K2 BFO Drift / Filter Alignment

[Douglas Furton]

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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[Don Wilhelm-4]

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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[K1FFX]

I'm wondering if there was any resolution to this problem, first posted just over a year ago,
since I observe what I think is the identical problem with my K2 S/N 6982.  A related
symptom was obvious BFO shift when pressing down on the top cover just behind the
A=B push button (for example, to hold the K2 steady while turning off AGC or checking
current XFIL setting, i.e., an operation that requires two button pushes).  Some loosening
and moving of the side panels seems to have resolved this second symptom, but the
BFO shift out of the filter center frequency persists.

I'm currently working on KPA100 construction, and would really like to resolve this
before installation (still several days away ... now looking at T1 installation!) and
would appreciate any thoughts on this BFO shift.  Meanwhile, have started tracking
BFO freq/Pin 14 voltage per Don's suggestion.

Thanks!

- Bruce

Don Wilhelm-4 wrote

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.


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.
>

[g3ymc]

Cannot see any shift when trying to replicate this on my K2, #2498. I
suspect it is a poor joint or similar, or maybe even a torroid with
loose wires.

73 Dave G3YMC

On 5 Jan 2011 at 12:32, K1FFX wrote:

> 'm wondering if there was any resolution to this problem, first posted
> just over a year ago, since I observe what I think is the identical
> problem with my K2 S/N 6982. A related symptom was obvious BFO shift
> when pressing down on the top cover just behind the A=B push button (for
> example, to hold the K2 steady while turning off AGC or checking current
> XFIL setting, i.e., an operation that requires two button pushes). Some
> loosening and moving of the side panels seems to have resolved this
> second symptom, but the BFO shift out of the filter center frequency
> persists.


http://www.davesergeant.com

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[Don Wilhelm-4]

  Bruce,

The K2 BFO will shift slightly if the bottom cover is deformed, but that
does not happen under normal conditions.  Check to see if your K2 is
sitting on its feet properly.  If you have something under the K2 or
have it on an uneven surface, it may be deforming the bottom cover
slightly when you press on the top.

73,
Don W3FPR

On 1/5/2011 3:32 PM, K1FFX wrote:

> I'm wondering if there was any resolution to this problem, first posted just
> over a year ago,
> since I observe what I think is the identical problem with my K2 S/N 6982.
> A related
> symptom was obvious BFO shift when pressing down on the top cover just
> behind the
> A=B push button (for example, to hold the K2 steady while turning off AGC or
> checking
> current XFIL setting, i.e., an operation that requires two button pushes).
> Some loosening
> and moving of the side panels seems to have resolved this second symptom,
> but the
> BFO shift out of the filter center frequency persists.
>

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[K1FFX]

Hi, Don -

Thanks for the hint.  Yes ... the "shift when press" symptom is definitely,
now I see, due to bottom panel interaction.    I believe the bottom panel
is under stress from force exerted by the tilt stand (which I had a heck
of a time installing back when I built the K2).  I will look into this further ...
maybe if I can resolve the "shift when press" symptom, the apparent
"shift by itself" symptom will also be solved.

Thanks!

- Bruce

Don Wilhelm-4 wrote

  Bruce,

The K2 BFO will shift slightly if the bottom cover is deformed, but that
does not happen under normal conditions.  Check to see if your K2 is
sitting on its feet properly.  If you have something under the K2 or
have it on an uneven surface, it may be deforming the bottom cover
slightly when you press on the top.

73,
Don W3FPR

On 1/5/2011 3:32 PM, K1FFX wrote:
> I'm wondering if there was any resolution to this problem, first posted just
> over a year ago,
> since I observe what I think is the identical problem with my K2 S/N 6982.
> A related
> symptom was obvious BFO shift when pressing down on the top cover just
> behind the
> A=B push button

[Don Wilhelm-4]

  Bruce,

Make certain that the two standoffs are mounted on the RF board and that
the bottom cover is installed with all 6 screws.

When properly secured, and with the tilt bail in either the up or down
position, the bottom cover should not move with pressure on the K2 top.  
Be certain there is no "rocking" motion with the tilt bail extended - if
there is, it could cause your problem - loosen the bail feet screws a
bit and reposition to correct any "rocking".

73,
Don W3FPR

On 1/5/2011 4:23 PM, K1FFX wrote:

> Hi, Don -
>
> Thanks for the hint.  Yes ... the "shift when press" symptom is definitely,
> now I see, due to bottom panel interaction.    I believe the bottom panel
> is under stress from force exerted by the tilt stand (which I had a heck
> of a time installing back when I built the K2).  I will look into this
> further ...
> maybe if I can resolve the "shift when press" symptom, the apparent
> "shift by itself" symptom will also be solved.
>
> Thanks!
>
> - Bruce
>
>
> Don Wilhelm-4 wrote:
>>    Bruce,
>>
>> The K2 BFO will shift slightly if the bottom cover is deformed, but that
>> does not happen under normal conditions.  Check to see if your K2 is
>> sitting on its feet properly.  If you have something under the K2 or
>> have it on an uneven surface, it may be deforming the bottom cover
>> slightly when you press on the top.
>>
>> 73,
>> Don W3FPR
>>
>> On 1/5/2011 3:32 PM, K1FFX wrote:
>>> I'm wondering if there was any resolution to this problem, first posted
>>> just
>>> over a year ago,
>>> since I observe what I think is the identical problem with my K2 S/N
>>> 6982.
>>> A related
>>> symptom was obvious BFO shift when pressing down on the top cover just
>>> behind the
>>> A=B push button
>
> -----
> Bruce Rosen
> K1FFX
>

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[K1FFX]

I think I've nailed it down as an interaction between the bottom cover and
L33, which mounted on that rubber spacer on the bottom side of the RF
board.  I can get the same BFO shift effect by barely pushing on L33
with an insulated tool, for example.   I took the cover off and measured
clearances, and L33 seems to be well below the height of the two spacers.
However the cover has a bit of flex to it (maybe made worse by the tilt
stand), and pushing on the bottom cover does cause the BFO to shift,
although it does not, AFAIK, actually touch L33.

Not clear that there is anything to be done about it.  Now that I know,
I'll just need to avoid pushing on the case, particularly from the bottom.

Will realign filters once more and see if they stabilize.

Thanks for the help!

[K1FFX]

I thought I would post what I think (hope!) is my resolution for this.

I wrote to Elecraft's support line.  Gary responded promptly (as always), suggesting essentially that I remove L33 and the rubber bumper and do a better cleaning job beneath the bumper, thereby moving L33 further away from the bottom cover.

I looked it over and concluded that was a task I didn't want to do.  So, I went the other way.  I put 3 leftover washers from the K2 kit on top of the two front 2-D fasteners and the middle standoff (near L33), and then replaced the bottom cover.  This moved the bottom cover further away from L33.  As far as I can tell, this has solved the problem.  Pressing on the top right of the case has no effect and pressing on the bottom of the case, short of really bearing down on it, also has no effect.  Now just to keep an ear on BFO drift now that the drift-when-pressed problem is apparently fixed; I'm hoping there will be no further drift.

Thanks to all for your help.

- Bruce