[Elecraft] K2 BFO problem

Mon Jul 19 10:06:00 EDT 2010

Ken,

You are likely chasing a bad solder connection - things that change by 
themselves is usually an indication of a solder problem.
You are getting close if it was the Q24 drain voltage changing.  Check 
RF Board R98, and then go to the Control Board and check the components 
shown on the Control Board schematic area marked "ALC" (at the top of 
the page).  Double-check the position of the components soldered on the 
back of the board.   When in CAL FCTR, tapping BAND+ should result in a 
high voltage on the VALC line, and tapping BAND-, it should drop to 
zero.  Don't forget to check the connectors between the Control Board 
and the RF Board.

73,
Don W3FPR

Ken Mohler wrote:
> Don -
>
> It gets weirder and weirder.  I did the test of the Q24 drain voltage.  When
> I tapped Band+ I got 3.4V.  BAND- dropped to a little less than a volt.
> Then I went on to check the following things you suggested.  Now here's
> where it got weird... I went back to redo the Q24 drain voltage and now I
> get around 0 volts no matter which BAND button I tap.  I didn't change
> anything - honest!  I'm less convinced that the problem lies within Q24.
> Perhaps Q24 is not getting the right signals from the other side. 
>
> My first K2 went together with nary a glitch, and I have been even more
> deliberate and careful with this one.
>
> If you still have the patience, suggest what I ought to do next.
>
> 73,
> Ken
> K0WKM
>
> -----Original Message-----
> From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
> Sent: Saturday, July 17, 2010 7:57 PM
> To: Ken Mohler
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K2 BFO problem
>
> Ken,
>
> The "fuzzy" waveform on the 'scope is normal - but how much so is 
> difficult for me to say whether yours is normal or abnormal.
>
> Check the value of C169 - it should be 390 pF (marked 391).
>
> OK, the BFO is oscillating, so look at Q24 carefully - is it oriented 
> correctly?  If so, enter CAL FCTR and tap the BAND+ button while 
> measuring the voltage on the drain (see the schemastic key page if you 
> are not familiar with the pinout).  The voltage should go high(almost 8 
> volts), and then tap BAND- and the voltage should go nearly to zero.
>
> Other things to check - look for a solder bridge between pins 1 and 2 of 
> U10;, check the SMT1B for correct orientation; re-check the orientation 
> of D36, and check the soldering of R98, C168, D36, R99 and SMT1B.
>
> If all the above is correct, try replacing Q24.  If you have a J309 
> around, try that (if you have the KDSP2 kit, there are J309s in there).
>
> 73,
> Don W3FPR
>
>
> Ken Mohler wrote:
>   
>> Don -
>>
>> You have me on the right track.  I have a valid frequency reading at pin 6
>> of U11 and I can follow it back through L33 and C169 to the gate of Q24.
>>     
> The
>   
>> waveform gets fuzzier and has lower amplitude past L33 (I do have a scope)
>> but is still countable at C169.  Past Q24 I don't have anything
>> recognizable.  I have reflowed the solder on everything around there.  D36
>> appears to be installed correctly. I don't have an extra J310 to swap out
>> Q24, but will get one if you think that's the next step.  BTW, I measured
>> the voltages at U11 and all are what you told me to expect except pin 8 is
>> around 6 volts.  Still, the BFO seems to be working, just not getting
>> anything as far as TP2.  Thanks for the help and please keep it coming.
>>
>> 73,
>> Ken
>> K0WKM
>>
>> -----Original Message-----
>> From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
>> Sent: Friday, July 16, 2010 6:49 PM
>> To: Ken Mohler
>> Cc: elecraft at mailman.qth.net
>> Subject: Re: [Elecraft] K2 BFO problem
>>
>> Ken,
>>
>> Did you obtain a good frequency reading when you did the PLL Reference 
>> Oscillator test back on page 36?  If so, check the probe again by 
>> putting the probe into either TP3 or TP1.  If you see a valid frequency 
>> displayed, your probe is OK.
>>
>> The most common problem builders have are soldering problems.  Re-flow 
>> the soldering in the BFO  and U11 area with a hot soldering iron (750 
>> deg/F).  Also check carefully for solder bridges.  Unfortunately, one 
>> lead of both X3 and X4 are under the pad for L33, so ignore those for 
>> the time being (if nothing else works, Lift one end of the resistor 
>> securing L33 and re-flow the soldering at those leads).
>> Do you have near 8 volts at U11 pin 8?  Is U11 pin 1 and 2 close to 1.4 
>> volts?  Do you have about 6 volts at U11 pin 6?
>>
>> Touch the counter probe to U11 pin 6 (careful not to short to an 
>> adjacent pin).  Do you have a valid frequency reading?  If you do, then 
>> the BFO is working, and the problem is with Q24 and its associated 
>> components.  Be certain you have mounted D36 (the SMT1B) part correctly, 
>> and the soldering of Q24, RP6, R98, and R99 is good.
>>
>> If still no progress, either assemble the parts provided in your K2 kit 
>> for the RF Probe (schematic in Appendix E) or let us know that you have 
>> an oscilloscope available.
>>
>> 73,
>> Don W3FPR
>>
>> Ken Mohler wrote:
>>   
>>     
>>> Hi Gang -
>>>
>>>  
>>>
>>> I'm building my 2nd K2 and am at the point of Alignment and Test, Part
>>>       
> II.
>   
>>> All of the tests and alignment steps through the VCO alignment have been
>>> completed successfully.  Everything has been right in the center of the
>>> expected parameters.  Then I got to the BFO test.  I'm getting a reading
>>>     
>>>       
>> of
>>   
>>     
>>> 0000.00.  Here's what I know so far:  D37 and D38 are the correct types
>>>     
>>>       
>> and
>>   
>>     
>>> oriented properly.  I have measured the resistance across the resistor
>>> holding L33 down (or up?) and get a reading of 1.5 ohms, indicating to me
>>> that L33 is intact and effectively soldered to the resistor leads.  I
>>>       
> have
>   
>>> checked the positioning of Q24 and it looks correct.  The right crystals
>>>     
>>>       
>> are
>>   
>>     
>>> installed at X3 and X4 and grounding the cases went smoothly, so I don't
>>> think I overheated them.  I have reflowed the solder on the parts around
>>>     
>>>       
>> the
>>   
>>     
>>> BFO area.  A symptom that may or may not mean anything: the S-Meter is
>>> showing S5.  It was properly adjusted back in Alignment and Testing Part
>>>     
>>>       
>> I.
>>   
>>     
>>> What should I do next?
>>>
>>>   
>>>     
>>>       
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>
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