[Elecraft] K3 RF Feedback Problem

[Stewart Baker]

Have you tried just running the mic into your K3 without the 
external station controller ?
What is that controller ?

73
Stewart G3RXQ
On Sat, 1 Nov 2008 07:57:10 -0600, Dr. James C. Garland wrote:
> Gang,
>
>
> I’m pulling my hair out over an RF feedback problem with my 
K3/100, and wonder if
> anybody has any ideas? I use an external station controller that 
switches key,
> microphone, CAT, line in/out, etc. between different rigs.  The 
microphone audio
> connects to the K3’s rear panel microphone jack and when the 
K3’s power exceeds about
> 15W (in SSB), the audio becomes greatly istorted. It does this 
when all the other
> cables to the K3 (except the microphone and power cord) are 
unplugged. It also does it
> when the audio is fed to the K3 via the rear Line In jack. 
Everything in my station is
> well-grounded. Here are some things I’ve noticed:
>
>
> 1.  The feedback still occurs when a military-type completely 
shielded 100W dummy load
> is screwed directly to the K3’s antenna port. This suggests the 
RF from the K3 is
> getting into the audio line from some other source than 
radiation from the coax cables
> in the station, most likely the 12V power cord. To support this 
conjecture, I also
> notice that the feedback threshold (15W) is not changed when the 
K3 drives a linear
> amplifier.
>
>
> 2.  I normally use a 30A Kepco 12V power supply to run the K3 
and my other station
> accessories. The RF feedback problem is improved but not 
entirely eliminated if I use a
> separate 12V supply for the K3. A common mode choke on the 12V 
line doesn’t appear to
> make any difference.
>
>
> 3. I can see some modulation-induced fluctuationss on the 12V 
line with an oscilloscope
> that increase with the K3’s power setting. I haven’t looked at 
these fluctuations
> closely enough yet to know whether it’s RF noise or just 
audio—frequency  fluctuations
> caused by the K3’s modulating current draw from the power 
supply.
>
>
> To summarize, my conclusions so far are that (1) the 12V power 
cord is a source of RF
> leakage from the K3,  even though the K3 has a filter at its 12V 
power connector to
> minimize this leakage; and (2) The audio/DSP circuitry in the K3 
is very sensitive to
> RF – much more so than other transceivers in my station.  (I’ve 
not had this problem
> with other rigs.)
>
>
> Because of this RF sensitivity, one evidently has to be very 
careful  when hooking
> accessories to the K3. The front panel microphone jack is (to 
me) wired in a curious
> way, with the Mic ground and PTT ground (shield) connections 
floating above the K3’s
> chassis with a 100uH rf choke. Similarly, both the “hot” and 
“shield” side of the rear
> panel mic input jack has series 100uH rf chokes, which isolate 
the input from chassis
> ground.
>
>
> The problem with this arrangement is that most accessories that 
would connect to the
> rear panel audio input jacks have single-ended outputs, with a 
shielded cable that is
> directly tied to chassis ground at the accessory end of the 
cable. Thus, unless one is
> very careful, it is easy to have a situtation where the signal 
grounds in the audio
> circuitry of the K3 can fluctuate with respect to the K3’s 
chassis ground, and this may
> be the source of the RF feedback sensitivity.
>
>
> Unfortunately, I can’t figure out the detailed mechanism for the 
feedback closely
> enough to figure out a solution. Foir example, I don’t know 
whether it’s better to
> leave the minus side of my 12V power supply floating, or to tie 
it to the chassis
> gound..  And even if I left it floating, it would be tied to the 
chassis anyway by the
> other accessories hooked to it, and this might cause more 
problems than grounding it at
> the power supply terminal. Somehow, it seems like a wiring 
change in the ground
> configuration of the K3’s audio circutis to improve RF isolation 
may be needed to lick
> the problem completely.
>
>
> 73,
>
>
> Jim Garland W8ZR