[Elecraft] Strange KPA500 problem on 6M

[David Olean]

Last summer a friend of mine got a new K3s and had problems with it on 
just six meters. As soon as he raised power above about 10 watts,  the 
SWR reading went crazy and the rig gave high VSWR warnings. He checked 
his six meter yagi with another SWR analyzer and the antenna checked 
good. After goofing around with it and getting nowhere, he brought it 
over to my place and we tried it on my six meter array. We saw the same 
problem here, high VSWR,  but with different combinations of antennas, 
the K3s actually worked OK.  It also worked fine into a dummy load.  I 
got suspicious and tested the K3s with a spectrum analyzer. Sure enough 
the rig had a huge parasitic oscillation on about 27.4 MHz and with most 
of the power there, you could see how the radio would show a high SWR. 
It only appeared when the internal 100w PA kicked in.  He did not have a 
KPA-500. This was just a K3s.

He sent the K3s back to the factory along with some pictures of the HP 
spectrum analyzer screen. I am not sure if they duplicated the problem 
at the factory,  but they applied a few changes to the PA and now it is 
all OK.   If the K3s was to have an unwanted oscillation, it would sure 
mess up the KPA-500 and show high VSWR and maybe damage some filter 
components.  If all else fails, that may be a place to look.

Dave K1WHS


>
>
> On 12/4/2016 6:30 AM, Mike va3mw wrote:
>> As quoted
>>
>> If the amp works on a dummy load, then look beyond the amp at this 
>> time until you have eliminated everything else.
>>
>> Somewhere else you are getting a voltage breakdown that is arcing and 
>> tripping off the amp due to high swr. An MFJ analyzer will not show 
>> that.
>>
>> Mike va3mw
>>
>>> On Dec 4, 2016, at 12:54 AM, Jim Brown <jim at audiosystemsgroup.com> 
>>> wrote:
>>>
>>>> On Sat,12/3/2016 9:41 PM, Vic Rosenthal wrote:
>>>> Look at any cables, switches and connectors that are common to the 
>>>> various antennas.
>>> Yes. Lots of "off-brand" coax is junk. So are most un-branded 
>>> connectors. If the PL29-style connector doesn't say Amphenol 83-1SP, 
>>> it's probably junk. How well is the connector soldered or crimped? 
>>> If crimped, was it done with the "official" crimper for the 
>>> "official" crimp connector that matches THAT specific coax? If not, 
>>> it's suspect.
>>>
>>> Are you using any of those shiny adapters that are sold at hamfests 
>>> for a few bucks? They're JUNK! Ask me how I know. :) They have 
>>> caused me many problems that were tricky to diagnose.
>>>
>>> IMO, the only GOOD coax connectors say "Amphenol" on them, or are 
>>> ancient MIL-spec parts with MIL numbers stamped into them. I snap up 
>>> stuff like this when I find it at ham flea markets.
>>>
>>> 73, Jim K9YC
>>>