[Elecraft] K3 RF in SSB audio on 6M

[Jim Brown]

On 10/8/2011 5:26 AM, Howard Sherer wrote:
> Only on 6M when I operate above 250W, I have RF getting into my SSB
> audio, using either the the R or F mic connector but slightly worse with
> the R mic connector. I have installed Fer chokes on the audio input
> lines with no improvement. Any suggestions.
>

More followup on this issue.  Today, I made up cables to interface two 
KRC2s to two K3s. In doing so, I studied how cable shields are 
terminated in the K3 and KRC2.  What I learned isn't pretty.  Both the 
KRC2 and the K3 have "screaming" Pin One Problems at their DB9 and DB15 
connectors. In the K3, there are RF chokes between the designated shield 
contacts and the shielding enclosure!  To provide shielding, a shield 
must be continuous. The RF chokes make this impossible. In the KRC2, the 
DB9 connectors are insulated from the shielding enclosure by paint. To 
get to the shielding enclosure, the DB9 shields must go through wiring 
on the PC board to a single point on the far side of the enclosure where 
the PC board makes intentional contact with the chassis -- IF the inside 
tooth washers manage to get through the heavy layer of paint on the 
inside of the chassis.  Not only that, but the top and bottom sections 
of the chassis are insulated from each other by paint, even with the 
hardware that mounts them together tightly screwed down.  I didn't build 
the KRC2s, they belong to N6RNO, a member of our CQP team.

The ONLY proper connection for a cable shield is the SHIELDING ENCLOSURE 
of equipment. To solve these problems, I took the two KRC2s apart, 
scraped the paint from the inside of the chassis where the DB9s are 
mounted, where the PC board is designed to be tied to the shielding 
enclosure, and where the two pieces of the shielding enclosure are 
mechanically tied together with those nice rectangular mounting parts 
that Elecraft uses. Not wanting to modify internal wiring of the PC 
board, I've terminated cable shields directly to the DB9 shells.

So far, I haven't had time to chase down details of how (or whether) DB 
connectors contact the shielding enclosure of the K3,but I'm not 
optimistic about what I'll find. For about seven years, I've been using 
serial cables that connect signal returns and cable shields (if the 
cable is shielded) to the DB9 shells, and I have yet to find a radio or 
computer where that doesn't work, so there's a DC path there somewhere. :)

So -- two of the first cables I would choke thoroughly to solve that RFI 
problem would be any cables connected to those DB9 connectors.  I have 
also seen evidence of RF noise coming in on the main DC power connector!

73, Jim Brown K9YC