[Elecraft] K2 VCO Alignment Issue

Sun Apr 28 18:00:29 EDT 2019

Don,

Finally got back to the K2 this afternoon. I have confirmed that T5 is on
the yellow core, with 16 turns through the center on the red and 4 turns on
the green. I mentioned earlier that I got all the toroids from the Toroid
Guy, so I felt confident that it was correctly wound. I checked all the
relay pins on K13, K14, and K15 and did not see any solder bridges on any
of them. Next, I pulled T5 and removed one turn and re-installed.
Unfortunately, nothing has changed. I'm still getting that same 8.37 V at
R30. Incidentally, I'm getting the 8.37 V on either side of R30. Is that
normal? I can also pull T5 and wind a new one with the parts that came with
the kit if you think that might help, but something tells me that the
problem lies elsewhere. I also thought about pulling C72 and replacing it
with another one, but unfortunately there was only one of that value in the
kit and I don't have anything like it it my parts stash. Any new thoughts?

de John, WB4GLJ

On Sat, Apr 27, 2019 at 6:00 PM Don Wilhelm <donwilh at embarqmail.com> wrote:

> John,
>
> Both frequencies are too low.
> You have either too much capacitance or too much inductance in the VFO.
> Count the number of times the red wire goes through the center of the T5
> core - it should be 16.
>
> Check relays K13, K14 and K15 for solder bridges.
>
> Make sure the core of T5 is yellow.
> You might have to try removing a turn from T5 if you can find no other
> cause - there have been a few K2s in the long past where the
> permeability if the T5 core was higher than expected, and removing 1
> turn from T5 corrected things - I have not seen that recently, but I do
> mention it 'just in case'.
>
> If all else fails, you may have to "wind your way" through the relay
> connections for the VFO Range Select area using the Relay Table on the
> Schematic Key page as your guide for the paths through the relays.
>
> 73,
> Don W3FPR
>
> On 4/27/2019 5:37 PM, John Wingard wrote:
> > Hi Don,
> >
> > Thanks for the quick response. I did check C72 again. It is a 271
> > value. I then spread the red turns on T5 as far as I could. I'd say
> > they cover about 90+% of the toroid. I put the L30 slug at mid-point
> > of travel and measured voltage at R30 and it is still right at 8.37
> > V.  While there I checked the frequency at TP1 and it was 8247.90,
> > which is considerably lower than the 8913 that you expected.
> >
> > Next I went up to 7100 and measured the voltage at R30. Still right at
> > 8.37 V, no change. Frequency at TP1 @ 7100 was 11558.11.
> >
> > So, it appears that the frequency at 4000 is not within the expected
> > range, and likely not at 7100 as well. Hope this tells you something
> > because I'm stumped.
> > I've got some plans for this evening so I may not get back on it until
> > tomorrow afternoon, but if you've got any more things for me to try I
> > will do them then.
> >
> > John, WB4GLJ
> >
> > On Sat, Apr 27, 2019 at 1:57 PM Don Wilhelm <donwilh at embarqmail.com
> > <mailto:donwilh at embarqmail.com>> wrote:
> >
> >     John,
> >
> >     That high voltage is trying to reduce the capacity of the varactors.
> >     Which means there is either too much capacity or too much
> >     inductance in
> >     the circuit.
> >
> >     Check the value of C72 first - it should be marked "271". Then set
> >     the
> >     L30 slug to about mid-position and spread the red turns of T5
> >     enough so
> >     the starting and ending turns are adjacent.  What is the voltage when
> >     tuned to 4000 kHz?  Measure the frequency at TP1 - it should be near
> >     8913 kHz.
> >
> >     Then change to 40 meters - 7100 kHz - What is the R30 voltage and
> >     what
> >     is the VFO frequency measured at TP1.
> >
> >     The answer to those questions will help in determining what to do
> >     next.
> >
> >     73,
> >     Don W3FPR
> >
> >     On 4/27/2019 1:32 PM, John Wingard wrote:
> >     > Working on my 2nd K2 (S/N 7869) and have run into a problem with
> >     the VCO
> >     > alignment. I'm getting a constant 8.37 V at R30  at 4.000 MHz..
> >     I cannot
> >     > change it with the full range of L30. Correct type inductor is
> >     installed at
> >     > L30.  Dropping the VFO down to 3.500 doesn't change it either. I
> >     have gone
> >     > back through the archives and read just about every post
> >     concerning this
> >     > problem but nothing I have tried has made any difference. I have
> >     double
> >     > checked all correct parts placements and orientations, re-flowed
> all
> >     > connections in the quadrant of the board around the VCO section.
> >     T5 is
> >     > wound as per the manual (from the Toroid Guy) and installed
> >     correctly. I
> >     > have tried both spreading and compressing the turns on T5 with
> >     no change
> >     > either way.
> >     >
> >     > Regarding the thermistor board,  I have 4.18 V on pin 5 of U6.
> >     >
> >     > Test in the previous section were within specs:
> >     >
> >     > 4 MHz Oscillator Cal using CAL FCTR was 12099.61 and confirmed
> >     with an
> >     > external counter.
> >     > Range of the PLL reference oscillator: 12099.61 - 12086.01 =
> >     13.59 kHz
> >     > VCO oscillator test: 8162.60 MHz
> >     >
> >     > I'm about out of ideas at this point. What should I start
> >     checking next?
> >     >
> >     > Thanks for any help!
> >     >
> >     > 73 de John, WB4GLJ
> >     > ______________________________________________________________
> >     > Elecraft mailing list
> >     > Home: http://mailman.qth.net/mailman/listinfo/elecraft
> >     > Help: http://mailman.qth.net/mmfaq.htm
> >     > Post: mailto:Elecraft at mailman.qth.net
> >     <mailto:Elecraft at mailman.qth.net>
> >     >
> >     > This list hosted by: http://www.qsl.net
> >     > Please help support this email list:
> http://www.qsl.net/donate.html
> >     >
> >
>
>