Tuning capacitors

[Murphyv310]

Hi.
It's a long time ago but when I was at college it was generally taught that aluminium takes on a corroded surface as soon as its subjected to the atmosphere. Thereafter the increase in corrosion is slow, all dependent on conditions. After 60, 70 or more years especially in damp conditions I've seen corrosion on aluminum of far more than 1 micron. One cap I mentioned was bad enough to cause shorts, the pitting and bumps being quite easy to see with the naked eye.

[Crackle]

In my examples I have seen the tuning capacitor has to be set to a higher value. (set the dial to 1700m to tune to 1500m) It would seem that the capacitance of the tuning capacitor has decreased, requiring the vanes to be meshed more.
So to my reasoning if a layer of oxide can effect the total value then it must decrease it.

The above is true for long wave, but does it also explain why I have to reduce the trimmer capacitor for MW, yes I think so.

Maybe the LW coils have changed in value, they cant add turns to themselves, but could anything else cause them to decrease in inductance, covering wax absorbing moisture etc..
If the LW coils are impregnated with wax then that could explain it as they have far more turns of finer wire, so the damp wax will have more effect on the value.

Does that make any sense?

Mike

[Murphyv310]

Never had it do that Mike. I sprayed the tuning cap I spoke about earlier with IPA aerosol and blew it with an air can, this reduced each section by about 40pf, not a full fix but a start.
Could your coils be impregnated with dampness? Long Wave will always fare worse with moisture ingress.

[Murphyv310]

The only way capacitance can drop on a tuning cap is if the vanes were eaten away so the gap was seriously widened. If that was the case I'm sure the set would be in very poor overall condition with rusty steel and rotten cabinet.

[Crackle]

Aluminium oxide is classed as an insulator, resistivity is >10 to the power of 14 ohms/cm
As opposed to solid aluminium 2.82 × 10 power of −8 Ω/m ("metre" no idea why the units are different)
according to the www.
So if the solid metal is turned into a good insulator in effect the gap has increased, I suppose?
Mike

[pwdrive]

So far as I know the layer of oxide between the plates of an air spaced capacitor increases the dielectric constant, increasing the dielectric constant increases the capacitance, have a play around with this calculator, dielectric constants for various materials are also given in the link:

http://www.daycounter.com/Calculators/Pl...ator.phtml

Lawrence.

[Murphyv310]

Hi.
Good link Lawrence, certainly what I've found in practice, capacitance rises with oxide.

[Murphyv310]

With regard to Mike's initial question I'm wondering if the problem is moisture laden coils? This has an adverse effect on tracking and more so on long wave. Also don't disregard any capacitor in the tuned circuits, although some types of small value caps are reliable they can still change value or even leak.
Perhaps check these items too.

[Kalee20]

So far as I know the layer of oxide between the plates of an air spaced capacitor increases the dielectric constant, increasing the dielectric constant  increases the capacitance...

It does.

But that aluminium oxide has to come from somewhere, and that somewhere is the plates. When aluminium oxidises it occupies a similar volume to before, which is why it gives such good protection (unlike iron, which when it rusts, expands massively and flakes off).

So if you start with two aluminium plates 100 microns apart, spaced by air, and let them oxidise 1micron each, you have effectively two plates 102 microns apart, with the space between filled with 100 microns of air and 2 microns of alumina. Because the dielectric constant of alumina is about 3, these 2 microns of oxide are equivalent to 0.66 microns of air.

So capacitance has DECREASED in the ratio 100/100.66. If you think about it, you'll always get a decrease, even if the oxide had a dielectric constant of several thousand!

[pwdrive]

So, that's different to this then in Post#3:

""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%.

Now that is an over-estimate in capacitance shift. The oxide layer could be thinner, which would change the values, and of course the aluminium is a tiny bit thinner because some of it has been converted to oxide.

But the principle remains - could a visible oxide layer change the value of a tuning capacitor? Yes.""

Lawrence.