[R-390] COLLINS PTO

[Pete Williams]

G'day List... My efforts over several days to endpoint and linearise a
Collins PTO-- specifically one from a R-388 (51J-3) have been an abject
failure .--- procedures documented tried with limited  success with results
way out of useful spec.
At the risk of boring readers, what I have done is documented below  and
pose points requiring better engineering background to supply answers
/explanation.
DONE...1.
 trimmer coil turn removed -- end point acceptable  with slug right in, but
linearity at 100 kHz points(each 360 deg. urn) can vary from 8 to 12 kHz
from one point to the next,
 2.  . ... reducing the 20 pf cap to say 10 or 15 pf  across the coil
assists setting end points but linearity still varied per  above comment.
Small variations in that cap from 10 to 15 pf change all aspects - endpoints
and linearity but still unacceptable Even very small incremental changes up
or  down similar results .
 3. It is a fact that linearity starts with the  design of the main tuning
coil, but what is not obvious is that the total capacitance across the coils
IN CONJUNCTION WITH THE POSITIONING OF THE TUNING SLUG MUST DETERMINE THE
CORRECT CONDITIONS FOR LINEARITY OVER THE FULL 10 TURNS... is this
interpretation correct ? ? It is obvious then , that if the total
capacitance is reduced, the tuning slug must have to be moved that little
bit more within the tapered coil to achieve resonance--- thus changing the
design parameters . 
4. Therefore ..not tried is changing the 250 pf  pf capacitor, the  major
contributor to the tuning range. It probably determines the point coincident
with linearity over the range and where the slug is positioned within the
coil .
5,The shims set the frequency at any given point in the tuning range...
their effect is minimal and at any given point, the maximum change that is
possible is only  a few kHz to set that  point of rotation accurately .
They DO NOT have any major effect in providing linearity.
In conclusion...are there knowledgeable folks that can give an explanation
as to how linearity is  achieved  with especial reference to my comment in
point 3 above 
Apologies for being so pernickety !
     
Cheers
Pete VK3IZ