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K3 RF Feedback Problem

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Jim Garland

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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Stewart Baker

Re: K3 RF Feedback Problem

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,
>
>
> Im 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 K3s rear panel microphone jack and when the
K3s 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 Ive noticed:
>
>
> 1. The feedback still occurs when a military-type completely
shielded 100W dummy load
> is screwed directly to the K3s 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 doesnt appear to
> make any difference.
>
>
> 3. I can see some modulation-induced fluctuationss on the 12V
line with an oscilloscope
> that increase with the K3s power setting. I havent looked at
these fluctuations
> closely enough yet to know whether its RF noise or just
audiofrequency fluctuations
> caused by the K3s 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. (Ive
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 K3s
> 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 K3s
chassis ground, and this may
> be the source of the RF feedback sensitivity.
>
>
> Unfortunately, I cant figure out the detailed mechanism for the
feedback closely
> enough to figure out a solution. Foir example, I dont know
whether its 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 K3s audio circutis to improve RF isolation
may be needed to lick
> the problem completely.
>
>
> 73,
>
>
> Jim Garland W8ZR

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Dave, G4AON

Re: K3 RF Feedback Problem

In reply to this post by Jim Garland

You "can't" have RF feedback if you are using a dummy load, unless one
of the connectors/leads inside the K3 isn't properly connected -
allowing high levels or RF inside the cabinet.

As per Stewart's comment, have you tried a directly connected mic and
also are you sure that power supply isn't causing a problem?

73 Dave, G4AON
K3/100 #80
-------------------------------------
Have you tried just running the mic into your K3 without the
external station controller ?
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Jim Garland

RE: K3 RF Feedback Problem

In reply to this post by Jim Garland

Thanks to all for your replies and interest. To answer some raised
questions:

(1) The K3 works well (no feedback) with an MH-2 microphone connected to the
front panel jack. The problem comes about when any other audio source (e.g.,
another microphone, or W2IHY Eq Plus) is connected to the rear panel mic
jack. Unfortunately, I can't easily connect another microphone directly to
the rear jack, without routing it first through my station controller.
However, I _can_ route the EQ Plus directly into the rear panel jack,
bypassing the controller, and see no change in the feedback problem. I've
not had the feedback problem with three other rigs I've tested.

(2) My test with the dummy load connected directly to the antenna input on
the K3 was intended to establish that RF was not getting into the audio by
radiation from coax cables in the shack. My best guess is that RF is
"leaking" out of the K3's power cable into my 12V distribution panel, and
from there into the audio line of connected devices.

(3) I'm not 100% sure that the problem is actually caused by RF. It might be
caused by audio modulation of the 12V on my power distribution panel
resulting from fluctuations of the DC current when the K3 is voice
modulated. One way to check that possibility, which I plan to do today, is
to see if the problem persists when the K3 is operating in AM and the power
supply current is roughly constant. I also plan to look at the audio and 12V
lines with a scope. I'll let you all know what I find. However, since I've
experienced no other problems using the identical hookup with other rigs,
e.g., a Yaesu FT-2000D, I can't help but think the K3 is abnormally
sensitive to this kind of problem.

73,

Jim W8ZR

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P.B. Christensen

Re: K3 RF Feedback Problem

In reply to this post by Jim Garland

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: [hidden email]

To: [hidden email]

Sent: Saturday, November 01, 2008 9:57 AM

Subject: [Elecraft] K3 RF Feedback Problem

Gang,

Im 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 K3s rear panel microphone jack and when the K3s 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 Ive noticed:

1. The feedback still occurs when a military-type completely shielded 100W dummy load is screwed directly to the K3s 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 doesnt appear to make any difference.

3. I can see some modulation-induced fluctuationss on the 12V line with an oscilloscope that increase with the K3s power setting. I havent looked at these fluctuations closely enough yet to know whether its RF noise or just audiofrequency fluctuations caused by the K3s 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. (Ive 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 K3s 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 K3s chassis ground, and this may be the source of the RF feedback sensitivity.

Unfortunately, I cant figure out the detailed mechanism for the feedback closely enough to figure out a solution. Foir example, I dont know whether its 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 K3s audio circutis to improve RF isolation may be needed to lick the problem completely.

73,

Jim Garland W8ZR

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Jim Garland

RE: K3 RF Feedback Problem

Very interesting, Paul. I was aware of the isolation chokes, but hadn't known anybody to try jumpering them. I'd be interesting in hearing one of the K3 designers explain what they had in mind by wiring the audio inputs this way. Perhaps it has something to do with using balanced line audio sources. Minimizing ground loops can be a headache when one has multiple audio devices, powered by different power supplies.

73,
Jim W8ZR
________________________________________
From: Paul Christensen [[hidden email]]
Sent: Saturday, November 01, 2008 10:03 AM
To: 4CX250B; [hidden email]
Subject: Re: [Elecraft] K3 RF Feedback Problem

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:[hidden email]>
To: [hidden email]<mailto:[hidden email]>
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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James C. Hall, MD-2

RE: K3 RF Feedback Problem

In reply to this post by Jim Garland

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: [hidden email] [mailto:[hidden email]] On Behalf Of Dr. James C. Garland
Sent: Saturday, November 01, 2008 8:57 AM
To: [hidden email]
Subject: [Elecraft] K3 RF Feedback Problem

Gang,

73,

Jim Garland W8ZR

Jim Garland

RE: K3 RF Feedback Problem

Hi Jamie,
My station controller is a homebrewed unit that switches up to 25 different control and signal lines (mic, key, speakers, data, Line In/Out, amp relay, packet, etc.) for three radios and three amplifiers, in any combination. The controller also operates a separate RF relay unit that transfers all the RF-carrying coax lines (rated 3 kW). It was a year-long project to design and build. (Actually, I have three of them, and they can be networked together, allowing control of 9 rigs and 9 amplifiers.) An LCD display shows what radios/amplifiers are currently selected. At some point, I'll write an article on it for a ham magazine. The unit (controller and RF relay box) all use printed circuit boards, so other than the microphone jacks (two mics can be selected), there is no point-to-point wiring. I've been testing it out with various radios, ironing out a few bugs, but have found no real problems other than the aforementioned problem with feedback in the K3.
73,
Jim W8ZR
________________________________________
From: James C. Hall, MD [[hidden email]]
Sent: Saturday, November 01, 2008 11:13 AM
To: 4CX250B; [hidden email]
Cc: 'W. Douglas McDowell'
Subject: RE: [Elecraft] K3 RF Feedback Problem

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: [hidden email] [mailto:[hidden email]] On Behalf Of Dr. James C. Garland
Sent: Saturday, November 01, 2008 8:57 AM
To: [hidden email]
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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gm3sek

Re: K3 RF Feedback Problem

In reply to this post by Jim Garland

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.

I just measured the common-mode RF current on the K3's 12V power cord at
about 2mA at 7MHz, 100W into a dummy load. The background level was 1mA,
most of which was coming *up* the line from the SMPS.

The meter consists of an RF current transformer made from a clamp-on
ferrite core, feeding an AD8307 wide-range log detector. It responds to
the total common-mode current from all sources combined, with a
bandwidth from LF to several hundred MHz.

For perspective, about 1mA of assorted RF crud is a typical background
level on computer and monitor cables, including the filtered mains input
or DC output of a good-quality SMPS. Common-mode currents are also
installation-specific, because they depend on the impedance into which
the current is flowing, so in real-life installations it's only
justifiable to compare orders of magnitude (the location of the decimal
point) rather than obsess over exact values.

From that perspective, 2mA on the DC cord of the K3 is not a seriously
high level of RF leakage from a box that is generating 100W of RF. It is
also notable that the output cable leading to the dummy load has only a
few mA of common-mode current (contrast that with 1.4A of
differential-mode RF current flowing *inside* the shield). My IC-746
gave very similar results to the K3.

Therefore it seems unlikely that K3s in general would have enough RF
leakage on the power cord to cause the problems you're seeing, Jim. That
doesn't rule out the possibility of a fault on your particular rig...
but maybe you should be looking for a broken shield connection or
something else that has been disturbed by puling out the previous rig
and installing the K3.

Most likely of all is a broken shield connection on a PL-259, which can
let out high levels of RF current, which then crawls over the whole
station.

>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.

Think very carefully about that treacherous word "ground"! In reality,
"ground" connections at physically different locations are *never* at
the same potential. Often it doesn't matter... but if the common
negative return for all the accessories is at the PSU, the voltage drop
along the negative return for the K3 will be injecting DC, RF and SMPS
leakage into the entire audio system.

If the PS is allowed to float and the common negative return for all the
accessories is moved very close to the K3, that might solve the
problems.

>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.

The unwanted impedances in the internal "ground" paths of the K3 are a
weakness that might make a problem appear worse, but I don't think they
are the actual source of the problems.

--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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Joe Subich, W4TV-3

RE: K3 RF Feedback Problem

In reply to this post by James C. Hall, MD-2

Jamie,

We had to modify all microHAM interface cables for the K3
to move the PTT ground from Pin 8 to the shell of the Foster
(mic) jack to avoid the "Pin 1 problem" caused by the common
RF choke (L7) on the front panel board.

I suspect your multi-switcher also connects PTT to pin 2/8,
mic to 1/7 and a shield between the chassis of both units.
Moving the PTT ground from pin 8 to the shell of the Foster
plug (or connecting PTT ground to the chassis in the
multi-switcher and leaving pin 8 open) should eliminate
the problem completely.

As W9AC reports, replacing L7 with a jumper seems to be the
best solution but it requires disassembling the front panel
of the radio. A good "work around" is to install a jumper
between pin 8 and the shell of your mic connector.

73,

... Joe, W4TV

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of James
> C. Hall, MD
> Sent: Saturday, November 01, 2008 1:13 PM
> To: 'Dr. James C. Garland'; [hidden email]
> Cc: 'W. Douglas McDowell'
> Subject: RE: [Elecraft] K3 RF Feedback Problem
>
>
> 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: [hidden email]
> [mailto:[hidden email]] On Behalf Of Dr.
> James C. Garland
> Sent: Saturday, November 01, 2008 8:57 AM
> To: [hidden email]
> 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
>
>
>

Jim Garland

RE: K3 RF Feedback Problem

Thanks very much to Joe W4TV and Ian GM3SEK for their helpful comments.Here's what I've learned.

1. RF leakage from the K3 into the 12V power cord does not appear to be a problem, as Ian surmised. Furthermore, as Ian also noted "If the PS is allowed to float and the common negative return for all the accessories is moved very close to the K3, that might solve the problems." This was largely correct. When I floated the negative return of the 12V supply, the distortion in the audio disappeared, so long as no other accessories were powered by the same 12V power supply. I measured about 200 mV p-p voltage fluctuations between the negative supply voltage and the chassis of the K3, when I talked into the K3 at 100W output.

2. 12V accessories ordinarily have their negative power terminal tied to their chassis ground. This is true of all the accessories in my station, including a homebrew keyer, an Alpha 4510 wattmeter, an EQ Plus audio compander, a 60 stereo amp used to drive bookshelf speakers,and my homebrew station controller. When I connected ANY of these to the same 12V line powering the K3, the distortion returned. Evidently, the K3's internal circuitry really like to have its local circuit ground isolated from its chassis.

3. A complicating factor is that I use a RIGrunner 12V distribution panel (model 4010S) to distribute 12V around to all the items in the station using standard Powerpole connectors. I discovered that the frame of the RIGrunner is internally connected to all its negative (black) Powerpole connectors, making it difficult to run an isolated negative return on my 12V line.

My fix for all of these is to use two 12V power supplies: a 30A Kepco supply with floating outputs for the K3, and a 6 Amp Lambda supply to power the accessories, via the RIGrunner distribution panel. No other devices in my station, other than the K3, have any problem with the negative 12V line tied to their chassis.

I know it is good engineering practice often to isolate internal circuitry from chassis ground. Lab quality signal generators, for example, usually tie the shell of their RF output connector to the chassis via a (typically) 100 ohm resistor. This isolation minimizes ground loop problems and current imbalance in the coax cable. So I don't fault Elecraft for doing this with the K3. The problem is that doing can introduce compatibility problems with otherstation instruments, as in my case.

Thanks again to all for your suggestions and commens.

73,

Jim W8ZR
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Joe Subich, W4TV-3

RE: K3 RF Feedback Problem

Jim,

> When I floated the negative return of the 12V
> supply, the distortion in the audio disappeared, so long as
> no other accessories were powered by the same 12V power
> supply. I measured about 200 mV p-p voltage fluctuations
> between the negative supply voltage and the chassis of the
> K3, when I talked into the K3 at 100W output.

When initially trying to resolve the issues with microHAM
interfaces and the K3, I also thought this was the problem.
Several users went to the extent of powering their radios
from batteries in an attempt to prevent return currents from
finding their way back to the power supply via the common
chassis grounds on the accessory (microHAM) equipment.

While is was successful in some cases, the only universal cure
was fixing the "pin 1 problem" with the front panel microphone
connector.

> I know it is good engineering practice often to isolate
> internal circuitry from chassis ground. Lab quality signal
> generators, for example, usually tie the shell of their RF
> output connector to the chassis via a (typically) 100 ohm
> resistor. This isolation minimizes ground loop problems and
> current imbalance in the coax cable. So I don't fault
> Elecraft for doing this with the K3.

The case of the K3 is just the opposite of what you describe.
The internal circuitry is tied to the chassis but the external
circuits are "floated" on the RF chokes in the return leads.
Specifically, the rear panel PTT/Foot switch return, front and
rear panel mic returns, the common on the ACC connector and the
RS-232 return are all go through RF chokes before reaching the
circuit common AND chassis ground.

The cause of the problem is the use of a common choke for the
front panel PTT and mic returns. When external interfaces
connect the PTT return to the chassis that 70 mV of ripple
appears at the top of the common RF choke (L7) - here's the
problem - effectively in series with the microphone signal!
Anything that eliminates the common impedance (think
"modulation choke") - including moving the PTT return
connection from pin 8 to the shell (chassis) of the of the
front panel mic jack or replacing L7 with a jumper - resolves
the problem by eliminating the place the RF gets into the
system.

73,

... Joe, W4TV

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of 4CX250B
> Sent: Saturday, November 01, 2008 5:02 PM
> To: [hidden email]
> Subject: RE: [Elecraft] K3 RF Feedback Problem
>
>
> Thanks very much to Joe W4TV and Ian GM3SEK for their
> helpful comments.Here's what I've learned.
>
> 1. RF leakage from the K3 into the 12V power cord does not
> appear to be a problem, as Ian surmised. Furthermore, as Ian
> also noted "If the PS is allowed to float and the common
> negative return for all the accessories is moved very close
> to the K3, that might solve the problems." This was largely
> correct. When I floated the negative return of the 12V
> supply, the distortion in the audio disappeared, so long as
> no other accessories were powered by the same 12V power
> supply. I measured about 200 mV p-p voltage fluctuations
> between the negative supply voltage and the chassis of the
> K3, when I talked into the K3 at 100W output.
>
> 2. 12V accessories ordinarily have their negative power
> terminal tied to their chassis ground. This is true of all
> the accessories in my station, including a homebrew keyer, an
> Alpha 4510 wattmeter, an EQ Plus audio compander, a 60 stereo
> amp used to drive bookshelf speakers,and my homebrew station
> controller. When I connected ANY of these to the same 12V
> line powering the K3, the distortion returned. Evidently,
> the K3's internal circuitry really like to have its local
> circuit ground isolated from its chassis.
>
> 3. A complicating factor is that I use a RIGrunner 12V
> distribution panel (model 4010S) to distribute 12V around to
> all the items in the station using standard Powerpole
> connectors. I discovered that the frame of the RIGrunner is
> internally connected to all its negative (black) Powerpole
> connectors, making it difficult to run an isolated negative
> return on my 12V line.
>
> My fix for all of these is to use two 12V power supplies: a
> 30A Kepco supply with floating outputs for the K3, and a 6
> Amp Lambda supply to power the accessories, via the RIGrunner
> distribution panel. No other devices in my station, other
> than the K3, have any problem with the negative 12V line tied
> to their chassis.
>
> I know it is good engineering practice often to isolate
> internal circuitry from chassis ground. Lab quality signal
> generators, for example, usually tie the shell of their RF
> output connector to the chassis via a (typically) 100 ohm
> resistor. This isolation minimizes ground loop problems and
> current imbalance in the coax cable. So I don't fault
> Elecraft for doing this with the K3. The problem is that
> doing can introduce compatibility problems with otherstation
> instruments, as in my case.
>
> Thanks again to all for your suggestions and commens.
>
> 73,
>
> Jim W8ZR

Don Wilhelm-4

Re: K3 RF Feedback Problem

In reply to this post by P.B. Christensen

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:[hidden email]>
> *To:* [hidden email] <mailto:[hidden email]>
> *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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P.B. Christensen

Re: K3 RF Feedback Problem

In reply to this post by Jim Garland

> "I know it is good engineering practice often to isolate internal
> circuitry from chassis ground. Lab quality signal generators, for example,
> usually tie the shell of their RF output connector to the chassis via a
> (typically) 100 ohm resistor..."

Moreover, designers of instrumentation-grade circuitry often tie multiple
returns to one common chassis ground. I know of one test equipment product
that utilizes five separate return path references to ground (e.g., digital
ground, analog ground, power supply ground, control systems ground, etc.),
but in each case, the return path currents are controlled and reasonably
isolated for each of the stated purposes. Nevertheless, each of the five
returns circuits in this example are tied to exactly one common chassis
point. But the current for the respective circuits circulate among their
own returns and not with others. They share just one low-impedance common
point. By contrast, ground loops exist in other equipment where one common
return is used with multiple circuit bonding points and various DC, AF, and
RF current are allowed to share and flow past sensitive devices.

For example, if there's a high current return path in a control circuit
(e.g., open collector to a solenoid), then the ground returns on a sensitive
3-stage-input instrumentation op-amp circuit (e.g., a high-gain mic pre-amp)
should not be sensitive to the current demands of the solenoid. The
addition of a 100-ohm resistor in your example may be one such use of
limiting return current on circuit paths *within* a piece of equipment.

But what is occurring with the K3's mic returns is a total isolation of a
common, grounded return path of the front and rear panel mic jacks at RF.
Yes, from D.C. through audio frequencies the return path is unimpeded as a
result of L4 and L7. But in the presence of RF, at some frequency between
AF and RF, the ground reference looking into the K3 mic connectors is
completely lost.

> "Perhaps it has something to do with using balanced line audio sources.
> Minimizing ground loops can be a headache when one has multiple audio
> devices, powered by different power supplies."

Even in balanced audio systems, the same rules apply. Had the K3 been
designed with a truly balanced, 3-stage instrumentation input for its mic
pre-amp, the inclusion of L4 and L7 on the shielded return paths would have
the same effect. The saving grace in an instrumentation-input circuit (or
in the alternative, an audio transformer input) is the inherently large
common-mode rejection ratio (CMRR) across a very broad frequency span that
limits the presence of RF on a twisted-pair audio line, even in the total
absence of the cable shielding. For nearly 100 years, the Bell System and
its progeny have used unshielded twisted-pair balanced audio systems in the
presence of outrageously-high RF fields with no measurable detriment to
performance in many instances.

Paul, W9AC

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MontyS

Re: K3 RF Feedback Problem

I bit the bullet and removed L4 and L7 this morning. It is not difficult;
they are both on the back of the front panel board. I jumpered the pads
with wire. I am now getting superb audio reports from critical local
stations that were guiding me in reducing RF on my audio. My MC-60 mic is
back in good graces.

It works.

Monty K2DLJ

> Even in balanced audio systems, the same rules apply. Had the K3 been
> designed with a truly balanced, 3-stage instrumentation input for its mic
> pre-amp, the inclusion of L4 and L7 on the shielded return paths would
> have the same effect. The saving grace in an instrumentation-input
> circuit (or in the alternative, an audio transformer input) is the
> inherently large common-mode rejection ratio (CMRR) across a very broad
> frequency span that limits the presence of RF on a twisted-pair audio
> line, even in the total absence of the cable shielding. For nearly 100
> years, the Bell System and its progeny have used unshielded twisted-pair
> balanced audio systems in the presence of outrageously-high RF fields with
> no measurable detriment to performance in many instances.
>
> Paul, W9AC

KK7P

Re: K3 RF Feedback Problem

> I bit the bullet and removed L4 and L7 this morning. It is not
> difficult; they are both on the back of the front panel board.

L4 on the front panel board is related to the issues some folks are
having when using the front panel mic connector with various audio
routers. It can be shorted with a jumper from pin 8 of the mic jack to
ground.

L7 on the KIO3 audio I/O board can also be bypassed for the same reasons
when using the rear panel mic jack in similar installations.

L7 on the front panel board is for reduction of noise from the RS232
serial port lines and should definitely not be bypassed!

73,

Lyle KK7P

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MontyS

Re: K3 RF Feedback Problem

Lyle -

Thanks for the correction. I saw the description on the schematic and
thought I was doing the right thing. Back in L7 goes, and I will look for
L7 on the KIO3 board.

Monty

>> I bit the bullet and removed L4 and L7 this morning. It is not
>> difficult; they are both on the back of the front panel board.
>
> L4 on the front panel board is related to the issues some folks are having
> when using the front panel mic connector with various audio routers. It
> can be shorted with a jumper from pin 8 of the mic jack to ground.
>
> L7 on the KIO3 audio I/O board can also be bypassed for the same reasons
> when using the rear panel mic jack in similar installations.
>
> L7 on the front panel board is for reduction of noise from the RS232
> serial port lines and should definitely not be bypassed!
>
> 73,
>
> Lyle KK7P
>
>
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Tom Hammond-2

Re: K3 RF Feedback Problem

In reply to this post by MontyS

Hi Monty:

I just added a jumper around (well, actually over) my L4. No need to risk
possible PCB damage by completely removing the RFCs.

73,

Tom N0SS

At 17:50 11/02/2008, Monty Shultes wrote:

>I bit the bullet and removed L4 and L7 this morning. It is not
>difficult; they are both on the back of the front panel board. I
>jumpered the pads with wire. I am now getting superb audio reports
>from critical local stations that were guiding me in reducing RF on
>my audio. My MC-60 mic is back in good graces.
>
>It works.
>
>Monty K2DLJ
>
>>Even in balanced audio systems, the same rules apply. Had the K3
>>been designed with a truly balanced, 3-stage instrumentation input
>>for its mic pre-amp, the inclusion of L4 and L7 on the shielded
>>return paths would have the same effect. The saving grace in an
>>instrumentation-input circuit (or in the alternative, an audio
>>transformer input) is the inherently large common-mode rejection
>>ratio (CMRR) across a very broad frequency span that limits the
>>presence of RF on a twisted-pair audio line, even in the total
>>absence of the cable shielding. For nearly 100 years, the Bell
>>System and its progeny have used unshielded twisted-pair balanced
>>audio systems in the presence of outrageously-high RF fields with
>>no measurable detriment to performance in many instances.
>>
>>Paul, W9AC
>
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>Elecraft web page: http://www.elecraft.com

P.B. Christensen

Re: K3 RF Feedback Problem

> I just added a jumper around (well, actually over) my L4. No need to risk
> possible PCB damage by completely removing the RFCs.

Unless the RFC begins to move under heat and begins bridging to an adjacent
pad or trace. Even when being careful, SMT/SMD parts can slightly shift on
the PCB.

Just my $0.02, but I prefer the removal of the RFC and short the pads with
0-ohm resistors, or fine magnet/buss wire. Removal of surface-mounted
components is generally easier than that of through-hole components. Also
important is that owing to packaging density, some of the pads and traces in
the K3 are extremely delicate and it does not take much lateral pressure on
some SMT/SMD components to accidentally remove a pad or trace. No matter
what style of component work-around you use, treat the pads and traces with
utmost respect..

Also, as Lyle pointed out, L4 and L7 in question are on separate PCBs (Main
DSP Board and KIO3). Anyone contemplating the mod should first familiarize
themselves with the schematics or they may be removing the wrong component
since identical component designators are used in the K3 across multiple
PCBs.

Paul, W9AC

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Rick N6CY

Re: K3 RF Feedback Problem

In reply to this post by Jim Garland

Jim, I've been on travel so apologize for the delayed reply. All I can say is WOW...I thought I was the only one who had this symptom with the K3! I have a similar setup with a Heil boom-mic going through a DX Doubler SO2R switch box, and had horrible SSB distortion on bands above 14 MHz and power levels above about 12 watts (the point where the PA kicks in). I unplugged and replugged cables and changed orientation; somehow eliminating the distortion, but never really found a "smoking gun" or understood the distortion mechanism. The thing that was killing me was that the K3 simply replaced an IC756ProII in the same spot and I had not previously experienced such transmit distortion. Several calls and emails to K3 support had me check the power supply. However the only conclusion we could come to over time was some kind of a weird ground loop.

Other posts seem to be right on target. RF on the mic "shield" line isn't being taken to true ground...and it's finding it's way into the microphone amplifier. Thanks to you smart guys out there, and for Elecraft, how about a design review?

73 Rick N6CY