[Elecraft] Power oscillation problem (KPA500+KAT500)

[Rick NK7I]

Andy,

You've answered your own question.  If it doesn't happen when the entire 
rig system transmits into a dummy load, but has problems while connected 
to an antenna; you know where to look.

In that search, high on the list of suspects (right up there with 
failure of any antenna elements) is damage from tiny livestock (mice, 
rats, birds that may have gotten into the attic or bugs like mud wasps); 
chewed wires, cables and insulators.

However with your extended time using a 40M antenna on 80M even at 250 
watts, it's likely that some part of the antenna system (feed, chokes, 
transformers, wires etc) has broken down (high voltages are possible 
causes).  Something changed, you know where to look now.  Be ready to 
replace the antenna with the means to prevent another similar failure.

73,
Rick nk7i

On 11/20/2023 7:16 PM, Victor Rosenthal 4X6GP wrote:
> This is really strange.The only thing I can think of is some kind of 
> feedback to the TS-590s ALC circuitry caused by common mode current on 
> the feedline. An easy test would be to connect a common mode choke at 
> the output of the KAT500 and see if the problem goes away. Does the 
> antenna have some kind of choke or balun at the feedpoint? If not, 
> probably it should, just to help reduce local RFI.
> Why did this suddenly happen? I have no idea. Changes in grounding or 
> any conductors attached to the system can affect the path that the 
> common mode current takes.
>
> 73,
> Victor, 4X6GP
> Rehovot, Israel
> formerly K2VCO
> CWops no. 5
> http://www.qsl.net/k2vco/
>
> On 21/11/2023 1:20, Andy Durbin wrote:
>> I have a new power oscillation problem that is manifest only on 40 m
>> when using an attic dipole. Basic station configuration is Kenwood
>> TS-590S, KAT500, KPA500.  As drive power is slowly increased and
>> output power rises above about 100 W the power output falls to zero
>> then slowly rises again. If drive power is increased the frequency of
>> the oscillation increases but the peak output power does not change.
>>
>> At onset the oscillation frequency is 3.5 Hz.  While KPA500 output
>> power is cycling TS-590S exciter power remains constant, KPA500
>> remains keyed, and neither LP-100A, KPA500, nor KAT500 indicate any
>> SWR transient.  KPA500 and KAT500 do not fault.
>>
>> This 40 m attic dipole has been in use for several years and this
>> problem is new.  I have set any arbitrary 250 W limit for this
>> antenna and have, in the past, experienced no significant problems
>> running that power level.
>>
>> What could cause KPA500 to stop power output and then resume? The
>> KPA500 will output 500 W into a dummy load on the same frequency with
>> no oscillation so that seems to rule out a KPA500 hardware issue.
>>
>> Next suspect would be RFI but what feedback mechanism would cause
>> KPA500 to drop to low or zero power while drive remains constant and
>> key line remains active.  Why would I have RFI now if I didn't
>> before?
>>
>> Next suspect would seem to be some sort of breakdown in the antenna
>> system and that breakdown heals when power is removed.  But how could
>> any breakdown pull power to zero and not fault KPA500 or KAT500?
>>
>> Next step will be visual inspection of the attic antenna.  I'm
>> posting in case anyone has seen a similar problem and has any
>> suggestions as to the cause.
>>
>> Early draft notes and some scope traces here -
>> https://www.dropbox.com/scl/fi/rxidnm4bkv9pissfyln00/Power-Oscillation-40-m-draft-1.pdf?rlkey=446vn8bjoe2bv2xn4un86zpd2&dl=0 
>>
>>
>>  Thanks and 73, Andy, k3wyc
>>
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