Type of dielectric for NTC capacitor

[boater sam]

Hi all, need a little advice please.

I have a broken 4.7pf capacitor which must have a negative temperature coefficient.

What dielectric is suitable?

The existing is a green ceramic cylinder about 10mm long and 6mm in diameter, axial leads.

I have silver mica, polystyrene, and ceramic disc which I understand will not do.

So, which will be OK?

[ppppenguin]

I think polystyrene has a small NTC. Whether it's enough for your application is anyone's guess.

I have some temperature compensating ceramics but not at 4p7.

[Nick]

What's the application and how much of a negative temperature coefficient do you need?

IEC/EN 60384-8/21 and EIA-RS-198 Class 1 ceramic capacitors have a whole range of negative temperature coefficient types, from NP0 (no drift) to N1500 (-1500 ppm/°C).

[boater sam]

There  is the rub, I don't know.

It is to stop local oscillator drift with temperature in an AM radio. I needs to counteract the positive change in capacitance of a 2 gang tuning capacitor.

Looking through my stock I don't have any capacitors of 4.7pf  to experiment with.
If I knew what dielectric was likely to be suitable I could buy one to find out.
Sam.

[Crackle]

Are there no markings?
Mike

[boater sam]

Indistinct , a 4 and a 7 and I think a P  Its a horrible dark green dipped pot one.
Its the modification on a later KB OB10 radio across the oscillator section of the tuning cap.  C7a, 4.7pf on the Trader sheet, it does not appear on the manufacturers sheet.