[Elecraft] K2: RF Feedback Distortion Problem

[Don Wilhelm]

  Jon,

Important parameter - do you get distortion when you operate into a 
dummy load.
If so, it is distortion created in the K2/100, and if not, then it is 
either RF Feedback or a parasitic oscillation, although the KPA100s 
built in the last couple years are quite stable.
The fact that it gets worse with increased power is an indicator that it 
is RF Feedback.

If it is RF feedback, does it happen if you disconnect everything except 
the microphone and antenna from  the K2?  If so, connect back the 
devices one at a time to find out which one is causing the problem.

If it is RF feedback, then I have no explanation about why it does not 
show up on your other transceivers, but then the cables to the K2 are 
not the same ones as on the other transceivers either.

Is this a recent KPA100 kit?  If it is the older kit with red toroids 
near the middle of the board it should be updated with the KPA100UPKIT.  
If it is the newer design with blue toroids, have you added anything 
extra at the SO-239 jack (like an inductor or a resistor) - if so, it 
must be moved outside the K2 or there is a chance of a parasitic 
oscillation.

73,
Don W3FPR

On 1/10/2011 4:16 PM, Jon Perelstein wrote:
> I'm still having trouble with RF feedback distortion in my K2.  It is probably
> an error I made in build and I need help in figuring out how to attack the
> problem.
>
> K2 is serial 6550; no additional mods.  KSB2 and KPA100.  Windom antenna 25'
> away at closest OR Buddipole 50' away at closest (no difference).  KAT100 tuner
> or MFJ-941E tuner (no difference).  Definitely occurring on sideband (Elecraft
> MH2 mic) and all digital modes (Signalink USB).  Have not received any negative
> comments on CW, but I can't say for sure that there is no distortion there.  NO
> distortion (sideband or digital) when using a Yaesu FT-897 or Icom 746 in the
> exact same location in the shack with the same antennas, cables, Signalink,
> computers, etc.
>
> With the K2, using 15 watts with PA OFF, I get some distortion, but signal is
> readable.  Using 15 watts with PA ON, I get considerably more distortion than
> with PA OFF.  Distortion gets worse with increases in power (PA ON).  No
> difference in distortion when I turn off and unplug all computers and ancillary
> equipment in the shack.  Less distortion when transmitting into a dummy load,
> but it is there.  Distortion is worst on 80/75, pretty bad on 40, and
> increasingly less of a problem as I move to higher frequency bands.
>