AGC Modification to the K2

[Dave White-3]

I have created a small modification to the AGC circuit of the Elecraft K2.
It alters the AGC onset point of the MC 1350 IF Amplifier (U12), which
results in more gain at the IF stage, and subsequently more audio output.
The mod involves replacing RP 6 and R1 with a series of three
commonly-available potentiometers and adjusting the pots so that the RF Gain
control swings the AGC line voltage through new values.

I've authored a short 3-page article that describes the modification. I'll
send it to anyone who replies directly to me at [hidden email]. There
is NO parts kit, so please don't ask. If there is enough interest, I'll
submit it to Eric or Wayne at Elecraft to see if they want to post it on the
website.

THE USUAL DISCLAIMER APPLIES: This is NOT an authorized Elecraft
modification and you implement it at your own risk.  It worked for me, but
may not for you.

Regards

Dave White
VE6DRW
[hidden email]


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[Ingo Meyer, DK3RED]

Hello Dave,

Because your host mail1.trinustech.com [207.229.33.167] don't accept mails from
my address and send me an error 553, I try it via the list.

Please send me the AGC modification too. Thanks!

To all others: Sorry for the bandwidth.
--
72/73 de Ingo, DK3RED - Don't forget: the fun is the power!

     [hidden email] - www.qrp4fun.de
        DL-QRP-AG #824 - www.dl-qrp-ag.de
       QRP ARCI #11295 - www.qrparci.org
         DARC #2360404 - www.darc.de

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[G4BUE]

I too cannot e-mail you direct Dave (see the message below I got
back).  Apologies for contacting you via the Reflector.

Please send me your AGC modification article.

Thanks and 73 de Chris, N4CJ (G4BUE)

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On 24 Feb 2007 at 9:04, Dave White wrote:

> I've authored a short 3-page article that describes the
> modification. I'll
> send it to anyone who replies directly to me at [hidden email].

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[Darrell Bellerive-2]

Dave,

I have just read through your modification and I really like what you have
designed.

I have never been able to get my AGC set to where I felt it should be.
Regardless of where the AGC threshold trimmer (R1) is set, there is always
more receiver gain with the AGC off. And moving the AGC voltage down from the
recommended 3.8 volt setting reduces the range of the RF gain control.

Have you measured the overall receiver gain increase with this mod?

It would also be interesting to see Jack Smith, K8ZOA's AGC analysis on a
modified K2.

I am anxious to try this mod.

Darrell


On February 24, 2007 08:04 am, Dave White wrote:

> I have created a small modification to the AGC circuit of the Elecraft K2.
> It alters the AGC onset point of the MC 1350 IF Amplifier (U12), which
> results in more gain at the IF stage, and subsequently more audio output.
> The mod involves replacing RP 6 and R1 with a series of three
> commonly-available potentiometers and adjusting the pots so that the RF
> Gain control swings the AGC line voltage through new values.
>
> I've authored a short 3-page article that describes the modification. I'll
> send it to anyone who replies directly to me at [hidden email]. There
> is NO parts kit, so please don't ask. If there is enough interest, I'll
> submit it to Eric or Wayne at Elecraft to see if they want to post it on
> the website.
>
> THE USUAL DISCLAIMER APPLIES: This is NOT an authorized Elecraft
> modification and you implement it at your own risk.  It worked for me, but
> may not for you.
>
> Regards
>
> Dave White
> VE6DRW
> [hidden email]
>
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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[Dave White-3]

Hi Darrell:

I never measured the increase in IF gain, as it strictly depends on your AGC
line starting point - and component tolerances.  However, you can download
the Motorola data sheet for the MC1350 and make some rough approximations.
The MC1350 AGC threshold voltage seems to track with the supply voltage;
thus if you are using 12VDC (the Motorola design spec.), then you need to
reduce the AGC threshold voltage (nominally 5 volts) by a ratio equivalent
to your new supply voltage (ie: 8VDC).  This too, is dependent on the actual
voltage to the 8R rail on the K2 AND the IF Amp dropping resistor (R112). As
explained in the article, if you're trying to maximize the IF gain, then you
need to determine the onset point empirically.

The AGC curve of the MC1350 is not linear, but at the onset point, the slope
is about 18db/volt.  Thus - assuming your max-gain threshold voltage to the
MC1350 is 3.25 volts, and you're starting with 3.8 volts, you could expect a
gain increase of about 10db.

When examining the Motorla data sheet, pay careful attention to their test
circuit.  The AGC line does draw some current.  Thus, you need to measure
the AGC voltage on the supply-side of the dropping resistor, NOT directly at
Pin 5 of the MC1350. This through me off for a while until I realized
Motorola's measurement parameters. They specify a 5.1K series resistor in
the AGC line while the K2 uses a 5.6K value.  The difference is not
relevant - as at the onset point, it only represents a potential .05 volt
error in supply to the MC-1350 - most likely beyond the tolerance of most of
our measuring equipment.

For some "AGC Zealot", you could alter the AGC characteristics by
drastically altering the series resistor value (100K for example) and the
AGC onset voltage - but this would require a MAJOR design of the AGC
circuit.  That's way beyond what I was trying to accomplish; I think the
MC1350 does a pretty good job as is.

I've used the modification for about 7 months and it seemed to work well for
me.

Good Luck

Dave
VE6DRW

----- Original Message -----
From: "Darrell Bellerive" <[hidden email]>
To: <[hidden email]>
Cc: "Dave White" <[hidden email]>
Sent: Monday, February 26, 2007 7:21 PM
Subject: Re: [Elecraft] AGC Modification to the K2

> Dave,
>
> I have just read through your modification and I really like what you have
> designed.
>
> I have never been able to get my AGC set to where I felt it should be.
> Regardless of where the AGC threshold trimmer (R1) is set, there is always
> more receiver gain with the AGC off. And moving the AGC voltage down from
> the
> recommended 3.8 volt setting reduces the range of the RF gain control.
>
> Have you measured the overall receiver gain increase with this mod?
>
> It would also be interesting to see Jack Smith, K8ZOA's AGC analysis on a
> modified K2.
>
> I am anxious to try this mod.
>
> Darrell
>
>
> On February 24, 2007 08:04 am, Dave White wrote:
>> I have created a small modification to the AGC circuit of the Elecraft
>> K2.
>> It alters the AGC onset point of the MC 1350 IF Amplifier (U12), which
>> results in more gain at the IF stage, and subsequently more audio output.
>> The mod involves replacing RP 6 and R1 with a series of three
>> commonly-available potentiometers and adjusting the pots so that the RF
>> Gain control swings the AGC line voltage through new values.
>>
>> I've authored a short 3-page article that describes the modification.
>> I'll
>> send it to anyone who replies directly to me at [hidden email].
>> There
>> is NO parts kit, so please don't ask. If there is enough interest, I'll
>> submit it to Eric or Wayne at Elecraft to see if they want to post it on
>> the website.
>>
>> THE USUAL DISCLAIMER APPLIES: This is NOT an authorized Elecraft
>> modification and you implement it at your own risk.  It worked for me,
>> but
>> may not for you.
>>
>> Regards
>>
>> Dave White
>> VE6DRW
>> [hidden email]
>>
>>
>> _______________________________________________
>> Elecraft mailing list
>> Post to: [hidden email]
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
>>  http://mailman.qth.net/mailman/listinfo/elecraft
>>
>> Help: http://mailman.qth.net/subscribers.htm
>> Elecraft web page: http://www.elecraft.com
>
> --
> Darrell Bellerive
> Amateur Radio Stations VA7TO and VE7CLA
> Grand Forks, British Columbia, Canada
>

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[Darwin, Keith]

This is interesting.  I've noticed the K2 AGC doesn't seem to work like
"other rigs".  Specifically, the K2's AGC seems to be less aggressive in
reducing IF gain in the presence of a big signal.  This means the K2
produces more audio dynamic range than other rigs I've used.  The quiet
background noise (or signals) are indeed quieter than the louder strong
signals.  For casual QSOs this means I have a higher signal to noise
ratio for more pleasant copy.

Does this mod do anything to make the AGC more aggressive?

- Keith N1AS -
- K2 5411.ssb.100 -

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

I have never been able to get my AGC set to where I felt it should be.
Regardless of where the AGC threshold trimmer (R1) is set, there is
always more receiver gain with the AGC off. And moving the AGC voltage
down from the recommended 3.8 volt setting reduces the range of the RF
gain control.

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[Jack Smith-6]

>
> You might find my AGC measurements on the K2 useful, where I compare measured AGC response (audio output verus input signal level) for a K2, a Racal RA6790/GM, a Cubic R3030 and a Drake R7. The K2's curve is different and that may explain some of the "K2 sound" in my opinion. The data is presented at http://www.cliftonlaboratories.com/receiver_agc_curves.htm .
Jack K8ZOA
www.cliftonlaboratories.com



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