[Collins] KWM-2 Xtals

[Dr. Gerald N. Johnson, electrical engineer]

Most any reputable crystal manufacturer should either have that
information on hand already or be able to determine it from measuring
one or more original crystals. Judging by the frequency range the
crystals were probably a common AT cut which gives a reasonable
frequency vs. temperature characteristic, and a decent equivalent series
resistance. I've looked at the crystal oscillator circuits in a vintage
KWM-2, 32S1, 75S1, and 75S3B/C and I conclude that the load C is at
least 20 pf, probably not 32 pf. I added an auxiliary crystal socket to
my 75S3B using miniature coax (its at the top of the tuning rack tower)
and I remember changing capacitor values to keep the crystals close to
their marked frequency. As far as series resistance is concerned, I
don't remember ever finding a crystal that wouldn't oscillate in that
receiver circuit. The only thing that being ground for the wrong load C
causes is that the oscillator is not quite on the predicted frequency
and thus the dial calibrator may not have the range to reach it. Once
the load C is known, any crystal with a fine frequency vs. temperature
characteristic can be used. Most crystal makers charge extra for a
flatter curve at radio operating temperatures and that often accompanies
a crystal with a tighter calibration tolerance. The crystal is always
going to be at room temperature or a little warmer, never really hot,
never really cold, unless the radio is going to be run from atop a
glacier without operator conveniences such as heat or in the Sahara
without shade. Even then its possible to adjust the crystal calibrator
against a 15 MHz transmitted standard (such as WWV) and adjust the dial
calibration to compensate or to know the dial error which will stay
constant across each tuning range.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.