[R-390] PTO Rebuild

[JMILLER1706 at cfl.rr.com]

I went through a corrector stack adjustment last year and it took an entire weekend and some.  It had to be changed quite a bit from factory setting.  I suspect you have described the real cause of the change in linearity here.  My corrector stack now looks like a "sine wave" almost, big hump near one end.  Does anyone have an idea of what an ideal stack should "look like", or is it too hard to predict?   

----- Original Message -----
From: "Miles B. Anderson" <mbalaw at optonline.net>
Date: Wednesday, April 6, 2005 9:20 pm
Subject: [R-390] PTO Rebuild

> I'd like to caution against jumping to the conclusion that the 
> correctorstack 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 
> seriesinductor 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 
> oscillatoris 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 
> capacitancedifference 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 
> properlyat 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
> 
> 
> 
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