Re: FT243 crystal grinding.
Bry Carling <af4k@...>
Great story Rex - thank you for posting this, and the article!
73 - Bry AF4K
From: 4sqrp@... <4sqrp@...> on behalf of Rex Harper w1rex@... [4sqrp] <4sqrp@...>
Sent: Wednesday, December 28, 2016 10:08 PM
Subject: [4sqrp] FT243 crystal grinding.
The crystal grinding workshop at Ozarkcon was given by Darrel Brehm, WA3OPY,
Ron Hale, AC5UR, and me, W1REX. My friends Darrel and Ron both work at International Crystal Manufacturing
in Oklahoma City and I, of course,
'work' at QRPme. The boys at ICM provided
some frequency counting equipment while I supplied the FT243 crystals and
my special crystal oscillators.
My unique contribution to the art of crystal grinding was the
crystal oscillators. They were
based on the ubiquitous universal crystal oscillator circuit with a twist. I
oscillators inside tuna cans with a bare copper pcb on the top of the can. The entire
copper pcb was a grounding plate where you placed the crystal for
The second electrode for the FT243 crystal was a copper plumbing cap attached to a wire going a Manhattan pad which is also connected
to the pcb to the oscillator inside.
procedure was to
FIRST measure the
unmodified FT243 crystal
holding the FT243 to the Manhattan pad and ground plate.
you take apart the
FT243 and measure the frequency of the blank
it on the pcb ground plate and placing the plumbing cap electrode on top.
you can slowly add lead shot, BBs or any other
uniform small weights into the plumbing cap until the frequency
reads sufficiently CLOSE to the original unmodified frequency. Now you have the
fixture 'tuned' to the spring & pressure plates inside the FT243 holder.
can do some grinding...
we used Bon Ami on 1/2" plate glass grinding
surfaces. Clean the blanks
thoroughly with distilled water, dry with hot air and test in the cap fixture.
Once you get a reading you are happy with, you reassemble the FT243 using your new blank and you should then be fairly spot on to the desired frequency. I scored on a big batch of 6975 FT243s and we successfully moved them all to 7030 which was a hard to find crystal in those days...I wrote up the whole process and did a kit run of the oscillator at FDIM... It was my first attempt at a NO Solder Buildathon. We did it live over the Internet and it was billed as the FDIM Build-A-Long.