[Elecraft] AGC Modification to the K2

[Dave White]

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" <va7to at yahoo.ca>
To: <elecraft at mailman.qth.net>
Cc: "Dave White" <dwds at trinustech.com>
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 dwds at trinustech.com. 
>> 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
>> dwds at trinustech.com
>>
>>
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>
> -- 
> Darrell Bellerive
> Amateur Radio Stations VA7TO and VE7CLA
> Grand Forks, British Columbia, Canada
>