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

[Jay Rusgrove]

Tim

I went the route of removing one of the 10 mmF flat silver mica capacitors to restore end point
calibration(btw, this method appears to work well). The capacitor I removed measures 10.2 pF on the
General Radio 516C bridge.

Jay W1VD



----- Original Message ----- 
From: "Tim Shoppa" <tshoppa at wmata.com>
To: <R-390 at Mailman.qth.net>
Sent: Saturday, July 21, 2007 6:26 AM
Subject: [R-390] PTO: Why does endpoint shift over time?


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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