There is a video on youtube, where a guy re-caps and repairs and tests
out a late 1940's Precision Apparatus E-200 signal generator.

Video name:
"Repair of a late-'40's Precision E-200-C RF signal generator"

This sig gen has a built in audio tone modulator (oscillator). He
replaced a resistor in that circuit, which was out of tolerance. He
replaced it with a modern film type resistor. The audio modulation did
not work afterwards. After testing everything, he went and replaced that
resistor (again) with an old style carbon resisstor, and after that, the
modulation worked.

What the heck?????

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????