Tuning capacitors

[Cleggy]

I've just done some sums, and come up with a typical vane/stator spacing of about 0.3mm, which is about right for a 10 vane 500pf tuning capacitor. Now suppose that there is 0.005mm of corrosion (ie 5 microns), so the total on both sides of the gap will be 0.01mm. The corrosion will be aluminium oxide typically, which has a relative permittivity of 10. This increases the effective gap by 0.01 x 10 = 0.1mm. So the effective gap is now 0.3mm+ 0.1mm = 0.4mm. So the capacitance will go up from 500pF to 750pF, and the tuning frequency by the root of that - a shift to longer wavelengths of ~20%.

That's a very interesting calculation but I thinks it's hard to quantify what the OP describes as "minor surface corrosion". Bright, shiny Al (which has had no chemical treatment or corrosion) usually has a surface oxide layer which is <<1 micron. But anodised Al can have several microns of oxide on it and still look shiny. I'd guess that this is one of those issues where the visual appearance might be quite misleading and you'd have to measure the C values to know what they really are.

I have a 1941 HRO, which I bought in the 1960's, and the 4-gang tuning cap on that looks as though it has had some form of a thin coating of some waxy substance applied to it. It tunes as expected, and although it does not look particularly nice, I've never attempted remove the coating. I cannot recall having seen such a coating on any other radio (though I know they used to "tropicalise" military equipment), nor have I seen the same thing on various pictures of HRO's; can anyone comment on it?

C