[Elecraft] VE6DRW AGC Mod Installed

[Darrell Bellerive]

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