VE6DRW AGC Mod Installed

[Darrell Bellerive-2]

For those of you following the saga of my AGC obsession, here's the latest.

I installed Dave's mod with only one change. I left C3 connected to pin 5 of
U2 rather than moving it to the RF Gain line as Dave's diagram shows. I have
emailed Dave about this, but haven't heard back yet.

The mod works very well. I adjusted the AGC line to have maximum gain from the
MC1350 with the RF Gain control fully clockwise and minimum gain from the
MC1350 with the RF gain control fully counter clockwise. The RF gain control
now controls the entire AGC range of the MC1350. This measured 68 dB of gain
range on my K2.

In addition the difference between AGC on and off is now a little less than
0.6 dB. Before the mod it was a little more than 0.8 dB. An improvement of
about 0.25 dB.

I tend to be one of those operators that uses the RF gain control to adjust
listening levels. With this mod the AF gain can be set at maximum, and with
the RF gain at minimum the receiver is virtually silent. Nice work on the
audio section Wayne and Eric! I never expected it to be this quiet.

For those implementing Dave's AGC mod, I'll go into more detail of my
adjustment procedure as it differs slightly from what Dave suggests.

First off I determined the AGC line voltage that gives me maximum gain from
the MC1350. This is measured at pin 1 of U2.

The primary tool used was an audio analyzer. My PC is equipped with an M-Audio
Audiophile 2496 sound card and uses Baudline as audio analysis software. I
suppose this is similar to Spectrogram on Windows computers, but I have never
used Spectrogram.

The K2 was set to 28.002 MHz, attenuator on, AGC fast, CW mode, SSB OP1 filter
selected, and RF gain at maximum. A DL1 dummy load was connected to the
antenna jack. The audio signal to the PC was tapped off the speaker output
with an 8 ohm speaker connected. The AF gain on the K2 was adjusted to
produce a noise floor around 40 dB down as displayed in the Baudline spectrum
analyzer.

As one adjusts the RM2 pot, one can see on the spectrum analyzer, the
approximate point where the gain of the MC1350 hits maximum. The AGC line
voltage at this point was used as a starting point. As the MC 1350 nears
maximum gain, the change in gain slows, so it is difficult to see the exact
point of maximum gain.

Setting Baudline to measure the total power of the received noise floor from
the K2, and averaging this reading over 5000 FFT transformations yielded a
reading stable to within 0.05 dB.

The AGC line voltage was then set to a few increments of 100 mV above and
below the starting point voltage and the audio power level was measured at
each increment. This narrowed down to the 100 mV range where maximum gain was
reached. I then continued to narrow down the voltage by halving the voltage
difference until I found the point of maximum gain. On my K2 this was 3.325
volts on the AGC line. Interestingly this coresponds to a measured AGC
Threshold voltage of 3.43 volts.

As the accuracy of my voltmeter is not calibrated, for reference my voltmeter
reads the 8v rail as 8.24 volts and the 5v rail as 4.98 volts.

Next, I needed to determine the minimum gain AGC voltage. The noise floor of
the K2 is not sufficient to find this level. An Elecraft XG1 set to output a
50 uV signal on 7.040 was used. The K2 was set to 7.040, the preamp may be
necessary to get a good reading. The audio signal was viewed on the spectrum
analyzer and RM2 adjusted to find the point of minimum gain. This is nowhere
near as critical to determine and a voltage of 5.10 volts was measured on the
AGC line. This results in a total swing of 1.78 volts on the AGC line from
maximum to minimum gain of the MC1350.

Following Dave's procedure for adjusting RM1 and RM2 resulted in the RF gain
control changing the AGC line through the full range of 3.325 to 5.10 volts.

Dave's procedure for adjusting RM3 and the s-meter worked well. I set the S HI
setting to show S9 on the meter with a 50 uV signal from the XG1.

So far I am very happy with this modification. I can no longer hear any
difference in the noise floor with the AGC switched between on and off. I
also have an RF gain control with a far greater range. The gradual onset of
AGC action that I like so much in the K2 seems to be preserved. I wonder if
Jack Smith, K8ZOA, of Clifton Laboratories could be enticed into performing
this mod and posting the new AGC graph.

If others try this modification please post your results.

73,
Darrell VA7TO K2#5093


--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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[wayne burdick]

Darrell Bellerive wrote:

> For those of you following the saga of my AGC obsession, here's the
> latest.
>
> I installed Dave's mod ....

Darrel,

Thanks for your careful observations on this AGC range modification.
(And thank you to Dave, as well, for doing the original work.)

Whenever a modification is proposed, a lot of K2 owners call or write
to us asking if they should make the mod, and whether it will be
incorporated into the design. So, to head off potential confusion, I'll
offer my two cents.

My opinion is that this mod would only be recommended for those who
frequently turn AGC off, use the RF GAIN control a lot, *and* wish to
fine-tune AGC behavior under these circumstances. From previous
surveys, we've found that 95% of the time, RF GAIN is left at maximum,
and AGC is left ON. So, for most K2 owners, the existing circuit is a
good compromise between functionality and complexity. We would be very
unlikely to incorporate this change or offer a mod kit.

That said, I'd encourage you (and/or Dave) to write up the modification
in enough detail to be easily duplicated. We'll be happy to add it to
our K2 modifications page.

73,
Wayne
N6KR






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[Darrell Bellerive-2]

Wayne,

Thanks for following my saga. What other rig manufacturer would do this and
encourage it. Sure makes me glad I chose a K2.

I agree that this mod makes no real performance difference, but just suits a
few of us who tend to do things different from most. Plus playing around with
radio mods is just so much fun.

Darrell

On March 7, 2007 11:48 am, wayne burdick wrote:

> Darrell Bellerive wrote:
> > For those of you following the saga of my AGC obsession, here's the
> > latest.
> >
> > I installed Dave's mod ....
>
> Darrel,
>
> Thanks for your careful observations on this AGC range modification.
> (And thank you to Dave, as well, for doing the original work.)
>
> Whenever a modification is proposed, a lot of K2 owners call or write
> to us asking if they should make the mod, and whether it will be
> incorporated into the design. So, to head off potential confusion, I'll
> offer my two cents.
>
> My opinion is that this mod would only be recommended for those who
> frequently turn AGC off, use the RF GAIN control a lot, *and* wish to
> fine-tune AGC behavior under these circumstances. From previous
> surveys, we've found that 95% of the time, RF GAIN is left at maximum,
> and AGC is left ON. So, for most K2 owners, the existing circuit is a
> good compromise between functionality and complexity. We would be very
> unlikely to incorporate this change or offer a mod kit.
>
> That said, I'd encourage you (and/or Dave) to write up the modification
> in enough detail to be easily duplicated. We'll be happy to add it to
> our K2 modifications page.
>
> 73,
> Wayne
> N6KR

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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