[Elecraft] K2: RF Feedback Distortion Problem

[Jon Perelstein]

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.

In scoping the signal on PSK-31, I can see that the ovoidal envelopes (best I 
can describe) do not have smooth edges.  Instead there is a noticeable sawtooth 
pattern to the edges.  Sometimes the ovoids are not completely formed, or start 
out one size and then mid-ovoid become a different size.  It appears that the 
sawteeth are isoceles triangles and not sinusoids, but I can't say for certain. 
 The sawteeth seem to be at the same frequency no matter what band I'm operating 
on.

Scoping on sideband, all of the voice "pulses" (sorry, not sure what to call 
them) are convex -- bowed to the right -- instead of being symmetrical.  

I would appreciate suggestions for where I should start my testing and what I 
should test/how I should test it.

Thanks,

Jon
KB1QBZ