[Elecraft] [K2] VCO Not Oscillating

[Matt Maguire]

Ok, that makes more sense now, although we now have the Drain of Q17
at 0.08V connected to the Source of Q18 at 6.64V via the primary
(green) winding of T5. I would hazzard a guess that there is also a
clerical error with the Q18 voltages, and that the Source is at 0.08V,
same as the Drain of Q17, the Gate of Q18 is at 0V, and the Source of
Q18 is at 6.67V. The alternative would be that the T5 primary winding
is open circuit, but then the other voltages wouldn't make sense.

There is minimal ALC voltage on Q17 Drain, so this supports your
conclusion that the VCO is not oscillating, although it could also be
that either Q16 or the U3 buffer is not working. An RF probe to pin 3
of the U3 buffer will tell you if there is any signal there.

You might check that all the variactor diodes are in the right way
(D21-D26) and the right type (D21 & D22 are 1SV149, D23 & D24 are
MV209) and that L30 is ok (no red line, large adjustment slot). You
could perhaps try changing bands to toggle the K13/K14/K15 relays to
isolate the associated components and see if the VCO starts
oscillating.

As you say, if any joints look suspect, reflowing them is not a bad idea.

Good luck, let us know how you go.

73, Matt VK2RQ

The most exciting phrase to hear in science, the one that heralds new
discoveries, is not 'Eureka!' but 'That's funny...' -- Isaac Asimov

On Thu, May 1, 2014 at 11:59 AM, Joseph Baxley via Elecraft
<elecraft at mailman.qth.net> wrote:
> Matt,
>
> My reason for asserting that the VCO is not oscillating is two-fold.  One,
> there is no frequency at TP1 when I touch it with the probe, but there is
> when I touch TP2 and TP3.  Both of these test points show a correct
> oscillation of 4.9MHz and 12.09MHz respectively.  Second, when I applied my
> voltmeter to R30 and tried turn the slug for L30 there was no change in the
> voltage at R30.  I will recheck both of these assertions.
>
> I looked over my notes for Q17 and realized that I had them mis-labeled when
> I wrote the first email in this chain.  Looking at the notes I have with me
> the labeling is S: 0V, G: 7.14V, and D: 0.08V.  I will of course check these
> again when I go back to work on the radio as well as the part types, though
> I'm fairly certain they are correct.  I'm taking the night off though.
> Also, I did check U3 and U4 over several times and recall them being in the
> correct orientation.  Will look again here as well.
>
> The action plan for tomorrow is to double check all components in the VCO
> for correct value and orientation and then to re-heat the solder joints on
> them.  Hopefully it will be something simple and stupid like a misplaced
> part or a cold joint.
>
> Thanks,
> Joseph
>
>
>
> --
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