Re: [Elecraft] [K2 4922] IF Amplifier Alignment
Clark Macaulay wrote: build process was unable to get 3.80: my dmm reads 3.76 but only at the extreme end of R1's range. I forgot to mention that I can only get 3.79, but that seems to be a bug in the manual. If one assumes U2b (i.e. got the a and b mixed before) doesn't load the voltage, I think the end stop voltage with all components nominal is 3.82; that means that 3.76 is less than 2% out, so within the component tolerances. It looks as though the manual should really have said set R1 at the end stop. At first, I didn't think much of it and have started on the Phase II of the RF board. Your message would imply that the desired 3.80 is probably a minimum and I can't even reach that! Since I've always had It's a maximum. The original design didn't have a variable, but it was found that many people wanted to lower R1 slightly. The material that describes the mod to the old version gives a better description of how one should vary R1; the current manual simply says you may want to adjust it, without any clue as to why. -- David Woolley Emails are not formal business letters, whatever businesses may want. RFC1855 says there should be an address here, but, in a world of spam, that is no longer good advice, as archive address hiding may not work. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
Re: [Elecraft] [K2 4922] IF Amplifier Alignment
Don, I found your answer on the setting of R1 in the last reply of interest to me. I'm buiding K2 #6100 and when I got to that part of the build process was unable to get 3.80: my dmm reads 3.76 but only at the extreme end of R1's range. At first, I didn't think much of it and have started on the Phase II of the RF board. Your message would imply that the desired 3.80 is probably a minimum and I can't even reach that! Since I've always had some trouble chasing agc circuits, could you suggest one place to start looking for the trouble? As they say, it all LOOKS fine to me and the resistance checks were good. 73 de Clark ke4rq ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
Re: [Elecraft] [K2 4922] IF Amplifier Alignment
David, Jack Smith has characterized the K2 AGC curves (along with some other rigs) and you may find some useful information on his website: http://www.cliftonlaboratories.com/receiver_agc_curves.htm The IF AMP AGC input is really current driven, so trying to obtain a direct relationship between the voltage and the AGC action is problematic at best - yes, I know the data sheet expresses the AGC voltage in the curves, but that is based on the operating conditions stated in the data sheet. When you set up the AGC Threshold voltage in the K2 to 3.8 volts, you are not reading the voltage into the MC1350, but rather reading an indirect control point voltage - the actual AGC drive to the MC1350 is two active devices 'down the road' from where the AGC Threshold is set. What I can say for certain is that I have had to re-visit the AGC Threshold and S-meter settings after completing assembly of any K2 that I have built or repaired (I have built many and repaired many more) - for instance, the filter passband alignment will influence the S-meter response, so the S-meter should best be set after the final filter alignment. True, the manual does not explicitly state that one should re-adjust the S-meter settings after all the other alignment is done, but it does hint at it. One can 'play' with the AGC Threshold voltage setting to achieve better receive sensitivity with no input signal, but move it very far below 3.8 volts and the S-meter settings and response gets 'squirrelly', so if you choose to do that, plan on a lot of interactive 'playing' with the AGC Threshold, CAL S Lo and CAL S Hi - the manual settings are a compromise, but they do work and work quite well - vary from them as you wish, but do so at your own peril, the 'standard' settings can be easily restored. 73, Don W3FPR David Woolley wrote: I don't believe the manual says this, although I've only got to the 40m receive only stage. I'm actually having great fun trying to work out how the AGC system really works and really should be calibrated. One complication is that the MC1350 is only characterised for a 12 volt supply, but the K2 uses 8. If I understand the internals of the chip properly, the AGC range will be constant but the threshold will reduce by one volt for every two volts reduction in supply voltage, so the 5 to 7 volts range in the data sheet will correspond with 3 to 5 volts in the K2 (at the V AGC point, not at U2 pin 5). Is that right? Another question is how one gets 88dB+ of S meter range (6dB * (9 - 1) + 40dB) from an AGC system with a maximum range of 68dB, especially when the initial S meter calibration uses less than 1.3 volts of the available 2 volt control range? Another one is that the design seems to assume a gain balance between the AF and AGC IF chains, but the differential gain is subject to production spread in the NE612 chips (minimum to typical gain of 3dB and unspecified typical to maximum gain). It also depends, to a lesser extent, on the output impedance of the 612 in the AGC chain. The AF chain will depend on the loss in the second filter; I don't know enough about crystal filters to estimate this. There are probably some other factors. (I think there is a deliberate 6dB difference in that the nominal gain of the U2a op amp is about 22 (33k explicit over 1.5k output resistance), and the LM 380 has a nominal gain of 50). I wonder if the feedback resistor on U2a really could do with being a variable. Although you can get a similar effect with R1 (because the main part of the AGC response is exponential), you lose range on the RF gain control. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
[Elecraft] [K2 4922] IF Amplifier Alignment
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Colleagues, I've moved on from the BFO issue (now resolved) to an IF alignment issue. My problem is a constant S3 reading on the S-meter. RF gain control works and is all the way turned up. Birdie at 6.999.17MHz is clearly readable but is only strong enough to push the S-meter up from S3 to S5. When I move off the birdie frequency I hear nothing on the headphones. Mode:CW Filter: FL2 RF gain: full AF gain: 12 o'clock Preamp: on The S-meter increases as the RF gain is decreased (as expected). As the constant reading masks the S-meter reading it is hard to peak the IF inductor L34 for maximum signal. Where to go from here? Thanks, Ron - -- - -=-o---=- Ron Hahn, EI2JP| Grid Square: IO62TG Rose Hill | Fists: 10883 Rosslare Strand| QRP-ARCI:12584 County Wexford, IRELAND| K2: 4922 __+__ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.7 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGgtP3yJCeYjZnfkQRArYMAJ4ybS19sZJFhHvqG80a/ZmavNRvjgCgmOIP pGISv6HI0RaYnFMapGCQQMg= =F8Uk -END PGP SIGNATURE- -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
Re: [Elecraft] [K2 4922] IF Amplifier Alignment
Ron, Peak the IF amp inductor (L34) either by ear or by connecting an audio voltmeter (or DVM in AC mode) to the speaker connection (RF Board P5) to detect the maximum signal. Attempting to use the S-meter at this stage is not reliable. In any case, the peak is very broad. If you cannot discern a peak, set the slug 1 turn from the top of the can. You can (and should) re-adjust the S-meter menu settings CAL S Lo and CAL S Hi after you complete all other assembly and test items. 73, Don W3FPR Ron Hahn (EI2JP) wrote: I've moved on from the BFO issue (now resolved) to an IF alignment issue. My problem is a constant S3 reading on the S-meter. RF gain control works and is all the way turned up. Birdie at 6.999.17MHz is clearly readable but is only strong enough to push the S-meter up from S3 to S5. When I move off the birdie frequency I hear nothing on the headphones. Mode:CW Filter: FL2 RF gain: full AF gain: 12 o'clock Preamp: on The S-meter increases as the RF gain is decreased (as expected). As the constant reading masks the S-meter reading it is hard to peak the IF inductor L34 for maximum signal. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
