+++++++++++++++++ Date: Wed, 05 Jun 2002 13:05:08 -0400 From: frastephen at comcast.net To: elecraft at mailman.qth.net Subject: [Elecraft] K2 AGC Mod Hello All! Well, I am still having a ball with the "infant" K2 #2644. I did notice something that others have been discussing lately, and thought I would report my findings. Well, I found that the AGC was way to pronounced on my K2, as built per the manual. Even with just background noise the AGC was very pronounced. Turning off the AGC was like turning off the attenuator and turning on the pre-amp! I also noticed absolutely no difference between having the AGC set to FAST or SLOW (I think this is a good real world, simple, bottom line "test" for too much AGC!). So I went to the local Radio Shack and picked up some 33k resistors (bought the 1/2 watt flavor, DOH!). Swapped out the 51k R1 and replaced it with the 33k. Powered up and I seem to be doing much better. Turnnig the AGC on when there are no signals present only drops the hiss by a small amount. Now, when I kick on the AGC I notice a marked difference in the behavior of the AGC whether it is in FAST or SLOW mode. Bottom Line: I believe that this mod has improved my K2 experience. YMMV The FAST/SLOW test worked for me ... anybody else think this is a useful test for diagnosing over-powered AGC? Peace and 73, Stephen W3SMK K2 #2644 (now with a little less AGC hegemony) ++++++++++++++++++++ Date: Wed, 05 Jun 2002 18:32:52 -0400 From: frastephen at comcast.net Subject: Re: [Elecraft] K2 AGC Mod To: k6se at juno.com, elecraft at mailman.qth.net From: > Stephen, W3SMK wrote: > > "Turning off the AGC was like turning off the attenuator and turning on > the pre-amp!" > ---------- > That's normal -- especially on stronger signals. I guess I wasn't clear -- this was the situation with NO signals present in the passband. A dead band was generating a fair amount of AGC. That is NOT normal. If the background noise is pushing your AGC then something is not quite right. No???? Peace and 73, Stephen W3SMK K2 #2644 ++++++++++++++++++++ Date: Mon, 02 Dec 2002 10:46:36 -0800 From: Wayne Burdick Organization: Elecraft To: TL Cc: Elecraft Reflector Subject: [Elecraft] Re: AGC MOD Isaac, > The new AGC circuit has a 51k pot in series with a 5.1k res. to GND. > Why not just replace the 51K R1 with a bare 50K POT? (is this some sort of protection?) The revision B K2 has a 50 k pot. The extra 5.1 k resistor in series is there to adjust the range of resistance in this circuit upward by 10% (so that it's about 5 k to 55 k), which should cover every possible AGC threshold adjustment requirement. The vast majority of K2 revision A owners will NOT have to make any change to R1. To see a change is warranted, listen to background noise on a very quiet band, then turn off the AGC (by holding PRE/ATTN and AGC together). If the background noise comes up significantly in volume, R1 is probably slightly too large. If this is the case, you can temporarily install a high-value pot across R1, as you mentioned. Once you have determined the optimal value, you can replace R1 with a new fixed value resistor. Typically it will be between 39 and 51 k. There are two other things to keep in mind regarding a change to R1: 1. You should re-test AGC performance on large signals 2. You'll need to re-calibrate the S-meter (CAL S HI and CAL S LO) 73, Wayne ++++++++++++++ Date: Mon, 02 Dec 2002 12:56:57 -0600 To: TL , elecraft at mailman.qth.net From: Tom Hammond =?iso-8859-1?Q?N=D8SS?= Subject: Re: [Elecraft] AGC MOD Isaac: >The new AGC circuit has a 51k pot in series with a 5.1k res. to GND. >Why not just replace the 51K R1 with a bare 50K POT? (is this some sort >of protection?) You could. In fact that's what I've done. BUT... you muse be more careful with the adjustment of the pot then, because if you get to too close to the threshold, the AGC will 'swamp' and you'll hear nothing. It's a fine adjustment when you get down close the the threshold... having the resistor in-line helps to 'smooth' things out a bit. >Not having enough room on my board (REV A) for my 50k POT, and not wanting >to drill holes or cut traces etc', I thought of paralleling R1 with a >small 1M pot I have, thus only moving R1 to the bottom of the board, below >its original location. I used a small PC-mount vertical pot, bent one of the pins for the END of the resistance under the pot and then bent the other two pins (one end and the wiper) out a bit, to fit into the holes vacated when I removed R1. This allowed me to install the pot with the bent-under pin resting on the top of R5 (possibly R2 as well), but in a position where it would not short out against anything else. Paralleling R1 might work as well. 73, Tom N0SS +++++++++++++++ Date: Mon, 17 Mar 2003 12:52:54 +1200 From: Ron Willcocks To: Elecraft Reflector Subject: [Elecraft] pre series "B" AGC adjustment. Hi folk, I have been reading the mail about setting the AGC levels in the latest "B" series K2's and noted the comments therin about tuning to a quiet part of a band and switching the AGC ON/OFF and noting the whether the background noise has a marked change in level. If it does change quite a bit then apparently the AGC is acting too soon. I tried it on my K2 (pre "B" series no. 2204) and I had a very marked difference in level when the AGC was turned ON/OFF, so I had a look through my saved items of interest from the Elecraft team, and found the article put out by Gary Surrency AB7MY back in Dec 2001, relating to "Poor S-Meter action". The main content of the article is about changing a diode to improve the S-Meter action, but as a related alteration, Gary wrote:- "If you notice a fairely large drop in RX hiss when you turn off the AGC, then you may want to decrease the value of R1 (also on the control board) from 51k to 33k. This has the same effect as if you could turn the RF gain a little further clockwise, and RX sensitivity will be better than before on weak signals." Anyway, I did change R1 to 33k and it has made a very noticeable difference in the sensitivity of my K2. There is now only a very small change in "hiss" when the AGC is turned ON/OFF, and it sure beats turning the AGC off when copying weak signals as I had been doing. I did have to alter the "S LO" and "S HI" setting a wee bit, but the overall AGC action is unchanged. Cheers.....Ron ZL1TW ++++++++++++++++