Thanks to all who helped me work out this problem. I replaced all the BFO components (thanks to Elecraft for sending me the parts at no charge). Now from a cold start at 16C to steady operating temp at 20C, the K2 drifts about 50Hz, well withing spec. In the end, I think the problem was with L33. My original L33 was not wound as carefully as the replacement, and had a couple fewer turns. But who knows.
Interestingly, when I reassembled all the BFO components, the K2 did not pass the BFO range test. I reflowed solder, removed L33 and examined it closely, put L33 back in... tried various things to no avail. But then, something I did solved the problem and the oscillator worked fine. Gremlins (or poor soldering). Hope it doesn't revert back to its failed state. Anyway, thanks to all who replied. And, for the record, a properly assembled K2 starting at 16C running up to 20C still meets the published drift specs.
I struggled with this problem for quite a while, spent *a lot* of time working it out, and will admit to becoming more than a little frustrated and disillusioned with my K2. But my experience epitomizes the difference between buying and running a stock radio, and building and trouble shooting your own work. If all you want to do is work stations, a K2 is probably not for you; if you enjoy radio more broadly, then the K2 is ideal.
> -----Original Message-----
> From:
[hidden email]
> Sent: Wed, 20 Jan 2010 08:58:47 -0500
> To:
[hidden email]
> Subject: Re: [Elecraft] K2 Freq. Stability
>
> 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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