[R-390] Collins PTO #1 Final Score..!

[Larry Haney]

Hi Jordan,  Good work.  Miles made a very informative post about an 'error
bulge' in the linearity of PTOs in the PTO section of the Pearls.  I've
included it here.

Regards, Larry

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Date: Wed, 06 Apr 2005 21:20:16 -0400
From: "Miles B. Anderson"
Subject: [R-390] PTO Rebuild

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.
----------------------------------------------------------------------------------------------------

On Fri, Nov 15, 2024 at 3:27 PM Jordan Arndt <Outposter30 at shaw.ca> wrote:

>
> Hi again group...
>
> I'm finally 'finished' the end point work on the first of two Collins 390A
> PTOs I bought from Jacques Fortin awhile back.
>
> Both end points match rather precisely at the + and - 000 points with
> points
> in between varying from as close to zero hz as I can measure from +000
> to~700khz, increasing to roughly -300hz at 400khz, then within 100 hz down
> to -000 where it is also at zero hz.
>
> I'm happy with that as it's much much better than the Cosmos it has
> replaced...
>
> 73...Jordan VE6ZT
>
>
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