++++++++++++++++++ See Also K2 WOOF ++++++++++++++++++ Date: Thu, 22 Nov 2001 21:42:08 -0800 From: Vic Rosenthal Subject: Re: [Elecraft] K2 VFO Anomaly Ken Lotts wrote: > > I monitored my K2 on another radio and discovered that there are some > sections of my K2 dial where my transmitted signal sounds horrible.. > especially on 40 meters. This is a known problem. I couldn't find the note on the elecraft web site, so I'm reproducing the original post by Gary Surrency (4/4/2000) describing it. If there's more recent info, the Elecraft gang will probably jump in, but in any event, here's a copy of Gary's post: Good luck, Vic K2VCO Fresno CA - ---------------------------------------------- Background: Some K2 owners have noticed a peculiar "warble" quality to the received signals around 7185 kHz on the 40m band. Signals received close to this frequency may sound as though they are "passing through water", or similar to signals that are received under conditions of multipath or polar absorption. It is most noticeable on SW broadcast stations, that are using amplitude modulation (AM). In addition, other stations listening to the K2's transmitted signal may detect this warble quality. Someone coined this behavior as the "Weird at One Frequency" or WAOF problem. This document provides a method to correct this. Technical details are provided at the end for those who are interested. We will change the manual for the K2 shortly to include these changes to the construction steps. (Note: The VFO lockup present when the K2 is powered ON with a memorized frequency near 7185 kHz is a separate but related issue, and will be corrected in the next firmware release.) Changes Needed: 1. Locate D16 and C70 at the front left corner of the RF PCB. 2. D16 needs to be mounted flush to the circuit board. By heating both leads of D16 together with the side of the soldering iron tip, you can press D16 down against the PCB easily and with minimal heat. This is preferable to removing D16 and possibly damaging this component. 3. Check for solder bridges between the pads of D16 and clean as necessary. D17 does not seem to be involved in this problem, so it may be left alone. 4. C70 needs to have short leads and should be positioned close to D22. If necessary, remove C70 and scrape some of the ceramic coating from the leads of the cap so it can be installed very close to the PCB. Once C70 is mounted, bend it over slightly so that it just touches D22. This procedure has worked on every K2 serviced to date. Technical Details (optional): Capacitor C70 has the full RF voltage of the VFO tank circuit on it. As a result, it emits some RF that is easily absorbed by D16. Since D16 is in the gate circuit of the 12.096 MHz PLL Reference Oscillator, it is susceptible to RF feedback from C70. The result is that there is an RF feedback path from the tank circuit of the VFO into the PLL Reference Oscillator at Q19. Q19's output is one of two important signal inputs into the MC145170 Phase Locked Loop (the other input is F_in from the VFO's output), so any phase distortion or undesired feedback can affect the PLL's function. Short leads in this area help insure circuit stability. PLL chip U4's OSC_in and F_in inputs are at equal frequencies (approximately 12.096 MHz) when the K2 is tuned to a 5 kHz wide window around 7185 kHz, plus or minus small differences in displayed frequency because of BFO and PLL Ref. Osc. variations. These signal's phase relationships are compared in the PLL, and the output is fed into the PLL loop filter at U6B. This IC and its surrounding circuitry filter the pulses from PLL chip U4, and provide the tuning voltage to the varactor diodes in the VFO (actually a VCO, or Voltage Controlled Oscillator). Any noise that is present on U6B's output modulates the VFO frequency by a small amount. This is heard in the VFO's output as a warble in what should be normally be a steady signal. Once the VFO is tuned outside of this 5 kHz window, the PLL phase detector functions normally. If you have a wide bandwidth oscilloscope, you can monitor the waveform at U4's PD_out pin 13, and probably see a "fuzziness" on the trailing edge of the complex waveform until the corrective procedures described above are completed. Interested readers can obtain more information on the MC145170 PLL synth IC from one of several Motorola applications notes. Try: http://www.mot-sps.com/books/dl110/pdf/mc145170-2rev4a.pdf The MC145170 has 4,800 transistors internally. - --- ++++++++++++++++++ Date: Thu, 22 Nov 2001 21:23:04 -0800 From: "John Grebenkemper, KI6WX" Subject: Re: [Elecraft] K2 VFO Anomaly Ken; In the range you are seeing the problem, I believe that the PLL Reference Oscillator and the VFO are on exactly the same frequency, which is roughly 12095 to 12100 kHz. You can check this by tuning the radio to this range, enabling the CAL Fctr, and placing the probe on TP3 for the PLL oscillator and TP1 for the VFO. If both frequencies are the same, you are getting some interaction between the PLL oscillator and the VFO and this is getting into the phase detector and loop filter which are formed by U4 and U6B on the RF board. Check to make sure that all of your bypassing capacitors in this area are properly installed. Also check that you have the correct parts in the loop filter formed with U6B. The reason that this occurs over 5 kHz is that the PLL oscillator frequency is pulled by 5 kHz before the frequency divider divisor in the MC145170 is changed. Once the divisor changes, the two oscillators are no longer on the same frequency. If you have a scope, you can probe U6B pin 7. This should be a very steady voltage, but I suspect in your case that you will find it varying at the same frequency as your snarl. - -John KI6WX > I monitored my K2 on another radio and discovered that there are some > sections of my K2 dial where my transmitted signal sounds horrible.. > especially on 40 meters. > > There is one section *exactly* 5kHz wide that my K2 CW signal sounds like a > snarl. > > 7.182.28 - 7.187.27 > > Transmitting a CW signal into a dummy load, I can hear a defined snap every > 5kHz on the band exactly as I cross the .27-.28 so I am sure that something > happens in the VFO exactly every 5kHz > > I am very surprised that I had never noticed this.. now that I know what to > listen for I can hear the snarl (and the snap) in my received signal as > well.. I guess I had always attributed it to QRN. > > This aught to be correctable.. before I tear into this.. anyone have any > suggestions? > > > Ken Lotts > aa7jc +++++++++++++++++++ Date: Sat, 24 Nov 2001 19:30:07 -0700 From: "Ken Lotts" Subject: RE: [Elecraft] K2 VFO Anomaly Thanks for the help guys! While I was waiting for your help that evening, I did the obvious and retouched all the solder connections that were in the PLL circuit.. That was all it took. If it comes back though, I will definitely know a few more tricks thanks to you! Now that I have the SSB & ATU options installed, this little rig has really become quite a little gem! Had I of know how useful the ATU is, I would have installed it months ago! The SSB filter (with mods) has really made a big difference too! I built a QRP ATU (LDG) a few years ago to go with my "Sierra" rig.. It works ok (especially at low pwr) but, it does not compare to the performance of the ATU in the K2. K2 delivers! Guess I need to get the rest of the option boards installed to see what else I have been missing ;-) Ken Lotts aa7jc +++++++++++++++++++ Date: Fri, 7 Dec 2001 11:26:30 EST From: Unifiedtx at aol.com Subject: [Elecraft] K2 VFO Anomaly (WAOF) I have checked D16 and resoldered it to make sure it is flush against the PCB. I have reheated C70 leads and pulled it down flush with the PCB. The leads of C84 and C85 have also been pulled down on the PCB. I have reheated the pins of D23, D24, D25 and D26 and made sure they are mounted flush with PCB. I have leaned C70 over to touch D22. I Still have the "arctic flutter" condition between 7181.16 KHz and 7186.15 KHz. I can also hear a very faint click every 5 KHz multiple in this vacinity of the 40 meter band. This is a stubborn case of the WAOF problem. Has anyone else had a stubborn case like this and corrected it? I would like to know what else you did to correct it. BTW, I removed the 33 PFd cap in the RTC and replaced it with a 68 PFd and found my clock keeps very accurate time with this new replacement. Thanks for your suggestions on the WAOF problem. Roy Morris W4WFB k2 #2225 KSB2, K160M, KNB2, KAF2, KAT2, AMP-KIT. ++++++++++++++++++++ Date: Sat, 8 Dec 2001 16:30:45 EST From: Unifiedtx at aol.com Subject: [Elecraft] K2 VFO Anomaly (WAOF) Gary Surrency said that when all else fails the way to fix the stubborn "wierd at one frequency" problem on 40 meters is to swap C70 and C71. This cured my "Arctic Flutter" problem between 7181.16 KHz and 7186.15 KHz. This was a tough one, but Gary knows his stuff!! Roy Morris W4WFB - --- +++++++++++++++++++ Reply-To: "Sean Baxter" From: "Sean Baxter" To: "Joe Malloy" , Subject: Re: [Elecraft] K2 - 40m Distortion Zones FIXED!! Date: Fri, 1 Feb 2002 23:33:46 -0500 Organization: Sean Baxter Thanks to all. WAOF was fixed by adding the 10pf cap across U4 pins 1 & 2. Sean - K3XT #2311 ----- Original Message ----- From: "Joe Malloy" To: "Sean Baxter" ; Sent: Friday, February 01, 2002 8:17 PM Subject: RE: [Elecraft] K2 - 40m Distortion Zones > Sure thing, you've got WAOF syndrome -- "weird at one frequency" is a well > known and documented problem (of minor proportions!). Go here > > http://www.elecraft.com/Apps/Alert11_WAOF.html > > and find out how to cure it and what causes it. By the way, though I > followed the instructions there, it didn't clear up my WAOF problem, but, > hey, everything else works, so I'm not complaining! > > 73, > > Joe, W2RBA > > > -----Original Message----- > > > > In the testing phase of my K2 I noticed something strange happening on 40 > > meters. With the SSB and CW modes (all filter options) I hear > > (all received > > signals) and will transmit on the following frequencies (SSB & CW) what > > sounds like 60 cycle modulated hum (very loud). On USB the frequency range > > is 7180 thru 7184, in LSB the frequency range is 7182 thru 7187. > > Except for > > the SSB module no other options are installed. This happens with both an > > outside antenna and a dummy load. When I tune outside these frequency > > ranges all works well. > > > > Any ideas? > > Thanks, > > Sean - K3XT > > #2311 ++++++++++++++++++