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Ted Kendall · 13th Sep 2019, 8:50 am

Bit of a mouthful, but the query is this. I'm working on a bit of 60s test gear which has a multivibrator putting out a nominal 3150 Hz. This uses two polystyrene capacitors (5nF) and an NTC resistor to compensate for their drift with temperature, and there's a fair amount of that, despite being transistorised, as there's a lot of R/C smoothing in the power supply. Simply placing fingers on the caps causes the oscillator to drift up by several Hz, and over several hours it drifts by about 30 Hz, so the NTC isn't tracking the caps too well.

Now, without doing a wholesale rebuild of the stage, it seems to me that careful selection of capacitor type might give me something which reduces the drift substantially, either by bringing it closer to the curve of the NTC, or being sufficiently tight that I could dispense with it. Any ideas?

13th Sep 2019, 8:50 am	#1
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,675	Capacitor temperature coefficients Bit of a mouthful, but the query is this. I'm working on a bit of 60s test gear which has a multivibrator putting out a nominal 3150 Hz. This uses two polystyrene capacitors (5nF) and an NTC resistor to compensate for their drift with temperature, and there's a fair amount of that, despite being transistorised, as there's a lot of R/C smoothing in the power supply. Simply placing fingers on the caps causes the oscillator to drift up by several Hz, and over several hours it drifts by about 30 Hz, so the NTC isn't tracking the caps too well. Now, without doing a wholesale rebuild of the stage, it seems to me that careful selection of capacitor type might give me something which reduces the drift substantially, either by bringing it closer to the curve of the NTC, or being sufficiently tight that I could dispense with it. Any ideas?