[Hallicrafters] Determining Crystal Resonance, and Possible Replacement

[Mike Everette]

If you have a BC-221 or LM frequency meter, you can
use it to find the resonant frequency of a crystal. 
The procedure is, first, to let the freq meter warm up
for several hours so it's good and stable.  (And, I
assume you have zeroed the internal 1000 kc
calibration crystal with WWV at some point.)

Calibrate the freq meter dial to the frequency of the
check point (listed in the calibration book for the
meter) closest to the frequency of the crystal you are
testing.

Connect the crystal between the output post of the
freq meter and ground, in series with a diode
(preferably a high frequency detector type) and a
microammeter.  

Turn the "RF COUPLING" or "OUTPUT" or "GAIN" control
on the freq meter up just a little and rock the dial
across the frequency of the crystal.  You will see a
sharp peak on the meter as you pass through resonance.
 You might have to adjust the output from the freq
meter to see the peak, but you will see it.  I have
used an 0-100 microammeter with good success.  The
more sensitive the meter, the easier it will be to see
the peak, of course.

The dial reading at which the peak occurs is the
resonant frequency of the crystal.

Now as for replacing crystals in single-crystal IF
filters, I have not actually tried this but it seems
pretty logical that an FT-241A crystal would work
okay.    For those who don't remember or don't know
what they are, the FT-241A crystals were used in World
War II tank transmitters (as in Sherman).  These rigs
were FM and operated in the band 20-28 mc/s.  The
frequency control crystals lie in the range of roughly
370 to 540 kc in increments of a little over 1 kc. 
They are marked with the channel number and operating
frequency of the tank transmitter.  For example,
"Channel 70, 27.0 MC" (and this onem by the way, is
500.000 kc).

Several of these "channels" lie in the 450-465 kc
range.  I have a list of the actual frequencies some
place but will have to dig it up, if anyone is
interested.

These crystals are a little larger than an FT-243
holder, and the pins will fit into an octal socket
(every other pin) or a holder for an FT-243.  They are
usually black with white tops, lettered in black.  You
can still find them at hamfests if you look.  Finding
the "really interesting" frequencies may be
challenging though.  JAN Crystals in Florida used to
sell them (is JAN still around?).  Fair Radio in Lima,
OH may also still have them.

73

Mike
WA4DLF




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