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

Posted by K1FFX on Nov 03, 2015; 11:58pm
URL: http://elecraft.85.s1.nabble.com/K2-K2-filters-continue-to-drift-out-of-alignment-tp7609910.html

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