AGC Mod Query
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Hello everyone.
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? tnx. -- 73 Fernando N2FQ/NNNØJYM San Jose, CA <http://pages.sbcglobal.net/n2fq> _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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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? ---------------------------------------------- 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 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Fernando Quinones
Fernando,
Yes, I would say that 3.37 volts is a bit too low, and yes it will make a difference in how the RF Gain responds - both the RF Gain control and the AGC adjustment pot are in the same resistive network (no active devices involved) - it is a 2 legged resistor network with the RF Gain in one leg and the AGC Threshold pot in the other leg. If you did AGC on/off evaluation with an antenna attached, you have set the AGC to not activate on band noise - the proper setting should be to set it so no AGC action occurs with internal receiver noise, so do it with the antenna disconnected - or if you want to be particular, with a dummy load attached. My guess is that you will end up with a voltage on U2 pin 5 of somewhere between 3.6 and 3.75 volts - at least that has been my experience. 3.8 volts is usually too high but a lot depends on the actual gain of the IF amplifier chip. If you end up with something as low as 3.37 volts using internal receiver noise only, you should investigate to see what is causing the excessive receiver noise. 73, Don W3FPR > -----Original Message----- > > 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? > > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.323 / Virus Database: 267.6.6 - Release Date: 6/8/2005 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Ron, Don
thanks for the notes and explanation. I went back and set the U2 Pin 5 back to 3.8, monitored this voltage as I changed R1, did the AGC ON/OFF while I stepped down the voltage until I couldn't hear no difference at about 3.70vdc. RF gain back to normal. Much appreciated. take care. -- 73 Fernando N2FQ/NNNØJYM San Jose, CA <http://pages.sbcglobal.net/n2fq> _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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