[R-390] Replacing carbon comps with metal film resistors

[Jacques Fortin]

Hi Perrier, Bob, and everyone else

This is an experiment I performed a lot of times before, but just to check again....

The Q swamping resistors inside several R-390A IF transformers are 39K, 10%, 1/2W, carbon composition...
I opened my parts drawer for resistors of that value, and waked up my old trusty Hewlett-Packard 4192A LF Impedance Analyser.
And there are the values I measured for various resistors types at 455kHz (all those being rated at 1/2W) with the minimum connecting lead lengths by using the HP 16047C Test Fixture.

But just before you look at those results....
I hope that everyone remembers that in the equivalent serial representation, an impedance is a value of pure resistance, plus or minus a reactance.
Meaning, Z = R ± j X
If the measured reactance is positive, you have an inductance in series with the resistive part, and if it is negative, you have capacitance in series with the resistive part.
In theory, a perfect resistor will exhibit a measured reactance of ZERO, or, expressed otherwise, smaller is the measured reactance, more the part is close to the perfect model.

Ohmite Carbon Composition:
40100 -j 1370

Philips Carbon Film:
38710 -j 950

Sanyo Carbon Film:
38850 -j 780

Corning Glass Works (CGW) Metal Oxide Film:
39470 -j 870

Philips Metal Film:
38980 -j 400

In this test, at 455kHz, the metal film resistor wins.
But the surprise is that the Carbon Composition turned out to be the more "imperfect" of the lot !

73, Jacques, VE2JFE