General info question about FT-243 crystals

[KC9KEP]

Hello

I've collected dozens of what I'd consider to be non-standard frequency FT-243 crystals.

I'm familiar with the idea of using the fundamental frequency and doubling/tripling so
that a specific crystal can be used for higher frequencies.

But I seem to have numerous crystals in the 8 MHz range.  Is there any way to make these
useful for the Ham Bands?  I understand that one can etch crystals to raise them in frequency
but I also understand that there is a limit to how far the frequency can be moved.

What were these frequencies originally intended for?  Military frequencies?

Thanks & 73

--KC9KEP

[AA4PB]

We used to use 8MHz FT243 crystals for the 6M band using a multiplication of 6.

[W9GB]

The basic procedure for accomplishing the frequency changing grinding operation is covered in:
 "Radio Handbook, Twenty-Second Edition" by William I. Orr, W6SAI (1981, Howard W. Sams & Company, Inc, Indianapolis, IN 46268), on page 11.7:

http://home.netcom.com/~wa4qal/crystal.htm
Crystals may be raised in frequency by grinding them to smaller dimensions.
Hand grinding can be used to raise the frequency of an already finished crystal and the can be accomplished without the use of special tools or instruments.
In the case of the surplus FT-243 style of crystal, the blank may be raised in frequency up to several hundred kilohertz, if it is a fundamental-frequency cut.

YouTube video of grinding process
http://www.youtube.com/watch?v=j7CS4A8wB1E

Crystal Grinding Without Tears
Helpful Hints on Bringing Blanks to Frequency
BY FRANCIS R. COWLES, * W1AOK
*c/o Crystal Research Laboratories, Hartford, Conn.
(From QST magazine, April 1946. pages 48-50)
http://www.bliley.net/XTAL/docs/misc/XTAL_grinding/grinding.html

CRYSTAL GRINDING FT-243 Crystal Blanks
by Karl Kanalz - W8TIF © 2001
http://www.electronics-tutorials.com/oscillators/crystal-grinding.htm

[WB6BYU]

A lot of 8 MHz crystals were multiplied up to 6m (8.333 to 9 MHz) or 2m (8 to 8.222 MHz).  It does
require more multiplier stages, but actually is an advantage for FM, where it gives better deviation. 
(If the 8 MHz signal is modulated, then the deviation gets doubled or tripled along with the frequency.)
Ones around 8.3 MHz would multiply to 12m, and others could be ground upwards for the 17m band.

You can also combine crystals of different frequencies in various ways:  for example, if you have a
20 MHz oscillator and mix that with crystals in the 8 - 9 MHz range you can cover 28 - 29 MHz. 
Another common approach was to build a VFO at a low frequency (where stability isn't as much of
a problem) and hetrodyne it to other bands by mixing it with a crystal.  For example, using a 9 MHz
crystal with a VFO that tunes 5 - 5.5 MHz gives you both 14 - 14.5 MHz and 4 - 3.5 MHz.  There
are many other possible combinations.

And if you have multiple crystals on the same frequency you can use them to build a crystal
filter.

[N2EY]

The FT-243 was one of a family of holders used by the military during WW2. In their time, they were considered "miniature" (!) both for size and because they used much less quartz than the older, larger holders.

They were made for various radio sets, and I have seen frequencies from the AM BC band to beyond 9 Mc. The ones you have were popular with hams for 6 and 2 meter use. They may also be useful for converters and filters if you have the right frequencies.

They can also be moved higher by etching or grinding. WA4QAL did some tests a few years back and his results are still online:

http://home.netcom.com/~wa4qal/crystal.htm

Although not orginally designed for overtone operation, it is possible to get those FT-243s to operate on the third and sometimes the fifth overtone by use of a circuit supplying the needed feedback. There was a one-tube converter in QST that used this trick. Thic can be useful for receive converters covering the bands above 20 meters. For example, an 8.333 MHz xtal operating on its third overtone will give you 25 MHz, which is just right for converting 15 meters down to 80/75 meters. Use of the overtone rather than the harmonic eliminates false responses from the oscillator fundamental and other harmonics.

73 de jim, N2EY

[VU2NAN]

Years ago I ground 8MHz crystals for my 2m homebrew transmitter.

http://nandustips.blogspot.in/2011/03/blog-post.html

73,

Nandu.

[W5LZ]

And to cover the other side of that 'grinding' thingy, marking them with a graphite pencil would move them down in frequency a bit.  A little soap and water would take you back to the original frequency (sometimes!).  Dang, that brings back memories...
 - 'Doc

[G3RZP]

Another trick to lower the frequency a bit was to rub soft solder on the plate. The increased mechaniacl loading reduced frequency, but unfortunately, the activity as well.