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K2: RF Feedback Distortion Problem

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jperelst

K2: RF Feedback Distortion Problem

I'm still having trouble with RF feedback distortion in my K2. It is probably
an error I made in build and I need help in figuring out how to attack the
problem.

K2 is serial 6550; no additional mods. KSB2 and KPA100. Windom antenna 25'
away at closest OR Buddipole 50' away at closest (no difference). KAT100 tuner
or MFJ-941E tuner (no difference). Definitely occurring on sideband (Elecraft
MH2 mic) and all digital modes (Signalink USB). Have not received any negative
comments on CW, but I can't say for sure that there is no distortion there. NO
distortion (sideband or digital) when using a Yaesu FT-897 or Icom 746 in the
exact same location in the shack with the same antennas, cables, Signalink,
computers, etc.

With the K2, using 15 watts with PA OFF, I get some distortion, but signal is
readable. Using 15 watts with PA ON, I get considerably more distortion than
with PA OFF. Distortion gets worse with increases in power (PA ON). No
difference in distortion when I turn off and unplug all computers and ancillary
equipment in the shack. Less distortion when transmitting into a dummy load,
but it is there. Distortion is worst on 80/75, pretty bad on 40, and
increasingly less of a problem as I move to higher frequency bands.

In scoping the signal on PSK-31, I can see that the ovoidal envelopes (best I
can describe) do not have smooth edges. Instead there is a noticeable sawtooth
pattern to the edges. Sometimes the ovoids are not completely formed, or start
out one size and then mid-ovoid become a different size. It appears that the
sawteeth are isoceles triangles and not sinusoids, but I can't say for certain.
The sawteeth seem to be at the same frequency no matter what band I'm operating
on.

Scoping on sideband, all of the voice "pulses" (sorry, not sure what to call
them) are convex -- bowed to the right -- instead of being symmetrical.

I would appreciate suggestions for where I should start my testing and what I
should test/how I should test it.

Thanks,

Jon
KB1QBZ
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Don Wilhelm-4

Re: K2: RF Feedback Distortion Problem

Jon,

Important parameter - do you get distortion when you operate into a
dummy load.
If so, it is distortion created in the K2/100, and if not, then it is
either RF Feedback or a parasitic oscillation, although the KPA100s
built in the last couple years are quite stable.
The fact that it gets worse with increased power is an indicator that it
is RF Feedback.

If it is RF feedback, does it happen if you disconnect everything except
the microphone and antenna from the K2? If so, connect back the
devices one at a time to find out which one is causing the problem.

If it is RF feedback, then I have no explanation about why it does not
show up on your other transceivers, but then the cables to the K2 are
not the same ones as on the other transceivers either.

Is this a recent KPA100 kit? If it is the older kit with red toroids
near the middle of the board it should be updated with the KPA100UPKIT.
If it is the newer design with blue toroids, have you added anything
extra at the SO-239 jack (like an inductor or a resistor) - if so, it
must be moved outside the K2 or there is a chance of a parasitic
oscillation.

73,
Don W3FPR

On 1/10/2011 4:16 PM, Jon Perelstein wrote:

> I'm still having trouble with RF feedback distortion in my K2. It is probably
> an error I made in build and I need help in figuring out how to attack the
> problem.
>
> K2 is serial 6550; no additional mods. KSB2 and KPA100. Windom antenna 25'
> away at closest OR Buddipole 50' away at closest (no difference). KAT100 tuner
> or MFJ-941E tuner (no difference). Definitely occurring on sideband (Elecraft
> MH2 mic) and all digital modes (Signalink USB). Have not received any negative
> comments on CW, but I can't say for sure that there is no distortion there. NO
> distortion (sideband or digital) when using a Yaesu FT-897 or Icom 746 in the
> exact same location in the shack with the same antennas, cables, Signalink,
> computers, etc.
>
> With the K2, using 15 watts with PA OFF, I get some distortion, but signal is
> readable. Using 15 watts with PA ON, I get considerably more distortion than
> with PA OFF. Distortion gets worse with increases in power (PA ON). No
> difference in distortion when I turn off and unplug all computers and ancillary
> equipment in the shack. Less distortion when transmitting into a dummy load,
> but it is there. Distortion is worst on 80/75, pretty bad on 40, and
> increasingly less of a problem as I move to higher frequency bands.
>

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jperelst

Re: K2: RF Feedback Distortion Problem

Don,

As always, thanks for your reply.

I can confirm that I *DO* get distortion when operating into a dummy load. I
can also confirm that I have the distortion when operating with nothing but the
microphone and antenna connected to the K2.

The K2 itself is Rev G dated May 2007, with errata to G-5 (March 2008). The
KPA was purchased with the K2 in July 2008, and is Rev E, dated Jan 2007. The
KSB2 was also purchased with the K2, and is Rev E dated Jul 2004. According to
the Elecraft web site, the KPA100UPKIT is for KPAs prior to Rev D.

ALL cables used for my testing were exactly the same for the K2 and the FT-897
and the IC-746. That includes the power supply cables, the coax from the
antenna to the tuner, and the coax from the tuner to the rig. I used the same
Signalink with the same USB cable (although, of course, the mic cables were
different).

I have not added any additional components to the Elecraft products. They
contain only those things that Elecraft included as specified in the Elecraft
assembly instructions.

Jon
KB1QBZ
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Don Wilhelm-4

Re: K2: RF Feedback Distortion Problem

Jon,

Thanks for that preliminary information.
Try two things, testing again for the distortion after each one.
First, remove the right side panel and tuck the KPA100 ribbon cable up
in the space near the top of the control board. Do the same for the
speaker cable - in other words, dress those cables as far away as
possible from the SSB board. Put the side panel back on and re-test.

With the KPA100 still connected, set the power at 20 watts and use an
external wattmeter to measure the KPA100 output. Is it near 20 watts or
is it over 110 watts?

If the first step does not cure your distortion, then remove the KPA100
completely and test only the base K2. Try it at 5 watts, 10 watts and
at maximum power.

Let us know what you find.

73,
Don W3FPR

On 1/10/2011 9:53 PM, Jon Perelstein wrote:

> Don,
>
> As always, thanks for your reply.
>
> I can confirm that I *DO* get distortion when operating into a dummy load. I
> can also confirm that I have the distortion when operating with nothing but the
> microphone and antenna connected to the K2.
>
> The K2 itself is Rev G dated May 2007, with errata to G-5 (March 2008). The
> KPA was purchased with the K2 in July 2008, and is Rev E, dated Jan 2007. The
> KSB2 was also purchased with the K2, and is Rev E dated Jul 2004. According to
> the Elecraft web site, the KPA100UPKIT is for KPAs prior to Rev D.
>
> ALL cables used for my testing were exactly the same for the K2 and the FT-897
> and the IC-746. That includes the power supply cables, the coax from the
> antenna to the tuner, and the coax from the tuner to the rig. I used the same
> Signalink with the same USB cable (although, of course, the mic cables were
> different).
>
> I have not added any additional components to the Elecraft products. They
> contain only those things that Elecraft included as specified in the Elecraft
> assembly instructions.
>
> Jon
> KB1QBZ
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>