The shaft of the pot is semi-circular with a flat notch on one side
and is shorter than the that of the replacement pot. The shafts of the
replacement pots must be ground or cut to match those the old pots.
First, the shaft is measured from the point that it exits the threaded
sleeve, not from the body of the pot. It is better for it to be too
long after cutting and to grind it down later if necessary, so be
conservative and keep testing for fit. The angle of the
flat is fairly critical. Examine the diagram below carefully before
proceeding. Ideally, the resistance extremes of the modified pots will
be 5 and 100 K
.
Note that all the pots are cut the same except the one for the
throttle. The throttle pot is mounted and wired backwards, so the
orientation of the flat notch on it is opposite that of the others.
The correct angle for the flat is determined by turning the shaft all
the way against the stop in the direction indicated above and matching
the shaft to the shaft of the pot that was removed from the
transmitter. A pencil or marker is used to mark where the flat should
be cut.
The face must be as flat as possible and as
near the correct angle as possible. Care must be taken to keep the
metal filings from getting inside the pots. The new shaft must be
a snug fit in the cam the same way the
original shaft was. The fit of the shaft should be tested frequently
as the flat nears completion. After the flat is cut, fine sandpaper is
used to remove any burrs.
The completed pot should be sprayed liberally with contact cleaner to remove any metal dust that may have gotten in it. The pot can now be reassembled using a little lubricant between the cams and the case and on the spring. The electrical connections on the new pot must face in the same direction that the old ones faced. The pot case and the cams must be aligned and centered with the assembly with the trim in the center. All parts should be aligned in the same way as the original. Ensure that the stick moves freely and centers when released. If it sticks or binds, more grease may be required or a screw may be tightened too much. With the new pots, the stick may not spring back as positively as before but that is okay so long as it returns to center when released. After both sticks have been modified, they are remounted in the case.
Next month, we'll see how to modify the wiring to complete our controller.