[Elecraft] [K2 4922] BFO Range Problem Continues..

[Ron Hahn (EI2JP)]

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Don Wilhelm wrote:
> 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
> 

Don,

Updated data below.  No real progress with the padding.  :-(

	A	B	10pf	20pf	47/122pf
BFO Hi	4917.62	4917.63	4917.11	4916.8	4917.39
BFO Lo	4913.59	4913.49	4913.3	4913.16	4913.02
Range	4.03	4.14	3.81	3.64	4.37

A is the original set of BFO crystals
B is the replacement set of crystals

Try 1: 10pf from pin 3 - 6 of U11
Try 2: 20pf from pin 3 - 6 of U11
Try 3: 47pf across C174, 122pf across C173

As you can see by the data padding the junction of the crystals only
reduced the upper range.

So I then tried the recommended padding across C173 and C174, and, as
you can see, it still fails to bring the lower range into spec.

Where to go from here?

Thanks a mil for all the help.

Ron
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