[Elecraft] K3 RF Feedback Problem

[James C. Hall, MD]

Hi Jim:

 

I'm wondering what external station controller you are using. I'm using a
NCS Multi-Switcher and I ran into very similar issues. I certainly felt (as
did Doug at NCS) that it was a ground loop problem. I used transformer
isolated mike cables and found that it was NOT a ground problem. I can give
you my long details, but the bottom line is that the mike gain in the
Multi-Switcher (internal) needed to be driven a bit higher than normal, and
the mike gain on the K3 need to be quite low (5 or 6) to avoid the 'growl'
effect. I'm forwarding this to Doug at NCS as he is quite interested in
what's going on that might be causing this.

 

73, Jamie

WB4YDL

 

  _____  

From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Dr. James C. Garland
Sent: Saturday, November 01, 2008 8:57 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] K3 RF Feedback Problem

 

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

 

 

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