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

[Barry Scott]

Yes, congratulations, Jordan.  Good work.

Thanks, Larry, for reposting that.  After adjusting the endpoints, I
think my Collins PTO is very close throughout the ten turns; however,
if I find it is not, this gives me something to check.

Thanks again,
Barry - N4BUQ

On Sat, Nov 16, 2024 at 1:54 AM Larry Haney <larry41gm2 at gmail.com> wrote:
>
> 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
>
> ---------------------------------------------------------------------------------------------------
> 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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