Posted by
Don Wilhelm-4 on
Dec 31, 2009; 2:57pm
URL: http://elecraft.85.s1.nabble.com/K2-BFO-Drift-Filter-Alignment-tp4236424p4236623.html
Doug,
That is indeed a strange problem - I have not seen one like that, but
there is always a first. It has the flavor of a bad solder connection
in the BFO area - one that you are not likely to find by inspection, you
will have to re-flow the solder with a hot soldering iron. It could be
a crystal, or a varactor, or a capacitor, or it could be that the
voltage output from the DAC is changing due to a bad bit in the DAC.
I do have a suggestion - make a series of tests over a period of several
days before doing anything to try to fix it.
You think the BFO is shifting, so find out for certain. Take one filter
that you are not likely to use (like LSB FL4), and do NOT change that
one. Instead, select that filter, enter CAL FIL, tap BAND-, then tap
DISPLAY to see what the BFO frequency is. Record the BFO frequency over
a period of days and see whether it really is changing. At the same
time you make the frequency reading (and without changing filters)
record the VBFO signal voltage - that can be found at Control Board U10
pin 14.
Reasons for this testing? Well, you need to know whether the BFO
circuit themselves are the result of the frequency change or the voltage
that is used to drive the varactors in that circuit is changing. One
could guess a lot and eventually find the cause, but the tests I have
outlined will narrow the search.
After making these measurements, you can go ahead and re-align the other
filters, but do not re-align that one you are using for the test.
73,
Don W3FPR
Douglas Furton wrote:
> I'm having a problem with my K2 (SN 6761 10W/CW only): I have to align the crystal filters almost daily. It seems the BFO shifts by 200 Hz or more almost daily.
>
> This may have been covered in the reflector at various times, but I've not seen this exact problem described, so I've decided to go ahead and post a note again to get a fresh opinion.
>
> I'll align the filters with noise in the band-pass using "Spectran" and all is good for the day. The next day, the filters are all out of alignment -- say 200 Hz low. The filters are all low by that amount. So, I realign the filters, all is good, until it happens again. Next time, the filters will all be maybe 200 Hz high. The realignment amounts to shifting the BFO 1-3 steps using cal fil. And, again, all the filters shift by about the same amount.
>
> I see what I would call *normal* BFO drift with warm up, but not enough drift to cause the filters to change by as much as 200 Hz. The ambient temperature is about 65F and doesn't change all that much from day to day.
>
> Any suggestions?
>
> Doug
> K8EXB
>
>
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