[Elecraft] K2 RF Feedback

[Stewart Baker]

On Sun, 11 Jul 2004 19:19:10 +0100, David Pratt wrote:
> Stewart - Did you ground the shell of the mic connector on the K2? I
> found that the r.f. feedback that I was getting was completely cured by
> the suggested mod on the Elecraft web site.
> 73 de David G4DMP
> In a recent message, Stewart Baker <stewart at baker.nildram.co.uk>
> said....
>> Given a few hours free I decided to investigate a problem with RF feedback
>> that
>> I had been getting with my K2.
>> Occasionally on 80m I had received reports that the TX audio was not 100%
>> and
>> after backing everything off, things improved. It was only when I noticed
>> that
>> on 40m I could provoke the RF feedback that I have gotten around to doing
>> something.
>> After checking the mic leads etc I decided that the RF was getting into the
>> input of the speech amp/compressor IC.
>> The cure was to fit a 1mH RF choke (out of the junk box) in series with C32.
>> This provided an immediate and complete cure. Hope this helps someone.

David,

Yes, I did that some time ago. It certainly makes a difference under certain 
circumstances. The problem I was getting,  however seemed to be that despite a 
good screened mic lead, plug and socket,  there was sufficient RF getting onto 
the compressor IC input to make an effect.

The only RF suppression on the AF line is 0.02uF (1 cap on the control board, 
and 1 cap on the KSB2). These C's provide a shunt to RF which for most 
operations should be OK, however as the overall system gain is high any RF that 
gets into the KSB2 input will cause a problem. Adding the 1mH RFC in series with 
the mic signal gives a high impedance path to RF, and a low one to the mic 
audio. I have also added a 0.001uF cap directly across the compressor input 
pins. This PI filtering should stop any RF "dead in it's tracks".

73
Stewart G3RXQ