Try your experiment on a RC made with a better capacitor (non-polar
mylar or poly-ester).   If you really want small TC change with
temperature, look for the "temperature coefficient (TC)" in the data
sheet.  Look at an "NPO" type.
Typical bypass caps used on VCC lines can change a lot!

You can also use a better resistor, they have TC ratings too.

Sometimes you might have a "leaky" capacitor.  Test it with an ohm-meter
when hot.  It should have infinite (very-very high) impedance.

Good Luck

Mike, WA6ISP
Hagen Engineering
mike@HagenEng.com

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