[Elecraft] K3: transmitted RF feedback with condenser mics?

[Jim Brown]

On Sun, 24 Jan 2010 14:53:28 -0500, Brian Machesney wrote:

>The problem headset is designed for aviation use and, as far 
as 
>I can tell, uses a shielded mic cable.

Since I'm an RFI/EMC specialist, I'll offer some ideas for you 
to try.

1) Reduce the input gain as much as practical while still 
giving you good modulation. There's a high a low gain setting. 
Try to make the low gain setting work.

2) Pay careful attention to where you're connecting the shield. 
It should go to the chassis, NOT "mic ground." The latter is a 
classic "pin 1 problem" and can be a major cause of RFI (RF 
feedback). I've been told by someohe who has studied the 
circuit board that there IS a pin 1 problem at the front panel 
jack. There is also a potential pin 1 problem at the rear panel 
jack (in the form of an RF choke in series with the mic 
shield). 

3) Try a small multi-turn choke on the mic cable. A single turn 
through a clamp-on is not likely to make a dent, but MULTIPLE 
turns can help (by reducing the RF current on the cable). 

All of the above is addressing RFI TO the K3. There's a small 
chance the the RFI might be to the mic itself. The choke might 
help that too.

You didn't mention what bands or how much power. A good test 
when diagnosing things like this is to back off on the power 
until the problem goes away. That reduction in power is the 
degree of suppression (in dB) that you need. 

>Has anyone else had and resolved this problem?

I had it with a dynamic mic on my FT1000MPs, and solved it with 
chokes, as described. In that case, it was a pin 1 problem at 
the front panel mic jack. 

73,

Jim Brown K9YC