[R-390] PTO Rebuild

[Barry]

Hmmm.  I'm familiar with setting the endpoint, but don't remember anything
about setting the start point.  I simply crank the shaft until I get the
lower frequency, crank it 10 turns and adjust the endpoint to get 1Mc.
Rethinking this, I may have been doing this backwards as well.  Perhaps I'm
supposed to be setting the endpoint at the lower end, not the upper end.
Maybe it doesn't make any difference?

At any rate, I'd like to read more about setting the start point.  Can
someone point me to the document for this?

Thanks!

Barry - N4BUQ


> 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
>
>
>
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>