Re: [Elecraft] K2 Freq. Stability

Posted by Don Wilhelm-4 on Jan 20, 2010; 1:58pm
URL: http://elecraft.85.s1.nabble.com/K2-Freq-Stability-tp4425453p4426608.html

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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