[Elecraft] K2 BFO problem

Sun Jul 18 23:52:56 EDT 2010

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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