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

[Tim Shoppa]

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