems to work fine on all the other bands. All those other antennas are resonant and don't go thru the antenna tuner and zepp that I use for 80 meters.

Any ideas on what might be going on?

Thanks, Jeff  KU8E

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