Jim,
This may, or may not be the root cause of your RFI problem, but it is important
to note that the K3 elevates the ground potential on both the front and rear
panel MIC connectors. This is not a trivial issue. L7 is a 100 uH choke that
isolates the ring of the rear panel TRS connector (J20) from chassis ground.
In the same manner, L4 (another 100 uH) RFC) isolates ground pins 7 & 8 on the
front panel MIC jack from chassis ground. The net effect of these chokes
results in an "ungrounding" of the MIC inputs.. The addition of the chokes is
something completely opposite of what is required in this part of the circuit.
When conducting other K3 audio mods, I went in and removed both L4 and L7, and
jumpered the solder pads with 24 AWG buss wire.
In my case, RFI problems 100% solved.
Paul, W9AC
----- Original Message -----
From: Dr. James C. Garland
To: [email protected]
Sent: Saturday, November 01, 2008 9:57 AM
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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