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

[W3FPR - Don Wilhelm]

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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