[Elecraft] K2 Freq. Stability

[Don Wilhelm]

Doug,

The BFO is obviously the culprit here.
What serial number is this K2?
Do you have the new L33? with many turns of #36 wire.  If this is an 
older K2, install the BFO upgrade kit.
If you already have the upgraded parts, then I suggest you equip 
yourself with all of the parts in the BFO circuit and just replace 
them.  You can replace them one at a time until you find things more 
stable, but I believe it is easier to replace them wholesale and see 
what happens.  You will need
2 BFO crystals X3 and X4
C173 and C174
D37 and D38
L33
Replacing L33 will require a high value 1/8 watt resistor to mount the 
toroid - the K2 lists this as R116 PN E500086.

73,
Don W3FPR

Douglas Furton wrote:
> Hello, all
>
> I've been bothered with the CW tone/filter setting
> on my K2 for some time now.  I seem to need to run
> CAL FIL daily, at least to have the BFO freq. read
> into memory.  I observe that the CW tone starts low
> and drifts upwards as the rig warms up when I tune a
> signal to the center of the filters (maxing the
> S-meter).
>
> Here is some data characterizing the freq. drift
> in my K2 (#6761).  I measured the PLL and BFO freqs
> with a nice frequency counter.  I measured these
> shifts with the K2 locked at 7050 kHz, and computed
> the total shift = dPLL - dBFO (all referred back to
> the initial (T=18.5C) measurements.  I measured the
> temp. with a thermocouple in contact with the PLL
> thermistor board. I also measured the  PLL and BFO 
> control voltages at various times and found them 
> to be constant.
>
>
> 	Cumulative freq. Shifts
> T	PLL	BFO	Total 
> (C)	(Hz)	(Hz)	(Hz)
> 18.5	0	0	0
> 19.3	9	-17	26
> 19.6	13	-34	47
> 20.2	17	-57	74
> 20.8	19	-70	89
> 21.3	24	-79	103
> 21.6	26	-87	113
> 22.0	28	-92	120
> 23.3	37	-124	161
> 24.0	40	-136	176
> 24.5	46	-164	210
> 25.0	53	-205	258
> 25.5	62	-256	318
> 25.5	70	-310	380
>
> (I hope the table above remains tabulated
> properly.  Best viewed with a monospace
> font.)
>
> It seems the thermistor board compensates the PLL
> fairly well (although maybe I could tweak RA upwards 
> a bit), but the BFO drifts considerably, and
> since the PLL and BFO drift in opposite directions, 
> the total drift is exaggerated.
>
> I may measure the drift on a second band, but these
> data are just from 40 m.  
>
> I would really like to improve the temp. stability
> of my K2.
>
> Does anyone have any suggestions?
>
> Thanks,
>
> Doug Furton
> K8EXB
>
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