[Boatanchors] RE: [National] Do Inductor values ever increase with age??

[F W Mooney]

Cal, Bryce, John, Bob, Larry, and Duane:

Thanks for the many suggestions on my original posting.

I received a lot of good advice and redid the tests of the huge =
inductors in the audio LPF of my National RBL-6. =20

With the test setup basically consisting of a source AC voltage =
connected to the series combination of the inductor and a carbon =
potentiometer, I adjusted the potentiometer until I had equal voltages =
across the resistor and the inductor.  At that point "R" of the =
potentiometer should equal "Z" of the inductor.  L is then calculated as =
R/( 2 * pi * f ). =20

I measured the inductance of the three coils at various =
frequencies--audio frequencies this time.  Here's the results I measured =
for the 7.7 Henry inductor:
=09
500 Hz:		5.2H
1000 Hz:	5.6H
3000 Hz:	8.2H
6000 Hz:	8.5H
10000 Hz:	1.6H

Per the manual, this particular coil should have a resistance of 400 =
ohms, and I measured 360 ohms.  This resistance should be negligible at =
these frequencies, so I could assume the impedance of the inductor was =
pure reactance. =20

Overall, I'm convinced the thing is acting like an inductor, and over =
the range of frequencies it's used at, it's in the ballpark. =20

However, any ideas why my results varied with frequency?  According to =
the manual the inductor is 4600 turns of wire on an iron core.

And are there better ways to measure inductance?

- Fred M.  KA1DGL
=09