[R-390] PTO Rebuild

[Miles B. Anderson]

I'd like to caution against jumping to the conclusion that the corrector
stack needs adjustment whenever the PTO can't be made to track at the
intermediate points. I tried once to adjust the corrector stack on a Collins
PTO and made such a hash of it that the whole assembly wound up in my junk
box.

I later discovered that the real problem is often much easier to solve. The
problem in my case was that one or more of the little unencapsulated mica
capacitors went west.

There is no way to adjust the shunt capacitance of a Collins PTO. All you
can do is adjust the start point of the tuning slug and the little series
inductor that is used to trim the end point. This means that there is a
unique shunt capacitance which will make the tuning equation come out right
at both ends and the middle. If that shunt capacitance changes because of
aged components, no amount of fiddling the inductances will make the tuning
linear anywhere except at the end points.

The procedure I followed was to adjust the start point and the end point as
per the manual. Then tune the PTO to the midpoint (500). If the oscillator
is high at the midpoint, add more shunt capacitance. If it is low, remove
shunt capacitance.

Adjust both end points again per the manual and check the error at the
middle. If it is still off, repeat the process.

We are only talking about 10 to 30 pf difference, but that small capacitance
difference can knock the daylights out of the linearity by putting an "error
bulge" in the middle.

When you think you have the middle and both endpoints "spot on," check the
tuning error every 100 kHz. If there are two "error bumps" (at, say, 300 and
700) these can be washed out by simply over-compensating the middle so the
error runs in the other direction.

By the way, I replaced the inner shield cover before each measurement.

The main thing to bear in mind is that the corrector stack was set properly
at the factory. The main coil is heavily doped and is not likely to move or
change. What IS likely to happen is a change in the shunt capacitance.

The shunt capacitors are intended to be temperature compensating, but this
never bothered me much. Ordinary NPO ceramics seem to work fine.

Miles Anderson, K2CBY