K2 RF Feedback
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Given a few hours free I decided to investigate a problem with RF feedback that
I had been getting with my K2. Occasionally on 80m I had received reports that the TX audio was not 100% and after backing everything off, things improved. It was only when I noticed that on 40m I could provoke the RF feedback that I have gotten around to doing something. After checking the mic leads etc I decided that the RF was getting into the input of the speech amp/compressor IC. The cure was to fit a 1mH RF choke (out of the junk box) in series with C32. This provided an immediate and complete cure. Hope this helps someone. 73 Stewart G3RXQ _______________________________________________ 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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Stewart - Did you ground the shell of the mic connector on the K2? I
found that the r.f. feedback that I was getting was completely cured by the suggested mod on the Elecraft web site. 73 de David G4DMP In a recent message, Stewart Baker <[hidden email]> said.... >Given a few hours free I decided to investigate a problem with RF feedback that >I had been getting with my K2. >Occasionally on 80m I had received reports that the TX audio was not 100% and >after backing everything off, things improved. It was only when I noticed that >on 40m I could provoke the RF feedback that I have gotten around to doing >something. >After checking the mic leads etc I decided that the RF was getting into the >input of the speech amp/compressor IC. >The cure was to fit a 1mH RF choke (out of the junk box) in series with C32. >This provided an immediate and complete cure. Hope this helps someone. 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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On Sun, 11 Jul 2004 19:19:10 +0100, David Pratt wrote:
> Stewart - Did you ground the shell of the mic connector on the K2? I > found that the r.f. feedback that I was getting was completely cured by > the suggested mod on the Elecraft web site. > 73 de David G4DMP > In a recent message, Stewart Baker <[hidden email]> > said.... >> Given a few hours free I decided to investigate a problem with RF feedback >> that >> I had been getting with my K2. >> Occasionally on 80m I had received reports that the TX audio was not 100% >> and >> after backing everything off, things improved. It was only when I noticed >> that >> on 40m I could provoke the RF feedback that I have gotten around to doing >> something. >> After checking the mic leads etc I decided that the RF was getting into the >> input of the speech amp/compressor IC. >> The cure was to fit a 1mH RF choke (out of the junk box) in series with C32. >> This provided an immediate and complete cure. Hope this helps someone. David, Yes, I did that some time ago. It certainly makes a difference under certain circumstances. The problem I was getting, however seemed to be that despite a good screened mic lead, plug and socket, there was sufficient RF getting onto the compressor IC input to make an effect. The only RF suppression on the AF line is 0.02uF (1 cap on the control board, and 1 cap on the KSB2). These C's provide a shunt to RF which for most operations should be OK, however as the overall system gain is high any RF that gets into the KSB2 input will cause a problem. Adding the 1mH RFC in series with the mic signal gives a high impedance path to RF, and a low one to the mic audio. I have also added a 0.001uF cap directly across the compressor input pins. This PI filtering should stop any RF "dead in it's tracks". 73 Stewart G3RXQ _______________________________________________ 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 Stewart Baker
On Mon, 12 Jul 2004 11:50:46 EDT, [hidden email] wrote:
> Hello Stewart, > I have experienced "fuzzy" transmitted audio with my K2/100,KAT100-1 on 75 > meters also. My KSB2 has all the KI6WX mods and has a bandwidth of 2.5 Khz. > It looks great on Spectrogram, and the received audio is excellent. I have > two K2s (both with KI6WX mods) and they both sound fuzzy so I suspect it is > an RF feedback. I removed the Elecraft MH2 (electret) mike and removed the > 5.6K resistor across pins 1 and 6 of the mike connector and connected the > Kenwood dynamic mike that came with my TS-850. This mike seems to clear up > the problem and I am getting good clean audio reports. This seems to mean > that the RF is getting into the mike input. I also did the mod to ground the > mike connector, but I don't think much of that mod, and don't think it helped > much if at all. Now I will have to tackle the same problem in my older K2. > BTW, my hamshack is on the second floor of my garage. This can present some > challanging grounding "opportunities". Anyway, I thank you for sharing your > remedy with us by using the RF choke in series with C32 on the KSB2 board. > Roy Morris W4WFB Roy, Thanks for your mail. Although I am using a balanced antenna system, on some bands particularly 40m there is still a significant amount of RF in my shack. Your grounding situation will be a lot worse (been there, done that, got the Tee shirt !) It only needs a small trace of RF getting into the audio side to make your transmission sound "fuzzy". The RFC mod I did got rid of all the RF feedback problems I was getting, without affecting the TX audio frequency response one jot. Anyway, putting it in can do no harm. 73 Stewart G3RXQ _______________________________________________ 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 Stewart Baker
Hi John,
The capacitor goes between pins 1 & 4 of U3 (SSM2165). It should be mounted on the bottom side of the KSB2 board. The RF choke value is not critical and can be anywhere between 1mH and 3mH. If you use one of a reasonable size it can be easily fitted by unsoldering and lifting out one end of C32 . I then put a small length of component wire into the empty hole and soldered the RF choke between the floating end of C32 and the component wire. The reference to PI filter is because the new circuit when drawn resembles the Greek letter PI (can't find it on my keyboard !) The circuit consists of C33 as the left hand bar of the character, the RF choke as the crossbar, and the new (0.001u) capacitor as the right hand bar. Most Amateur radio textbooks will show examples of PI circuits, the most famous being the PI network useful for impedance matching. I have found that RF feedback can be a difficult beast to track down. Sometimes it can occur when the antenna/feeder arrangement changes only very slightly. A setup that works well one day may play up the next if for example if you tidy up the the shack wiring (an activity never performed here ). The other area that the K2 is very sensitive to is magnetic coupling from Power supply transformers. This can badly affect both transmitted and received signals. It is worth looking at earlier posts on this subject. 73 Stewart G3RXQ On Wed, 14 Jul 2004 10:41:40 -0500, John Rader wrote: > Hello Stewart, > Thanks for the suggestion. > I occasionally get RF into my audio as well. I am going to put a choke in > series with C32 as you suggest. How critical is the chokes value? > Most audio reports are good, but occasionally I recieve a bad one. >> I have also added a 0.001uF cap directly across the compressor input pins. >> This PI filtering should stop any RF "dead in it's tracks". > Do You remember which pin numbers on the compressor chip you placed the cap > across? Can you explain the PI filter as well. > Thanks, > John K5XTX > _________________________________________________________________ > MSN Toolbar provides one-click access to Hotmail from any Web page FREE > download! http://toolbar.msn.click-url.com/go/onm00200413ave/direct/01/ _______________________________________________ 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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