I find that I am having problems obtaining proper dial calibration. I
can generally set the high end of the band using the trimmer cap. But
after I set the low end by adjusting the coil, the high end is off
again. Repeated efforts don't significantly narrow the gap.

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It is possible to get the local oscillator (LO) set on the wrong side
of the signal frequency when aligning the higher frequency bands. If
you get the LO on the high side of the signal frequency on one end of
the band and on the low side on the other end, the calibration and
sensitivity will never be good. Continued re-adjustment with the LO
on the wrong side of the signal will result in the problems you are
having. One of those hand-held frequency counters can "sniff out" the
LO and tell you exactly where it is. Lacking that, another receiver
with good calibration can be used to identify the LO and determine if
it it is on the high side of the signal frequency where it belongs.

My SX-42 dial calibration is quite accurate, so bad calibration is not
to be expected with that receiver.

Best Regards,

Ed Engelken
Canyon Lake, TX