[Elecraft] K2 BFO problem

[Ken Mohler]

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?
>
>