Cyclone 40 PTO Modification
Some builders have experienced movement of the Cyclones PTO when transmitting,
generally only 100 to 200 cycles. While not enough to prevent QSOs,
some found it disconcerting. This is due to minor heating of U1 while transmitting.
The temperature rise is just few degrees and is almost undetectable to the touch.
This simple mod will reduce the drift considerably, and in some cases
eliminate it for all practical purposes. Beta testing has been most encouraging. Four State is providing free mod parts to
all Cyclone purchasers. If you haven't built your kit yet, incorporate these
changes in your initial build. To perform the mod, do the following in the order listed.
1. Remove the top and bottom covers.
2. Remove C15, the 330p maroon colored silver mica capacitor in the PTO circuit
at this location. *
3. Solder the 150p NP0 capacitor leads in the now empty C15 holes.
Leave the leads long above the board, as in this picture.
4. Reassemble the bottom cover.
5. Glue the 180pf polystyrene capacitor to the top of U1.
and let it cure completely, either gel type super glue or epoxy work well. Use plenty of glue to
ensure good heat transfer, but don't allow it to spread over the sides and
onto the legs. Refer to these pictures.
At least one builder encapsulated the entire capacitor, including the leads, in super glue with no ill effects to the circuit. **
If you have U1 in a socket, and choose to remove it to glue the poly cap, use an IC puller only, not a screwdriver, and
orient the notch correctly before reinstalling.
Both cap leads are oriented over the notch end and point toward the center of the board.
6. Tack solder the poly cap leads to the leads of the NP0 capacitor
that you installed in the C15 holes. No need to wrap the leads around each other.
refer to this picture.
That's it, you're done. Your installation should now look
Snip the excess lead lengths and reassemble the top cover.
Click here to see more reference pictures.
Note: The rig's frequency may (or may not) shift a little after this mod.
If it does, you may wish to retune the PTO coil or add a small NP0 cap (~10p) in parallel
with the new capacitors.
* It may be best to heat the solder joints on the bottom of the board and
push the leads back up through the board with a small paper clip or similar object.
** Use cyanoacrylate glues with adequate ventilation and
safety glasses, the fumes can be irritating.
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