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

[Bob Camp]

Hi

There is a very small gap between the turns in the coil and the core  
that moves in the coil. That magnifies the impact of the dimensional  
change.

The material in the tuning slug oxidizes over time. For some  
interesting reason that changes it's magnetic properties.

Bob

On Jul 23, 2007, at 3:44 PM, Dan Merz wrote:

> Hi, whoops,  that's 5 mils on 1 inch diameter,  not 0.5 mils. Must be
> getting old,  Dan.
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net [mailto:r-390- 
> bounces at mailman.qth.net]
> On Behalf Of Dan Merz
> Sent: Monday, July 23, 2007 12:04 PM
> To: 'Tim Shoppa'; 'Bob Camp'
> Cc: R-390 at mailman.qth.net
> Subject: RE: [R-390] PTO: Why does endpoint shift over time?
>
> 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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