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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