Paul,
Although the inductors may be at fault (and IMHO should be jumpered
out), there are other considerations that enter into the picture. The
most predominate is the power supply. If external boxes have independent
powering, that can often cause problems because the power supply common
is not at the same potential for all the supplies. In addition, most
Astron power supplies (and others too), connect the negative output
terminal back the the AC safety ground which can cause ground currents
to flow and give rise to many 'strange happenings'. A good check for
power supply grounding problems is to temporarily run the K3 from a 12
volt battery (an automotive type will do for the K3/100).
73,
Don W3FPR
Paul Christensen wrote:
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 <mailto:[EMAIL PROTECTED]>
*To:* [email protected] <mailto:[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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