Mike Dennison wrote:
Andre,
I will be getting a Ropex Tx as a Christmas present for my portable
work. Please let me have details of the crystal mods.
Here are the modifications I have now performed on 3 different units:
The Ropex "The First" LF transmitter comes with a
crystal, soldered in place, for operation on 136,533 kHz.
To change the frequency of operation, you may proceed as
follows:
a) obtain a new crystal for parallel operation into 20 pF, at 48 times the
desired frequency. For instance for operation on 136,750, I ordered a crystal
for (136,750 x 48) = 6,564,000 Hz.
b) remove 4 Phillips screws, two on either side of the
unit.
c) lift the green lid.
d) remove 2 Phillips screws located on bottom of unit.
(These screws hold the internal chassis to the bottom of
the box).
e) remove 4 Phillips screws holding the front panel to
the frame.
f) bring the front-panel slightly forward
g) at the fuse-holder end, unsolder the thick red wire
going from the fuse holder to the printed circuit board.
(Note: the fuse holder is somewhat flimsy, and you may
decide to replace it with a sturdier unit). The various
parts of the assembly can now be moved about.
h) Unscrew and remove the inch-long screw that holds
the two power Fets against the back of the aluminum chassis.
With tweezers or long-nose pliers, remove the small tapped
rectangular plate into which fitted the screw.
i) unscrew and remove the 4 self-tapping Phillips screws
located at the 4 corners of the printed circuit board.
This will release the 4 stand-off spacers/bushings that
support the printed-circuit board. The printed circuit
board is now free from the aluminum chassis.
j) on the component side of the printed-circuit board,
notice that the crystal is soldered to the ground plane.
With solder wick, or a vacuum pump, remove that solder
point.
k) on the trace side of the printed circuit board,
unsolder the two pins of the crystal, and remove the
crystal.
l) cut off a 3-pin length of a regular (not machined) dip
IC socket. Remove (or cut-off, flush) the middle pin.
Insert that socket in place of the crystal. Secure with
a _small_ drop of 5-minute epoxy (Araldite).
m) you will now need to manufacture a crystal sub-
assembly. (This approach allows for the easy replacement
of crystals and fine tuning). Cut off a 3-pin length of a
machined (not regular) dip socket and glue it to the
bottom of a 0.5 by 1 inch piece of scrap perforated
board or printed circuit board. Now glue another 3-pin
length of regular (not machined) dip IC socket about 0.5
inch from the bottom socket. The new crystal will fit in
this upper socket. On the other side of the board, attach
an adjustable 30-100 pF capacitor. Wire the capacitor and
the new crystal socket in series with the bottom socket.
Insert the sub-assembly in the socket located on the
main printed circuit board.
n) now re-attach the printed circuit board with the 4
self-tapping screws and bushings, then re-insert the
screw that holds the 2 power Fets to the aluminum
chassis. Reassemble the box in the inverse order
described above, leaving off the lid. Running the
transmitter in a test load, adjust the trimmer capacitor
until the exact frequency of operation is obtained. This
frequency will drift a few hertz as the unit warms up
(with the cover on).
Good luck.
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