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. |
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