Crystal Grinding

Grinding crystals, both from FT-241/243 blanks as well as HC-6/U holders!

First item needed is a piece of plate glass, about 3/8 inch [10 mm] thick, about 7 or 8 inches square [175 - 200 mm]. Don't use fragile window-pane glass.

The initial abrasive used by many was Bon-Ami scouring powder, but Ajax and other similar scouring powders will also work. For a finer abrasive, you can use "Soft Scrub", allegedly a "non-abrasive", but it really is - just a bit finer than the heavy-duty stuff. For still more gentle action, you can use toothpaste, not the gel stuff - get old-fashioned Colgate. For the really, really fine touches, get some jeweler's rouge.

Let's assume your crystal is from a FT-243, say it's about 20 to 25 kHz below your desired operating frequency. It could be lower than that, it will just require more grinding on your part. First you should check the crystal in a test oscillator or xtal controlled transmitter, jot down the frequency on a piece of paper for future reference.

Carefully remove the screws that hold on the cover plate of the FT-243. Try not to destroy the neoprene gasket that seals the cover plate to the body of the crystal holder. Inside, you'll see a compression spring and a, usually, brass plate that has a little "tail" that runs off to one of the crystal holder pins, provides one of the two electrical connections to the crystal blank. G-e-n-t-l-y lift up that plate, just enough that you can tip out the crystal blank from between the two brass plates, there's one on the back side also.

On the heavy-duty glass plate, make a thickish paste of water and some of that scouring powder. Use a pretty good glob of it because you will be spreading it about as you grind your "rock". Place the crystal in the scouring power paste, and using just your index and middle finger, apply gentle pressure on "diagonally opposite" corners.

Using this "grip", begin to move the crystal in a figure eight, "not circular", pattern over the glass base plate. After about five or six "figure eights", grasp the other two diagonal corners in the same fashion and continue with the figure eight pattern, same number of laps.

Now, turn the crystal over and grind the opposite side the same number of turns, using first one pair of diagonal corners and then the other pair of corners, as described above. If you need to, add some more scouring powder / water to the mess you have on the glass plate.

The number of "figure eights" you'll need to get the rock on the desired frequency is a function of how low in frequency it is, relative to the desired operating frequency, how hard you press down as you grind, and just how abrasive your scouring powder is. Only experimentation and years and years of doing this will give you a "feel" for the technique.

Carefully remove the quartz blank from the grinding compound and in a slow stream of cool water, thoroughly rinse off the grinding paste, you can gently apply a toothbrush to aid in this process if you want. Leave the grinding paste on the glass grinding plate, you'll probably need it again.

With clean fingers, hold the crystal blank "by its edges" and reassemble the blank in the FT-243 holder - just slip it in between the two brass plates. Don't forget to re-install that compression spring! Put the cover plate back on the assembly and re-install all cover screws. Torque them down pretty well, since the amount of pressure on the crystal affects its operating frequency.

Install the assembled FT-243 crystal in the test oscillator and measure the new operating frequency. Knowing how many figure-eight laps you did on each side will give you a rough idea of how many kilohertz you can move the crystal blank versus the "number of laps" of grinding effort.

I doubt that just a few turns will move the rock very much, but I highly recommend you begin slowly and gently until you develop a feel for how much your efforts will actually move the frequency. Then you can be more aggressive in future "grindings".

Let's assume your FT-243 still isn't on frequency, it won't be, believe me, so disassemble the FT-243 again, remove the crystal blank and grind it some more - maybe this time, 20 or 30 laps on both corners, and 20 or 30 laps of the OTHER side. Again, thoroughly, reassemble the FT-243 and re-check the frequency in your test oscillator. "repeat as necessary".

Keep in mind that if you are checking the frequency in a test oscillator, the operating frequency may be slightly different from that of the actual transmitter in which the crystal is used. I'd recommend using the "real transmitter" as your test oscillator, to avoid the differences in circuit capacitance, oscillator type, et cetera.

To move a xtal down in frequency just use a graphite soft-lead pencil on the xtal, you have to be very careful when you do this.

Remember: Grinding moves the xtal up and lead or graphite moves it down in frequency.