[Elecraft] AGC Mod Query

[Ron D'Eau Claire]

Fernanco, N2FQ wrote:

Awhile back there was some mail about the AGC Mod on the K2. Specifically,
adjusting R1 on the Control board.

My K2 had a fixed resistor so I replaced it with a 20 turn
50k one. I also followed the write up about how to adjust it for apparent
equal noise with AGC ON/OFF.

Worked like a charm and the final voltage on U2 pin 5 turn out to be
3.37vdc.

The audio coming out the speaker booms now, BUT
the RF control is not as effective as it used to be.
Namely, I like to turn down the RF to lessen
the background noise. Now I can turn
the RF all the way ccw and see no apparent
reduction in background noise.

So I'm going back into the rig and play with it.

My question is this: Did I make a too aggressive
change on R1?

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Definitely yes! You've disabled the AGC. The RF GAIN is merely a manual AGC
voltage controller. It does with a knob what the AGC amplifiers do
automatically based on the received signal strength.  Clearly, the RF GAIN
no longer has sufficient voltage swing to turn down the I.F. amplifier gain.


Another way to demonstrate that the AGC isn't working is to tune in a
decently-strong signal, then turn the AGC off (press PRE/ATT and AGC buttons
simultaneously. "OFF" will appear on the LCD display). If the AGC is
working, you'll hear a huge increase in the signal level when you turn it
off. If the signal stays the about same, the AGC is already off. Press the
buttons again to see "On" appear on the LCD and you should hear the signal
level drop as the AGC takes hold. Also the S-meter bar graph will indicate a
signal strength.

When the AGC is on, you want it to start reacting to the background noise
just slightly. Of course that will vary from band to band. Most Hams live
with the most QRN on 160 or 80 meters. Also turning the PREAMP on will
increase the background noise. 

Ron AC7AC