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

[Don Wilhelm]

Matthew,

Great that you found the source of your problems.  Onward and upward.

When you get all finished with your K2 and want to do a good dial 
calibration, take a look at the K2 Dial Calibration article on my 
website www.w3fpr.com and you should be able to get it "right on" with 
that procedure.  I do several every week, and I can tell you that the 
method does work and works quite well.

73,
Don W3FPR

On 7/17/2012 3:35 PM, Matthew Ellis wrote:
> 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,
>>>
>>>
>>
>
>