[Elecraft] [K2] Output power not matching power setting and very unstable between transmissions

[Vic Rosenthal K2VCO]

Also, what about the power control pot itself?

On 10/5/2014 08:36, KY7K wrote:
> I forgot to mention that all testing is into a known good DL1 dummy
> load that I’m also using for my external power readings.
>
> Any wise insights are most welcome!
>
> Steve - KY7K ky7k at arrl.net Get OUT and play radio!
>
> On Oct 4, 2014, at 10:28 PM, KY7K <ky7k at cox.net> wrote:
>
>> I recently purchased a used K2 (Rev A) from a club that was not
>> working because it had been “hooked up backward”. That is the best
>> anyone can remember as to what happened to the radio ~7 years ago…
>>
>> Currently, the power output as displayed on the radio and on an
>> external power meter is very unstable and does not match the power
>> setting desired. This is not just a little off, I can select 1W on
>> the radio and then hit the tune button 5 times and might get .4W,
>> 6.7W, HI-CUR, 9.0W, 1.2W. This is the QRP K2 with SSB, NB, KIO2 and
>> DSP options. The problem occurs with or without the option boards
>> installed.
>>
>> Background: When I first tested it, it would power on but the only
>> function that was operational was the power control. Every other
>> knob and button was non-functioning as if the cpu was hung up. The
>> radio was not transmitting in this state. I had a old CPU (U6) from
>> my other K2 that I installed as a test and that seemed to fix the
>> problem, at least in receive. I, unfortunately, did not test it in
>> transmit. I ordered a FWK2MCIO kit to get a new cpu and also
>> ordered all the mod kits for the Rev A to Rev B upgrade, once they
>> came in I installed everything and while performing the the
>> calibrations after the upgrade discovered that the transmit power
>> was very unstable between transmissions. Other than the bad U6, the
>> only other issue I found was that the PCB trace from pin 5 of the
>> Mic connector on the front panel board was basically missing, from
>> getting shorted to +12V I assume, it disintegrated when touched...
>>
>> I’ve narrowed the issue down to the control board, since if I swap
>> the control board from my working K2 into the bad radio the power
>> issue disappears. I’ve checked VRFDET, VALC, and VPWR on the
>> control board and they are all OK best I can tell, and I’m now at a
>> loss where to look next. Note: the power level is stable once it
>> settles on some random output power. The power level is random from
>> one transmission to the next.
>>
>> The following links are images showing a tune cycle. The first
>> image shows the full scan with the radio set to ~1W and actually
>> putting out ~.4W. The second image is zoomed in on the start of the
>> cycle.
>> https://www.dropbox.com/s/i64sw2ue2405roe/k2_0_5w_full.png?dl=0
>> https://www.dropbox.com/s/qe297am756igbd4/k2_0_5w_zoom.png?dl=0
>>
>> This is with the power set to ~7W and the radio indicating hi-cur,
>> before settling down to ~6.4W
>> https://www.dropbox.com/s/tqxzi3yk13gye4g/k2_hicur_full.png?dl=0
>> https://www.dropbox.com/s/6qz10478zt66tjd/k2_hicur_zoom.png?dl=0
>>
>>
>>
>> Steve - KY7K ky7k at arrl.net Get OUT and play radio!

-- 
73,
Vic, K2VCO/4X6GP
Rehovot, Israel
http://www.qsl.net/k2vco/