[Elecraft] K2 audio images strong signals 40 meters

[Don Wilhelm]

Paul,

Thanks for those details.  I cannot attribute those offsets to anything 
related to the BFO, but on 40 meters near 7185 kHz, the PLL and VCO 
frequencies are quite close together, and interaction between the two 
can cause some "strange happenings".

I believe you may be hearing some subset of the effects from the "Weird 
At One Frequency" problem that was present in earlier K2s. Your K2 has 
the 'fixes' that usually clear it up included in the above SN 3000 board 
layout.
However, it is not entirely gone in *some* more recent K2s. Don't ask me 
why, it varies from one K2 to another, I really don't have an answer - 
it may have to do with construction practices and lead lengths above the 
RF board, but that is just a guess.

As a first pass attack on the problem, use flush cutters and trim 
everything in the vicinity of U4 so that nothing can act as a short 
'antenna'.
Look at the leads for RFC15, C91 and C88 which are soldered to the pins 
of U4 on the bottom of the board.  They should be mounted to the pins of 
U4 with the minimum lead length possible (the lead length on the side 
away from U4 is not as critical).

If you have components (particularly capacitors and resistors and 
varactors) with significant lead length above the board in the PLL and 
VCO areas, reduce that lead length to the minimum possible and flush 
trim the leads below the board.

Normal diagonal cutters will not cut sufficiently close to the board - 
If you do not have them, buy, beg or borrow true Flush Cutters for this 
task.

I can't promise that it will be a total cure, but it may reduce the 
intensity of the responses.

73,
Don W3FPR

On 8/6/2014 7:20 PM, paulb wrote:
> Hi again folks
>
>
> Update on K2 40 meter audio images around 7175;
>
> Serial number of build is 6534 with most options installed.
>
> Dummy load on Antenna RX 40 meters.
>
> Test signals received on 7170 approx 50uVolt, S9 on the K2.
>
> Tune to approx 7175 an image is heard. It is not inverted or
> reversed tune.
>
> Leave the K2 on 7170 and tune the test signal to 7184.
> It can be heard  on 7170.
>
> Best approx guess is 5 s-points down so probably not
> a big deal except when the band is quiet and strong signals
> are nearby.
>
> The calibration cable has been removed,
>
> Filter setting and mode have no effect.
> On CW the image signal tunes exactly the same, again
> about 5 s-points down.
>
> Noticed this oddity a while ago when listening to a couple of
> strong SSB stations during daylight hours at another QTH
> with low rf background noise.
>
> Suspect this has been the same since build several years ago,
> won't be loosing any sleep over it, more curious if it is
> something related to vco mixing products somewhere?
>
> I have noted the mods on earlier K2 builds, could well be
> in the same field.
>