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

[Dan Merz]

Hi,  dimensional change of the order of 1/2% over 50 years doesn't surprise
me for a composite material like a reinforced phenolic,  which is the amount
of dimensional change to account for 10 khz change.  That's on the order of
1/2 mil for a 1 inch diam and proportionately less for a smaller coil. But
I'm wondering what evidence pins the change on the coil form and not the
permeability of the core material that moves in the coil?  I suspect that
such drift in the other cores of the radio would mostly go undetected or be
of little concern but the pto drift has been the subject of a lot of
scrutiny because it relates directly to tuning accuracy.  Maybe we should be
grinding a little off the core instead of pruning the coil.  I pruned the
coil in mine.  The phenolic is probably paper or linen reinforced so this
may be the avenue to swelling over time.  Dan. 

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Tim Shoppa
Sent: Monday, July 23, 2007 6:27 AM
To: Bob Camp
Cc: R-390 at mailman.qth.net
Subject: Re: [R-390] PTO: Why does endpoint shift over time?

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.

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