Ron,
The problem is that your crystals have a Q which is high and they are
more difficult than the average to pull down in frequency.
Your best recourse is to pad the crystals down in frequency a bit. You
can first try a small capacitor from the junction of the crystals and
C174 (U11 pin 6) to ground. My rule is that the capacitor should not
exceed 20 pf. Too much capacity here can cause the BFO oscillation to
act 'strangely'.
If 20 pf will not lower the low frequency enough, then add a padder
capacitor to both C174 and C173. 47 pf across C174 and 120 pf across
C173 should do the job nicely and preserve the ratio of the two
capacitors - the ratio is important to control the feedback for the
oscillator circuit.
73,
Don W3FPR
Ron Hahn EI2JP wrote:
Colleagues,
Some time ago now I reported an issue with my K2 BFO being slightly out
of range. The advice at the time was to get a new set of BFO crystals
from Elecraft, which I did. The new results are as follows:
Original Replacement
BFO Hi Freq 4917.62 4917.63 must be >= 4916.3 OK
BFO Lo Freq 4913.59 4913.49 must be <= 4912.7 BAD
Range 4.03 4.14 must be >= 3.6 OK
I have done the following:
1) Re-calibrated the 4MHz oscillator using a known accurate counter.
2) Re-checked the PLL reference oscillator range (within spec).
3) Re-checked the VCO (within spec)
4) Re-aligned the VCO (all bands within spec)
5) Checked all components for the correct (marked) values (all ok)
6) Checked R116 for any possible shorts of L33
7) Checked L33's leads, which appear to be OK
Where I have fallen down on both sets of crystals is the lower BFO
range. Where to go from here??
Thanks for the help,
Ron
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