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

[Jordan Arndt]

Hi Larry...

I bought several of those uncased mica caps from SSN back when I was going through the 2 Cosmos PTO's I have, and I have several of them left over after solving many of the problems with those PTO's. If you recall, one of them was rather jumpy and both required a rebuild of the 3 turn coil, which corrected the 10 turn spread offset which were off by from 7 to 12 khz.

I don't think I'll be trying to correct the offset bulge in the first Collins PTO which is now installed. At least not any time soon...!

73...Jordan VE6ZT
  ----- Original Message ----- 
  From: Larry Haney 
  To: Jordan Arndt 
  Cc: r-390 at mailman.qth.net 
  Sent: Saturday, November 16, 2024 12:54 AM
  Subject: Re: [R-390] Collins PTO #1 Final Score..!


  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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