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K3 Amplifier Drive Inconsistency

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Bill Schwantes

K3 Amplifier Drive Inconsistency

I need help solving an amplifier drive inconsistency problem when I operate
using digital modes on 50 MHz. I noticed during a recent contest that the
K3 will sometimes key the amplifier but provide zero drive. I'm using K3 SN
669 on 50 MHz FT8, FT4 and MSK 144 and driving a KPA1500 to about 750 watts.

After several consecutive transmissions I notice that the amplifier
responds briefly to being keyed but the power drops to zero. It's keyed up
but has no output. The K3's output (drive) is set at 17 watts but there is
no output on the RF meter when this happens. The K3's red TX indicator is
on. This problem also occurs when driving a KPA500, though less often

I can correct this problem temporarily by switching the amplifier to
standby, which allows the K3 to change to "barefoot" output then switch
the amplifier back to "operate" which changes drive power back to it's
preset level. That's usually good for a dozen or so transmit cycles before
the problem reappears.

This problem does not occur when driving other amplifiers; nor does it
occur when using CW or SSB modes. It seems to have something to do with the
K3/KPA1500 command interface controlling the output power of the K3.

Any ideas?

73 Bill W7QQ
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donovanf

Re: K3 Amplifier Drive Inconsistency

Hi Bill,

I've never experienced your reported amplifier drive problem with
my K3 s/n #6100 driving 20 watts into my Lunar Link LA-62
1500 watt output 6 meter amplifier. Perhaps it unique to the
K3 - KPA1500 combination.

73
Frank
W3LPL

----- Original Message -----

From: "Bill Schwantes" <[hidden email]>
To: [hidden email]
Sent: Monday, August 5, 2019 11:24:54 PM
Subject: [Elecraft] K3 Amplifier Drive Inconsistency

I need help solving an amplifier drive inconsistency problem when I operate
using digital modes on 50 MHz. I noticed during a recent contest that the
K3 will sometimes key the amplifier but provide zero drive. I'm using K3 SN
669 on 50 MHz FT8, FT4 and MSK 144 and driving a KPA1500 to about 750 watts.

After several consecutive transmissions I notice that the amplifier
responds briefly to being keyed but the power drops to zero. It's keyed up
but has no output. The K3's output (drive) is set at 17 watts but there is
no output on the RF meter when this happens. The K3's red TX indicator is
on. This problem also occurs when driving a KPA500, though less often

I can correct this problem temporarily by switching the amplifier to
standby, which allows the K3 to change to "barefoot" output then switch
the amplifier back to "operate" which changes drive power back to it's
preset level. That's usually good for a dozen or so transmit cycles before
the problem reappears.

This problem does not occur when driving other amplifiers; nor does it
occur when using CW or SSB modes. It seems to have something to do with the
K3/KPA1500 command interface controlling the output power of the K3.

Any ideas?

73 Bill W7QQ
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Jim Rhodes-3

Re: K3 Amplifier Drive Inconsistency

Last night I noticed I had little to no output from my KX3/KXPA100 combo
especially on 40. I did a little experimenting and discovered that 3 turns
of the transmit audio line through an unknown mix clamp on ferrite fixed
the problem. Without it I was mildly affected on 20 and not at all on 15.
With that choke in place I was stable on 40 and all other bands I tried. I
am currently portable for work (in North Dakota) and am using a mobile
antenna clamped to the top of a 6 ft tall wooden fence. Without the choke
the ALC would go up when keyed, then drop off to near nothing. I know AFSK
is becoming a more preferred method of generating RTTY, but I have never
had this issue with FSK. So I learned a lesson about the little USB
soundcard I use for portable ops.

On Mon, Aug 5, 2019 at 6:34 PM <[hidden email]> wrote:

> Hi Bill,
>
>
> I've never experienced your reported amplifier drive problem with
> my K3 s/n #6100 driving 20 watts into my Lunar Link LA-62
> 1500 watt output 6 meter amplifier. Perhaps it unique to the
> K3 - KPA1500 combination.
>
>
> 73
> Frank
> W3LPL
>
> ----- Original Message -----
>
> From: "Bill Schwantes" <[hidden email]>
> To: [hidden email]
> Sent: Monday, August 5, 2019 11:24:54 PM
> Subject: [Elecraft] K3 Amplifier Drive Inconsistency
>
> I need help solving an amplifier drive inconsistency problem when I
> operate
> using digital modes on 50 MHz. I noticed during a recent contest that the
> K3 will sometimes key the amplifier but provide zero drive. I'm using K3
> SN
> 669 on 50 MHz FT8, FT4 and MSK 144 and driving a KPA1500 to about 750
> watts.
>
> After several consecutive transmissions I notice that the amplifier
> responds briefly to being keyed but the power drops to zero. It's keyed up
> but has no output. The K3's output (drive) is set at 17 watts but there is
> no output on the RF meter when this happens. The K3's red TX indicator is
> on. This problem also occurs when driving a KPA500, though less often
>
> I can correct this problem temporarily by switching the amplifier to
> standby, which allows the K3 to change to "barefoot" output then switch
> the amplifier back to "operate" which changes drive power back to it's
> preset level. That's usually good for a dozen or so transmit cycles before
> the problem reappears.
>
> This problem does not occur when driving other amplifiers; nor does it
> occur when using CW or SSB modes. It seems to have something to do with
> the
> K3/KPA1500 command interface controlling the output power of the K3.
>
> Any ideas?
>
> 73 Bill W7QQ
> ______________________________________________________________
> 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
> Message delivered to [hidden email]
>
> ______________________________________________________________
> 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
> Message delivered to [hidden email]
>

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Don Wilhelm

Re: K3 Amplifier Drive Inconsistency

Jim,

That sounds like RF getting into the audio. Try moving the ferrites to
the antenna feedpoint to choke off the RF on the coax shield. Yes, the
coax from a vertical needs a current choke. In fact, you may need a 2nd
choke at the shack entry point as well. The coax will try to be an
additional radial and that means it will have RF current on it - if not
from the antenna itself, the radiation from the radiator may couple with
the coax shield and cause RF on the shield (that is why the additional
choke at the shack entry).

The KX3 has one characteristic that may cause confusion. If the signal
to the mic input is too large, the KX3 will shut down the audio gain
giving the impression that there is not enough audio when in fact there
is too much signal at the input.

73,
Don W3FPR

On 8/5/2019 9:23 PM, Jim Rhodes wrote:

> Last night I noticed I had little to no output from my KX3/KXPA100 combo
> especially on 40. I did a little experimenting and discovered that 3 turns
> of the transmit audio line through an unknown mix clamp on ferrite fixed
> the problem. Without it I was mildly affected on 20 and not at all on 15.
> With that choke in place I was stable on 40 and all other bands I tried. I
> am currently portable for work (in North Dakota) and am using a mobile
> antenna clamped to the top of a 6 ft tall wooden fence. Without the choke
> the ALC would go up when keyed, then drop off to near nothing. I know AFSK
> is becoming a more preferred method of generating RTTY, but I have never
> had this issue with FSK. So I learned a lesson about the little USB
> soundcard I use for portable ops.

Jim Rhodes-3

Re: K3 Amplifier Drive Inconsistency

Thanks for the suggestion. The antenna is less than 25 feet from the rig.
The RF getting into the audio could be coming from the coax or the antenna,
no way to say which. There is 5-6 turns of coax, 6 inches in diameter, a
couple of feet from the rig where it comes into the apartment. Beads on
the coax might do the trick, but I am not near my parts boxes, they are
about a 12 hour each way drive away. And I am in very rural North Dakota,
would have to order something mail order and wait till pony express gets it
here. So I will stick with what works. I am in a tiny apartment with just a
little patch of weeds with a little grass on it and can't get the antenna
any further away. I had the same problem when I tried a short dipole with a
fair balun (elecraft of course) at the feed point. And, as I said, I now
have zero problems. Well, other than a crappy antenna setup.

On Mon, Aug 5, 2019 at 8:45 PM Don Wilhelm <[hidden email]> wrote:

> Jim,
>
> That sounds like RF getting into the audio. Try moving the ferrites to
> the antenna feedpoint to choke off the RF on the coax shield. Yes, the
> coax from a vertical needs a current choke. In fact, you may need a 2nd
> choke at the shack entry point as well. The coax will try to be an
> additional radial and that means it will have RF current on it - if not
> from the antenna itself, the radiation from the radiator may couple with
> the coax shield and cause RF on the shield (that is why the additional
> choke at the shack entry).
>
> The KX3 has one characteristic that may cause confusion. If the signal
> to the mic input is too large, the KX3 will shut down the audio gain
> giving the impression that there is not enough audio when in fact there
> is too much signal at the input.
>
> 73,
> Don W3FPR
>
>
> On 8/5/2019 9:23 PM, Jim Rhodes wrote:
> > Last night I noticed I had little to no output from my KX3/KXPA100 combo
> > especially on 40. I did a little experimenting and discovered that 3
> turns
> > of the transmit audio line through an unknown mix clamp on ferrite fixed
> > the problem. Without it I was mildly affected on 20 and not at all on 15.
> > With that choke in place I was stable on 40 and all other bands I tried.
> I
> > am currently portable for work (in North Dakota) and am using a mobile
> > antenna clamped to the top of a 6 ft tall wooden fence. Without the choke
> > the ALC would go up when keyed, then drop off to near nothing. I know
> AFSK
> > is becoming a more preferred method of generating RTTY, but I have never
> > had this issue with FSK. So I learned a lesson about the little USB
> > soundcard I use for portable ops.
>

Jim Brown-10

Re: K3 Amplifier Drive Inconsistency

In reply to this post by Jim Rhodes-3

On 8/5/2019 6:23 PM, Jim Rhodes wrote:
> I
> am currently portable for work (in North Dakota) and am using a mobile
> antenna clamped to the top of a 6 ft tall wooden fence.

That mobile whip is only half of an antenna. When used on a vehicle, the
metallic parts of the vehicle provide the other half, a path for return
current, which is called a counterpoise. If you provide no counterpoise,
return current will flow on the outside of the coax down into your shack.

The solution to your problem is to 1) provide that counterpoise,
possibly in the form of one or more resonant radials (more is better);
AND 2) put a proper ferrite choke at the antenna feedpoint so that the
coax doesn't carry return current. See http://k9yc.com/2018Cookbook.pdf
for detailed advice on chokes.

73, Jim K9YC

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