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

[Jordan Arndt]

Hi Barry et al...

The easy part was setting the end points...even though I did so by actually 
removing and re-installing the PTO assembly several times.  The hard part 
was aligning and adjusting the PTO itself properly on the mounting brackets 
so that it aligned properly in all three dimensions..

73...Jordan VE6ZT

----- Original Message ----- 
From: "Barry Scott" <72volkswagon at gmail.com>
To: "Larry Haney" <larry41gm2 at gmail.com>
Cc: "Jordan Arndt" <Outposter30 at shaw.ca>; <r-390 at mailman.qth.net>
Sent: Saturday, November 16, 2024 8:42 AM
Subject: Re: [R-390] Collins PTO #1 Final Score..!


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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