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
To: 4sqrp@...
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  built the 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 frequency measurement. 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.

The procedure was to FIRST measure the unmodified FT243 crystal holding the FT243 to the Manhattan pad and ground plate.

Then you take apart the FT243 and measure the frequency of the blank by placing it on the pcb ground plate and placing the plumbing cap electrode on top.

Now 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 cap fixture 'tuned' to the spring & pressure plates inside the FT243 holder.

You 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.

Link here:
Price: $0.00: © 2010 W1REX - QRPme Web design: N5DUX

Rex  W1REX

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