[K2]K2 filters continue to "drift" out of alignment

[K1FFX]

I posted a query about this a few years ago and it remained largely unresolved at the time.

The "end user" symptom was and is that filters would over time "drift" (there's probably a better technical term) out of alignment, requiring a CAL FIL operation.  I've had to do this several times since completing my K2 in late 2010 (S/N 6982).  I found that the last time I did this (maybe 4 months ago), aligning FL2 was almost not do-able ... peaking that filter got me right down to a DAC value of 000.

I had been off the air for a few months and when I turned the K2 on the other day I found that the filters were again out of alignment.  This time, I was unable to peak FL2 - FL4 ... I ran into DAC value of 000 before I could peak the filters.

While flipping through my notebook with the N0SS alignment instructions, I came across my notes from doing CAL FIL in late 2010.   What I now see is that what has apparently "drifted" are filter bandpass frequencies, as demonstrated by the "find the center frequency while tuning the DAC between 000 and 255" procedure.

Here are the center frequencies then and now:

FL1 (1.00)  2010: 120   Today: 74
FL2 (0.50)  2010: 112   Today: 60
FL3 (0.30)  2010: 110   Today: 53
FL4 (0.15)  2010: 110   Today: 51

Apparently, with the center frequency significantly shifted towards 000, there's not enough "room" left to peak the filters.

But the real question is, why the shift in center frequency?

I'd really appreciate any pointers as to where I should be looking to fix the current situation (3 out of 4 filters not alignable) and hopefully finally resolve this.

Thanks!!

- Bruce

[Don Wilhelm-4]

Bruce,

The bottom line is that the BFO frequency is 'drifting'.
The most probable cause is a defective solder connection in the BFO area.
Carefully lift L33 and reflow the soldering of the BFO crystals. Reflow
the soldering of C173 and C174, RP6. D37, D38, C169 and Q24.
Make certain when you replace L33 that the leads of the resistor are
well soldered and the leads of L33 are soldered to the resistor leads.

73,
Don W3FPR

On 11/3/2015 6:58 PM, K1FFX wrote:

> I posted a query about this a few years ago and it remained largely
> unresolved at the time.
>
> The "end user" symptom was and is that filters would over time "drift"
> (there's probably a better technical term) out of alignment, requiring a CAL
> FIL operation.  I've had to do this several times since completing my K2 in
> late 2010 (S/N 6982).  I found that the last time I did this (maybe 4 months
> ago), aligning FL2 was almost not do-able ... peaking that filter got me
> right down to a DAC value of 000.
>
> I had been off the air for a few months and when I turned the K2 on the
> other day I found that the filters were again out of alignment.  This time,
> I was unable to peak FL2 - FL4 ... I ran into DAC value of 000 before I
> could peak the filters.
>
> While flipping through my notebook with the N0SS alignment instructions, I
> came across my notes from doing CAL FIL in late 2010.   What I now see is
> that what has apparently "drifted" are filter bandpass frequencies, as
> demonstrated by the "find the center frequency while tuning the DAC between
> 000 and 255" procedure.
>
>

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[K1FFX]

Don Wilhelm-4 wrote

The bottom line is that the BFO frequency is 'drifting'.
The most probable cause is a defective solder connection in the BFO area.
Carefully lift L33 and reflow the soldering of the BFO crystals. Reflow
the soldering of C173 and C174, RP6. D37, D38, C169 and Q24.
Make certain when you replace L33 that the leads of the resistor are
well soldered and the leads of L33 are soldered to the resistor leads.

Thanks for the specific and detailed hints, Don (and thanks also to Matt and Bob for helpful e-mails).  I decided to defer handling L33 and the two BFO crystals.  I went ahead and reflowed the soldering for the other components.  This has fixed the immediate problem: the center frequencies moved "up" and was I able to easily align all the filters.  Hopefully, this will be a permanent fix and there will be no further significant BFO drift.  If I continue to see drift, then I'll go ahead deal with L33 (whose solder joints all looked fine to me yesterday) and reflow the BFO crystals' soldered leads.

Thanks again!

- Bruce K1FFX

[K1FFX]

I thought I would close the loop on this issue.

It turned out that my "fix" last Fall was just a partial or temporary fix and the BFO did, in fact, continue to drift.  So, I finally went ahead and did the procedure which you recommended, which I hesitated to do at the time.  I removed L33 and reflowed the solder for the two BFO crystals.  It turned out that removing L33 was actually straight-forward ... or maybe I'm just getting better at this.  It just took good magnifiers and a little patience.

In any case, this seems to have fixed the problem.  The "center frequencies" moved back to about where they were when I first built the K2 and receiver performance has been restored.

Thanks again for the advice.

Best -

Bruce K1FFX