[Elecraft] K3 SSB Tx problem

[G3SJJ]

I have been wrestling with this problem for a couple of weeks now and 
can't find a solution. Maybe someone has experienced something similar 
or can suggest where to look.

I am finalising my SO2R setup. Radio 1 is the K3, Radio 2 is my FT1kMP. 
They both feed phones, CW, PTT and mic into a Topten DX Doubler, from 
there into the mHam Keyer which provides the PC interface for Winkey for 
CW plus Voice Keying via the PC sound card and finally serial interface 
for the FT1kMP. Serial for the K3 is achieved through the KUSB. It all 
works well on CW but SSB is a different matter.

When I go to transmit on SSB on the K3, there is some kind of 
oscillation/ feedback. Very much like a rushing wind noise and is 
superimposed on the transmitted signal. If I press the footswitch, all 
is quiet but when I start speaking it appears to go into oscillation, 
modulates when I talk and stops when I stop. The FT1kMP does not have 
the same problem. Transmitted audio from the MP through the system is OK.

I have checked and re-soldered all connecting cables, and checked all 
ground connections. The problem is presnt even when transmitting into a 
dummy load and seems to reduce as I reduce the power control from 100w. 
When the 100w PA switches it out it has almost gone, just a slight 
trace. Turning the power up to the level where the 100w PA relays switch 
brings the problem back.

If I unplug the cable from the K3 to the DX Doubler and plug my Heil 
headset in everything works fine.

I have also tried unplugging different cables one by one but cannot come 
to any specific conclusion.

Chris G3SJJ