The stock oscillators are crap, it'll drift more than the SS Minnow
They're also quite difficult to adjust with any precision as Dave discovered - especially if there is any slack in the trimmer. Also, sometimes, any change can perturb the oscillator by orders of magnitude more than the adjustment you're trying to achieve.
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I'm surprised that the oscillator isn't voltage controlled with a DAC to generate the V
FC - that is not only a better way of doing it but lends itself to automatic calibration rather more easily than sticking a screwdriver into a trimmer. Especially as the tongue angle calibration on most units tends to be very poor
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I suspect that these were intended to be used with the Rubidium standard rather than the inbuilt oscillator.