[Elecraft] RFI in audio chain

[Jan Ditzian]

I have a problem that appears to be changing, but I could use some 
help.  The problem is RFI in the audio input (microphone input) when I 
use my vertical antenna on 40 meters.  Here is a rundown of what has 
happened:

I have a 67-foot elevated vertical that I can use on 80 and 40, and it 
works fine on CW.  However, on SSB, both bands, there clearly is 
feedback; there is no feedback on the other bands where I use a C3 
yagi.  Initially, the feedback was so bad that the rig would go into 
oscillation, and I had to turn the amp to standby.

I recently purchased the new KSYN3A and decided to install it.  I 
replaced the K3 with my IC-730 backup, using the hand microphone that 
came with the 730.  The 730 did not have any RFI.  I finished the 
modification and returned the K3 as the operating rig.  Now, the RFI on 
40 seems to have diminished substantially or disappeared, but it still 
happens on 75/80.  However, it seems to be much less there as well.  I 
do not suspect that the KSYN3A had anything to do with this, but perhaps 
I tightened connections better when I returned the rig to service.  I 
also redid some ground connections.

The microphone for the K3 is an Audio-Technica that works well 
everywhere else.  It has a long cord, though.  I put a few toroids on 
the cord near the microphone connector and that has possibly reduced RFI 
a little, but it is still there.

Is there a possibility that a bypass capacitor is bad, or has someone 
else had the problem and solved it externally to the K3? For instance, 
has anyone found that a long string of ferrite beads has cured this problem?

Despite decades of operating, I am hardly a troubleshooting hotshot, and 
I would appreciate guidance.

Thank you,

Jan, KX2A