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

[Bob Camp]

Hi

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 ....

Bob


On Jul 21, 2007, at 6:26 AM, Tim Shoppa wrote:

> Do we know why PTO endpoints shift so
> consistently in the same direction over time?
> (To the point where every PTO I've found has
> needed a turn or two removed from that little
> slug-tuned coil to get the endpoint right.)
>
> It must be either the fixed capacitors drifting in
> one direction, or the permeability of the inductors
> drifting over time.
>
> I do know that caps aren't truly constant in value.
> And that there is, for example, some silver migration
> in mica capacitors etc. but for mica capacitors,
> this migration is known mainly by me for crackly
> audio, not a slow term drift in value.
>
> I don't know what would make an inductor's slug
> drift so consistently in time. I know some are
> temperature sensitive, and some types of slugs
> go "bad" if over a certain temperature, but that's
> not what is happening here.
>
> How come, after 30-40-50 years, haven't things
> stabilized? I know for a lot of components there
> is some drift after manufacture and bake-out during
> use, but this period seems to be measured in
> days and weeks, not big chunks of a century!
>
> Tim KA0BTD
>
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