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

[Don Wilhelm]

Steve,

If "hooked up backwards" means that the DC power was connected with 
reverse polarity, that will normally cause no harm to the K2 because 
there is a series reverse voltage protection diode in the input power 
line.  The burned trace on the mic jack pin 5 suggests that something 
different happened to it.

Try replacing D9 with a 1N5711 diode and see what happens.  The power 
output into a good dummy load will normally be within 0.2 watts of the 
set power if all is working right.  You older K2 has a 1N34 diode at D9 
and it is not as accurate over the full frequency range as the 1N5711.

If you have not done so already, record the menu settings, especially 
the filter settings and do a Master Reset (hold the 4, 5, and 6 buttons 
in while powering the K2 on).  Then run CAL PLL and restore the menu 
settings.  The fact that the problem goes away if you swap control 
boards suggests that there may be some problem with the EEPROM contents, 
and a Master Reset should 'clean it out'.

Set CAL CUR to 3.50 instead of 2.50 and see if the HiCur messages go 
away.  If they continue, you may have to replace the PA transistors.  
Another thing to check is the DC voltage at the base of Q7 and Q8 during 
a TUNE - it should be in the range of 0.60 to 0.64 volts.  If not, 
replace Q11 and Q13.

73,
Don W3FPR

On 10/5/2014 1:36 AM, 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!
>>
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