[Elecraft] [K2] VCO Not Oscillating

[Don Wilhelm]

Joseph,

I don't know if you are mislabeling the leads of Q18 or not, but those 
voltages and the leads do not make sense.
The Drain of Q18 is connected to the 8B voltage rail through a 33 ohm 
resistor and should have almost 8 volts on it.
The Gate is biased by the rectification of the RF voltage by D13, and 
when oscillating should have a negative voltage (the fact that you show 
no negative voltage says Q18 is not oscillating).
The Q18 Source would normally have 2 to 3 volts on it, but since Q18 is 
not oscillating, there is no VFO RF being applied to D11, and that means 
Q18 will not conduct - that places the gate of Q17 at a high voltage and 
saturates the Drain to Source of Q18 which will produce 0 volts at the 
Q18 Source.

Refer to the Schematic Key page in the back of the manual for the proper 
lead callouts for the J310 transistor.

In other words, Q18 is simply not oscillating for you.  Look at the 
schematic for the RF board Sheet 1 in the upper right quadrant. Check 
each component shown there for proper values and good soldering.  Make 
certain Q18, Q17, and Q16 are oriented correctly and diodes D11, D8 and 
D13 are oriented correctly (look at the Parts Placement Diagram in the 
back of the manual).

Be certain varactors D21 thru D26 are oriented correctly.
If it still does not oscillate, rewind T5 being especially careful to 
make it look exactly like the diagram in the manual and make certain the 
leads are well stripped and tinned before installing it onto the board.
If all that is correct, it will oscillate at some frequency.  If the 
frequency is not correct, that is a different matter, but if everything 
above is correct, it will oscillate.

You could try another J310 at Q18.  There is one that should not be 
mounted yet in your build which would eventually be used at Q24.  If 
that fixes the VFO, ask parts at elecraft.com for a replacement J310.

73,
Don W3FPR

On 4/30/2014 11:44 PM, Matt Maguire wrote:
> 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
>>