[Elecraft] K2 VCO Alignment only 0.02V @ R30

[Don Wilhelm]

Jeremy,

Unless you have a typo, the 40 meter VFO frequency is too high while all 
the rest of the bands is too low.
Make certain that the value of C71 is 82 pF and re-flow its soldering.
Set the K2 to 7000 kHz and re-measure the VFO frequency - probe in TP1
Next (while you are still in CAL FCTR, turn the slug on L30 Without 
moving the VFO knob) to see if the frequency moves.  If it does not, 
re-flow the soldering on the pins of L30.  If the frequency does move, 
then T5 and L30 are properly connected, and that is what I wanted you to 
check.
Go to the secondary menu (tap DISPLAY) and check the parameter for D19 - 
it must be "n".

When re-flowing the solder, use a hot (750 degF) iron and add just a 
tiny dab of solder - you actually only need to add a bit of flux.
Watch the solder to be certain it flows out fully onto both the solder 
pad and the lead.  If you can see a distinct edge to the solder on the 
pad, that connection is suspect - it did not receive enough heat or was 
not heated long enough.

If all was as expected for the tests above, inspect the pins of relays 
K13, 14 and 15 carefully - you are looking for any whisker of solder 
that would form a bridge between two pins.

Next, move your search to the left of the VFO on the schematic, and 
check all the components in the U6B circuit for correct values and good 
soldering.  Make certain all components are in the proper holes - 
sometimes there is a via hole next to a solder pad and the component 
lead can be placed in the via hole instead of the solder pad.  When you 
have finished that inspection, power on and check the voltage at U6 pin 
5.  If it is not 4 volts, you have a problem on the small thermistor board.

73,
Don W3FPR


On 2/2/2013 11:31 AM, Jeremy Jones wrote:
> Hi Don,
>
> I set C22 to midway.
> PLL @ TP3 is 12099.12 - 12086.32.
> T5 is a yellow core, wound with 15 turns red and 4 green. I had to bring it down to 15 turns as recommended by Gary to get 8253.86khz @ TP1 when tuned to 4000.10 for the VCO test.  It was around 7694.39 with 16 turns.
> L30 is the correct part, and all diodes have been checked to ensure the correct part and placement.
> Soldering has been checked.
> Verified C67, C65, C64, C68, D13, Q18 for value and orientation.
> Relays click when changing bands.
>
> Still getting the following readings.
> 3500 = 8248.42
> 7000 = 12010.78
> 10100 = 14157.67
> 14000 = 16976.67
> 18068 = 18878.23
> 21000 = 15211.55
> 24890 = 16977.38
> 28000 = 18878.84
>
> I am at a loss for what to look for next. Everything seems to check out, but my readings are still off. Anyway I can try to isolate a faulty component?
>
> Sent from my iPad
>
> On 2013-02-02, at 10:38 AM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
>
>> Jeremy,
>>
>> I have read your exchanges with Gary,
>>
>> First, don't bother with the C22 setting - just set it to the midpoint (slot parallel with the long edge of the board) and leave it there until you have the rest of the K2 receiver working on all bands, and then go to my website www.w3fpr.com and do the procedures stated in the K2 Dial Calibration article there.
>>
>> Taking things one step at a time - is the PLL Range correct at TP3? Tap BAND+ to bring it to the highest frequency (about 12100 kHz) then tap BAND- to get to the lower limit (about 12087 kHz)  Those frequencies can vary by 2 to 3 kHz from the target values and still be OK.
>>
>> Once assured that the PLL Range is correct, then tackle the VFO frequencies.
>> You do have something wrong there because your frequencies are too low on all bands.  That indicates either too much capacity or too much inductance is present in the VFO circuit.  Look at the schematic for RF Board sheet 1.  The area in question is in the upper right corner - the input side of Q18 - as well as the VFO range Select components.
>>
>> Is T5 wound on a yellow core?  If wound on a red core, it will have too much inductance.  Does L30 have a slot across the entire diameter of the tuning slug?  It should.
>> Look at the tops of D23, D24, D25 and D26.  They should have a top shape like Q18 and about the same size.  D21 and D22 should have a smaller body profile when viewed from the top.  The wider flat side of all those varactors should be facing toward the rear of the RF Board.
>>
>> Your next step is to check the relays K13, 14 and 15.  The white stripe should be on the end toward the rear of the RF board.
>>
>> Examine the bottom of the board for the soldering of those components in the VFO and VFO Range Select areas to be certain there are no tiny solder bridges.
>>
>> If everything is correct, it *should* have close the frequencies listed in VCO Table on the Schematic Key page - within 3 to 5 kHz when everything is correct.
>>
>> 73,
>> Don W3FPR
>>
>>
>>
>> On 2/2/2013 9:43 AM, JeremyJones wrote:
>>> I'm in the process of building a K2 and am having trouble at the Alignment
>>> and Test part 2.  I've gone through at ton of previous posts here an have
>>> been talking with Gary at Elecraft but no luck so far.  The main issues are
>>> C22 only has adjustment less than 1khz (12085-12086) @ TP3, and R30 which
>>> should have 6V only has 0.02V.
>>>
>>> Below is the email chain (bottom to top) with the trouble shooting Gary and
>>> I went through.  Wonder if anyone has any ideas of what the issue might be,
>>> or further trouble shooting steps to isolate the problem.
>>>
>>> Thanks,
>>> Jeremy