[Elecraft] K2 BFO Test: BFO low freq is really low

[Fred Jensen]

Jason,

Visually check the two transistors that form the preamplifier for the
frequency counter (Q9 and Q10, I think).  If they have a shiny bluish
face on the flat side, Elecraft will send you two replacements (totally
black).  I had this exact problem caused by low or no gain in the
transistors.  Just a bad lot apparently.  See Elecraft Builders Alert
#21 of October 19, 2004.

73,

Fred K6DGW
Auburn CA CM98lw

W3FPR - Don Wilhelm wrote:
> 
> Jason,
> 
> Yes, 2456 kHz IS way too low - in fact it is quite out-of -bounds - and is
> close to only half the frequency that it should be.
> 
> First thing to check is soldering, then re-check the soldering, and when you
> are finished, check soldering again.  Use the schematic to identify those
> components in the BFO area and check them carefully for good solder joints
> and proper values.
> 
> Be certain the varactor diodes are the correct type, and be certain the
> crystal the correct one and is properly soldered.  The BFO is a VXO type
> circuit, and if everything is proper, it would be very difficult if not
> impossible to pull a good crystal by 50% of its frequency.  If everything
> else is correct, you may have a bad crystal (or perhaps it is shorted at the
> solder joints by excess solder) - consider removing the crystal, cleaning up
> the solder and putting it back in.
> 
> Lastly, look for something wrong with either the counter probe or the RF
> voltage at the BFO test point.  It is possible (but not probable) that the
> voltage is high enough to properly count the frequency at the high end, but
> drops off at the low end and counts only approximately half the pulses.  If
> you have a 'scope (or build the RF probe included with your K2 kit), it
> should be able to tell you if the RF voltage is really lower at the low
> frequency end.
> 
> The calibration of the 4 MHz reference may cause reading to be off by a bit,
> but not nearly the difference you are reporting.  If you set it with another
> receiver, you are close enough for now - you can refine it later when you
> have completed the K2 and are ready to do a precise dial calibration.  I
> recommend using the most recent method of zeroing WWV and comparing the BFO
> and VCO frequencies (previously known as the N6KR method).  Details are on
> the Elecraft website and can also be found in the K2 dial calibration
> article on my website www.qsl.net/w3fpr.
> 
> 73,
> Don W3FPR
> 
> ----- Original Message -----
> From: "Jason Marten" <martenjj at hotmail.com>
> 
> > I've come to the Alignment and Test, Part II.  I think I'm having a
> > problem under the BFO test.
> >
> > Following the instructions, I switch to 40m, hook up the frequency counter
> > to TP2, and get a reading of 4913.52.  The manual says I should get
> > something between 4908 and 4918.  So far, so good.
> >
> > Next, I go to the part where you determine the BFO range.  After hitting
> > Band+, I get a reading of 4917.54 kHz.  The manual says you need >=
> > 4916.3, and it's usually between 4916--4917.  Maybe a tad high, but looks
> > good to me.  After hitting Band-, I get a reading of 2456.11 kHz.  The
> > manual says it must be <= 4912.7 (which it most certainly is) but it is
> > usually about 4909--4912 (which is isn't, by a long shot).  This results
> > in a range of 2461.43.  The manual says it must be >= 3.6 (which it is, by
> > a lot), but it's typically 4--6 kHz.
> >
> > It looks like I'm meeting the letter of the law based on the >= and <=
> > requirements, but I'm nowhere near the typical ranges on the BFO low freq.
> > and range.
> >
> > Following this test in the manual is some info on what to do if the
> > frequencies are too high or too low:
> >
> > First off, I did not use a frequency counter to adjust C22 on the control
> > board,   I used a separate receiver and hooked up a wire from its antenna
> > jack to the 4 MHz crystal, as described in the manual.  This resulted in
> > me turning C22 less than 30 degrees counter clock wise (anti clock wise).
> > This seems reasonable to me, so I don't think C22 is off by very much.
> > However, I think I might want to try using a frequency counter, to get a
> > little more accuracy in calibration.  Could anybody recommend an
> > affordable frequency counter?  I've never shopped around for them, so I
> > have no clue as to the price, etc.
> >
> > The third point mentions I might have shorted out part of L33.  Looking
> > under a loupe, I did not see any evidence of a short, and in fact, I can
> > slide L33 underneath R116, it doesn't appear that heating R116 affected
> > L33.
> >
> > The fourth point mentions L33 could be broken.  I put my DMM on the tiny
> > leads attached to R116 and got 1.7 Ohms.  Does this sound reasonable?
> >
> > The final point says to check the capacitors and diodes in the BFO
> > circuit.  After another quick look, this appears to be OK to my eyes.
> >
> > Any help would be appreciated!
> >
> >
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