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" <[EMAIL PROTECTED]>
To: <[email protected]>
Cc: "Dave White" <[EMAIL PROTECTED]>
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 [EMAIL PROTECTED]
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
[EMAIL PROTECTED]
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--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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