[Elecraft] K2 Alignment and Test Part II: VCO Test fails

[Matthew Ellis]

Don,

Thanks to your pointing me in the right direction I appear to have 
tracked down my issue:  Capacitor R68 (10pf, next to K15) was duff 
(probably damaged when I inserted it as the holes on the pcb are wider 
than the lead spacing on the capacitor).  The final clue was when I 
broke an unrelated capacitor lead off which showed how easily they can 
be damaged (I've put hundreds of capacitors into circuits without issue 
before),   The other clue was that when I touched my frequency meter 
probe to TP1 I got a very brief reading before it dropped to zero, and 
while it would continue to read zero if I touched tp1 repeatedly, if I 
left it for a while (a minute or so) then touched the probe again, I 
once again got a brief reading.  This suggested to me that T5 was 
probably OK (your telling me it was a Hartley oscillator helped there as 
I then looked it up to get a better idea of the operation).

Lets hope it doesn't take me two weeks to get each of the various tests 
to pass!

Thanks again,

Matthew

On 15/07/2012 14:14, Don Wilhelm wrote:
> Matthew.
>
> Zero Hz. says that Q18 is not oscillating.
> To help you visualize a little better, forget about the PLL part and 
> concentrate on Q18 only.  That is just a Hartley oscillator.
>
> You have already shorted out the VFO ALC with the short across C60 - 
> and that is a good debugging step.  The tuned circuit inductance 
> consists of L30 in parallel with the red winding of T5 and the 
> capacitance is made up of the varactors and capacitors switched in and 
> out of the circuit by K13, K14 and K15.  That is really all there is 
> to it as far as basic oscillation is concerned.
> Once it oscillates, then you can consider the other "appendages" like 
> the R30 voltage and the VFO ALC.
>
> So, go over the circuit once again with that in mind.  There is a 
> reason Q18 is not oscillating -  Several possibilities:  the green 
> winding of T5 is wound backwards, the diodes are oriented backwards, 
> The varactors are mounted backwards,  Q18 is oriented incorrectly, Q18 
> is damaged,
>
> Note also that the path to TP1 includes U3 (which also could be 
> failing).  Since you have an external frequency counter, check for 
> oscillation at the source or Q18 rather than just at TP1.
>
> You should first be trying to just get Q18 to oscillate.  Once that is 
> working, the other parts come into play to make the frequency of 
> oscillation correct.
>
> 73,
> Don W3FPR
>
>
>
> On 7/15/2012 3:44 AM, Matthew Ellis wrote:
>> Hi,
>>
>> I'm struggling to track down an issue with the VCO test in part 2 of the
>> alignment for my K2 (page 64 in my manual).
>>
>> The 4MHz Oscillator calibration looks OK as the frequency on TP3 shows
>> as 12096hz stable (both on the internal test probe and an external freq
>> counter).  The PLL test is in range as well.  But I get nothing (0hz) on
>> TP1 for the VCO test with both the internal probe and a standalone
>> frequency meter.
>>
>> I've checked and re-wound/re-fitted Transformer 5, gone over my assembly
>> with a magnifying glass a few times (both top and bottom side of the
>> pcb), confirmed all the components in the VCO area are present and
>> correctly oriented.  I've also used a signal scanner on the i2c lines to
>> confirm the pll ic is getting the 'change freq' instructions.  Most
>> voltages appear good compared to the manual, although the voltage on Q16
>> C and Q17 G is off (initially 8v then dropped to near 0v after i
>> re-soldered the joints of resistor R9).  I have shorted C60 to
>> completely bypass the VFO ALC (suggestion from Elecraft).
>>
>> All prior tests were fine, everything in expected range.  Also the
>> correct frequency shows on pins 1 and 2 of U4 (MC145170), and voltages
>> look good on U3,4 & 6.
>>
>> I'm in the UK, so sending it back to Elecraft is not my preferred option
>> (time/cost).
>>
>> Has anyone got experience of issues in this area?
>>
>> Thanks,
>>
>>
>