Variable capacitor oxidation
 

Tips please for removing the white oxidation/corrosion on the vanes of a variable capacitor.

I have an ultrasonic bath, but not sure what cleaner or home made solution would be best for this particular task.

Thanks,

Ian

Re: Variable capacitor oxidation
 

How bad is it? Photos? Is any of it black in colouration [that type of aluminium oxide is nearly as hard as diamond]? Most of it will brush off, and warm soapy water will remove a lot if not all of it .... then, you may wish to consider 'preventative measures' e.g. a light spray with a silicone-based foam polish [all finished with proper drying and bearing lubrication etc.].

Re: Variable capacitor oxidation
 

It's not dreadful I don't think, but very awkward to get to with brushes, etc. The white 'crystals' are on fixed and moving vanes so touch at multiple points.

Thanks.

Re: Variable capacitor oxidation
 

A thin strip of paper slid between each pair of vanes works quite well with that sort of corrosion.

Re: Variable capacitor oxidation
 

In the past I've used a feeler-gauge slid between the plates to remove these whitish deposits.

(even slight amounts of the white 'fur' can be enough to cause corona-discharge/flashover in power-amplifier stages).

Re: Variable capacitor oxidation
 

You've got an ultra-sonic bath; that's the most powerful means of cleaning you could want for something like this; don't get involved with any more brutal method until you've done all you can with the bath.

As for the medium; always start with something weak, at a modest temperature for a short time...and then progress there. I'd start with 50/50 IPA/water which will clean off anything organic and may loosen lots of dirt and oxide as a starting point, but will probably not go far enough. 5 minutes at 35'C to start. Check to see that there's no adverse effect and ratchet up from there. In water, there's always the potential for galvanic effects causing bad things to happen, so keep an eye on it. Dry with hair drier - don't leave wet. Chemicals; start with vinegar?

Stay the heck away from silicones; death!

Just used my ultrasonic bath to clean the local Vicar's small container for Holy wine; it is 116 years old and has religious / sentimental value; 100 year-old dried wine is stubborn, but I got it! Started off with vinegar (advocated on the internet for wine deposits), which was not helpful and progressed exactly as described to more potent media.

B

Re: Variable capacitor oxidation
 

I've heard it said that a dishwasher is good for such deposits but haven't tried it myself... I've never really fancied a rolling pin right between the eyes!

Before we had such a device I did have to restore a tuning gang for a very poorly SAD94, and removed the rotor assembly so I could use various grades of wet and dry finishing paper (as used for car body/paint repairs) to sand away the crud between individual vanes, finishing with very fine grit (800 or 1000 grade).
The backing material is sufficiently strong to allow you to get a good push and pull sanding effect although it takes a while and many narrow strips (doubled over so sanding on both sides if need be) depending on the amount of corrosion.
It's also water proof, so can be best used wet, acting as a lubricant and to continuously clear away the removed crud.
Once cleaned of the large blisters back to the aluminium the thin film of oxide that immediately forms on the metal is sufficient to prevent further corrosion in the absence of damp.

Mine looked far worse than yours but after a good clean up and reassembly with greased bearings, after nearly 20 years it is still going well... in dry conditions.

It might be possible to use the same technique without removing the rotor, but I'd be worried about grit from the oxide or the wet and dry paper getting in the bearings.

Pete

Re: Variable capacitor oxidation
 

I would be very wary of putting anything that could be (or is) aluminium alloy in a dishwasher. Dishwasher detergent is essentially a strong alkali and will attack aluminium.

Re: Variable capacitor oxidation
 

Personally I try a stiff paintbrush first. I have a narrow one with long bristles that is very usefuo for this and other similar tasks.

Re: Variable capacitor oxidation
 

A piece of slightly thicker cotton string might help.

Re: Variable capacitor oxidation
 

This might sound silly and/or impracticable but I'm sure it's been confirmed once before on here. There was a tip in the RSGB mag [perhaps the Pat Hawker Column?] that the white deposit could be removed by boiling the item with a cabbage. I think this involved a full chassis though! I appreciate that we are approaching December 25th just now and not April 1st but apparently it works:shrug:

Dave

Re: Variable capacitor oxidation
 

Other useful implements [other than those described above] include ...... dental floss [preferably the flat/tape stuff]! This material is both fine and strong ..... and impregnated with a fine rouge-like abrasive [inmates are reputed to have cut through prison cell bars with it ..... but they have infinite 'time' {geddit?} and patience!].

Re: Variable capacitor oxidation
 

Fibre glass circuit board brush?

Re: Variable capacitor oxidation
 

Yes, I recall reading that, too. Boiling cabbage (same with other green vegetables*) will release oxalic acid*. Apparently, this causes a mild reaction with the aluminium and slightly etches the surface, thus removing the oxidation.
Citric acid monohydrate could also be worth a try: it does an excellent job of removing limescale from kettles.

* Rhubarb leaves are high in oxalic acid. Seems that the darker the green, the higher the yield.
* Please note that oxalic acid is harmful if ingested, even in small doses - usually causing kidney failure.

Al.