[R-390] PTO: Why does endpoint shift over time?

[Tim Shoppa]

Bob wrote:

>The PTO's in the R390's shift due to aging of the tuning coil.

>There was a *lot* of work done by Collins after the 390 was designed  
>to eliminate this problem. One of my ex-bosses was part of that  
>particular effort. When I asked him why they didn't incorporate the  
>"improved" coil in the later radios the answer was pretty simple -  
>"the contract didn't call for it".

>The two main components of the drift are the aging of the particular  
>material used in the tuning slug and shrink of the coil form that the  
>coil was wound on. Both essentially just keep moving forever and  
>ever ....

If I knew some metallurgy I might be able to make some
half-assed explanation for why it should drift and keep on
drifting over fifty years (and presumably, a hundred, and
two hundred, and...). As it is, I could only make fully-assed
explanations! Oxidation? Cumulative background radiation
causing crystal defects? I know, it must be that kryptonite
in the 0A2 that's doing it! (Sorry, that last one was more
than fully assed, but really the labels on 0A2's do talk about
some small number of microcuries of Krypton!)

Shrink of the coil form - I guess this would fall under
polymer science. In my delving into the innards of PTO's
the form seemed to be phenolic/bakelite. I do know that
lots of plastics will for example swell in the presence of
humidity but I do not know of any examples where the
swell continues on for half a century. Seeing as how we
have to take turns off the endpoint adjustment inductor,
I would guess that the direction is towards larger
inductance which would mean larger diameter which
is vaguely consistent with "swell", but you describe it as
a "shrink" Bob, so maybe I don't understand everything!

Tim.