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K3 "hum" problem - not the usual?

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K1LI

K3 "hum" problem - not the usual?

After enjoying my K3 for about nine months, I was recently alerted to
audible 60-Hz hum on my transmitted audio signal. After reading the many
"hum related" posts on the reflector, I began investigating. Step 1 was to
remove all connections to the rig except the DC power and the coax to the
external PA (I placed a direct short on the keying line input at the PA).
Without a mic connected, the indication of the problem is that the PA
registers output power with no audio input. The problem only occurs in an
SSB mode when the nearby PA is both on and enabled (i.e., not on "standby")
and when the K3 is both within about 8-in of the PA and parallel or at right
angles to it; rotating the rig about 15 degrees around the z-axis eliminated
the hum. All in the usual vein, so far.

There are several aspects of this problem that I find hard to understand:

- With all conditions apparently constant, the problem is at least somewhat
intermittent and will build up to full output over about a 3 second period
when it does occur
- When "hum" is being produced, the K3 output power meter reads zero while
the external PA is putting out something near full power! (Ordinarily, it
takes 20 watts of output power from the K3 to drive the external to this
power level)
- Lowering the output power setting on the K3 reduces the amount of "hum
power" registering on the PA output power meter; there is a marked reduction
in "hum power" when I pass the K3's internal PA power threshold and the "hum
power" goes to zero when the K3 power output is set to zero
- There is hysteresis in the output power registered by the external PA vs.
the output power setting on the K3; i.e., starting with K3 output power set
to zero, the K3 output power setting which produces the onset of "hum power"
as shown by the external PA is a larger value than the K3 output power
setting at which the external PA "hum power" decreases / disappears.

I am "space challenged" inasmuch as I have an SO2R setup, so moving the K3
will be difficult. Any thoughts on what's going on?

--
73 -- Brian -- K1LI
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Jim Brown-10

Re: K3 "hum" problem - not the usual?

On Mon, 28 Sep 2009 13:29:54 -0400, Brian Machesney wrote:

>The problem only occurs in an
>SSB mode when the nearby PA is both on and enabled (i.e., not on "standby")
>and when the K3 is both within about 8-in of the PA and parallel or at right
>angles to it; rotating the rig about 15 degrees around the z-axis eliminated
>the hum. All in the usual vein, so far.

Magnetic field coupling from the power supply to the K3's input circuitry,
probably due to the use of unshielded audio transformers in the K3. This is a
well known problem. One good band-aid is to go to the TXEQ1 settings and fully
cut the three lowest octave bands. There's little communications value to the
speech below 300 Hz anyway, and it burns TX power, so it's a win-win. Also,
the acoustic stuff in that lower frequency range includes breath pops and room
noise.

It's also possible to have magnetic coupling on input wiring if you've managed
to form it into a big loop, but the transformers are the more likely cause. I
ran into the loop problem once when I jury-rigged a headphone amp for SO2R. No
problem in TX audio, but it was in my phones. One of my neighbors had so much
magnetically coupled hum into the Line In transformer that it regenerated to
full output power when he tried to run RTTY in AFSK mode.

73,

Jim Brown K9YC

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Joe Planisky

Re: K3 "hum" problem - not the usual?

Would wrapping some of this "Ultraperm 80" shielding around the K3's
audio transformers be a solution?

(http://www.goldmine-elec-products.com/prodinfo.asp?number=G16600A)

73
--
Joe KB8AP

On Sep 28, 2009, at 12:06 PM, Jim Brown wrote:

> On Mon, 28 Sep 2009 13:29:54 -0400, Brian Machesney wrote:
>
>> The problem only occurs in an
>> SSB mode when the nearby PA is both on and enabled (i.e., not on
>> "standby")
>> and when the K3 is both within about 8-in of the PA and parallel or
>> at right
>> angles to it; rotating the rig about 15 degrees around the z-axis
>> eliminated
>> the hum. All in the usual vein, so far.
>
> Magnetic field coupling from the power supply to the K3's input
> circuitry,
> probably due to the use of unshielded audio transformers in the K3.
> This is a
> well known problem. One good band-aid is to go to the TXEQ1 settings
> and fully
> cut the three lowest octave bands. There's little communications
> value to the
> speech below 300 Hz anyway, and it burns TX power, so it's a win-
> win. Also,
> the acoustic stuff in that lower frequency range includes breath
> pops and room
> noise.
>
> It's also possible to have magnetic coupling on input wiring if
> you've managed
> to form it into a big loop, but the transformers are the more likely
> cause. I
> ran into the loop problem once when I jury-rigged a headphone amp
> for SO2R. No
> problem in TX audio, but it was in my phones. One of my neighbors
> had so much
> magnetically coupled hum into the Line In transformer that it
> regenerated to
> full output power when he tried to run RTTY in AFSK mode.
>
> 73,
>
> Jim Brown K9YC
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
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> Please help support this email list: http://www.qsl.net/donate.html

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Leigh L. Klotz Jr WA5ZNU

Re: K3 "hum" problem - not the usual?

Administrator

In reply to this post by Jim Brown-10

Jim K9YC wrote:
> Magnetic field coupling from the power supply to the K3's input circuitry,
> probably due to the use of unshielded audio transformers in the K3. This
> is a well known problem. One good band-aid is to go to the TXEQ1
> settings and fully cut the three lowest octave bands.
> ...
> One of my neighbors had so much magnetically coupled hum
> into the Line In transformer that it regenerated to full
> output power when he tried to run RTTY in AFSK mode.

Note that the upcoming firmware upgrade will reportedly eliminate TXEQ in
AFSK (and other audio-based digital modes), and it apparently may be some
time before another update provides true per-mode TX/RX EQ. So people
with hum will need another solution, except for the voice modes.

I tried wrapping the component side of the transformers in a sheet of
Mu-metal (actually, a competitor, not Mu-brand) and noted no difference.
It was probably a futile effort anyway as it didn't wholly enclose the
transformer; probably I need to complain to Maxwell about this.

Leigh/WA5ZNU

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wayne burdick

Re: K3 "hum" problem - not the usual? [Try CONFIG:AFSK TX = FIL ON]

Administrator

The K3 has a CONFIG menu entry called "AFSK TX". If you set it to "FIL
ON", a 400-Hz transmit-mode audio filter is inserted, centered around
the mark/space tones. This is only available when the rig is set for
AFSK A mode. It should be effective against low-frequency hum coming
in through the mic or LINE IN connectors.

73,
Wayne
N6KR

On Sep 28, 2009, at 2:45 PM, Leigh L. Klotz, Jr. WA5ZNU wrote:

> Jim K9YC wrote:
>> Magnetic field coupling from the power supply to the K3's input
>> circuitry,
>> probably due to the use of unshielded audio transformers in the K3.
>> This
>> is a well known problem. One good band-aid is to go to the TXEQ1
>> settings and fully cut the three lowest octave bands.
>> ...
>> One of my neighbors had so much magnetically coupled hum
>> into the Line In transformer that it regenerated to full
>> output power when he tried to run RTTY in AFSK mode.
>
> Note that the upcoming firmware upgrade will reportedly eliminate
> TXEQ in
> AFSK (and other audio-based digital modes), and it apparently may be
> some
> time before another update provides true per-mode TX/RX EQ. So people
> with hum will need another solution, except for the voice modes.
>
> I tried wrapping the component side of the transformers in a sheet of
> Mu-metal (actually, a competitor, not Mu-brand) and noted no
> difference.
> It was probably a futile effort anyway as it didn't wholly enclose the
> transformer; probably I need to complain to Maxwell about this.
>
> Leigh/WA5ZNU

Leigh L. Klotz Jr WA5ZNU

Re: K3 "hum" problem - not the usual? [Try CONFIG:AFSK TX = FIL ON]

Administrator

In the creeping features department, a similar high-pass filter (60Hz, 120Hz, 300Hz?) might be nice for PSK31 and other digimode hams who use panoramic display software since they don't transmit at a fixed offset.
Leigh/WA5ZNU

wayne burdick wrote

The K3 has a CONFIG menu entry called "AFSK TX". If you set it to "FIL
ON", a 400-Hz transmit-mode audio filter is inserted, centered around
the mark/space tones. This is only available when the rig is set for
AFSK A mode. It should be effective against low-frequency hum coming
in through the mic or LINE IN connectors.

73,
Wayne
N6KR

On Sep 28, 2009, at 2:45 PM, Leigh L. Klotz, Jr. WA5ZNU wrote:

> Jim K9YC wrote:
>> Magnetic field coupling from the power supply to the K3's input
>> circuitry,
>> probably due to the use of unshielded audio transformers in the K3.
>> This
>> is a well known problem. One good band-aid is to go to the TXEQ1
>> settings and fully cut the three lowest octave bands.
>> ...
>> One of my neighbors had so much magnetically coupled hum
>> into the Line In transformer that it regenerated to full
>> output power when he tried to run RTTY in AFSK mode.
>
> Note that the upcoming firmware upgrade will reportedly eliminate
> TXEQ in
> AFSK (and other audio-based digital modes), and it apparently may be
> some
> time before another update provides true per-mode TX/RX EQ. So people
> with hum will need another solution, except for the voice modes.
>
> I tried wrapping the component side of the transformers in a sheet of
> Mu-metal (actually, a competitor, not Mu-brand) and noted no
> difference.
> It was probably a futile effort anyway as it didn't wholly enclose the
> transformer; probably I need to complain to Maxwell about this.
>
> Leigh/WA5ZNU

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K1LI

Re: K3 "hum" problem - not the usual? [Try CONFIG:AFSK TX = FIL ON]

In reply to this post by wayne burdick

Wayne,

Thanks for that suggestion - it worked!! One starts to wonder if there is
any thing that this radio can't do!

73 -- Brian -- K1LI

On Mon, Sep 28, 2009 at 5:58 PM, Wayne Burdick <[hidden email]> wrote:

> The K3 has a CONFIG menu entry called "AFSK TX". If you set it to "FIL
> ON", a 400-Hz transmit-mode audio filter is inserted, centered around
> the mark/space tones. This is only available when the rig is set for
> AFSK A mode. It should be effective against low-frequency hum coming
> in through the mic or LINE IN connectors.
>
> 73,
> Wayne
> N6KR
>