Restoring a brass variable capacitor?

[Mattylad]

Hello all,

I have the urge (or more of a whimper) to finish something I started 20 years ago.
It's a brass variable capacitor, circa "before I was born" marked POLAR on the top inside and .0003 on the bottom inside.
It has 9 fixed vanes & 8 moveable vanes, all made in brass.

When I was nowt but a mere lad I thought it looked nice & would look even better if polished up & restored - I still do, this is why I have stored it in a bag in my box of variable caps for so long since I took it apart then.
The ball bearings are in the bag.

The vanes are the problem because they are a tad pitted as you can see in the pics.
A lot of hand polishing using T-cut (also 20+) does bring it to a bit of a shine but getting rid of the pitting on the edge takes a heck of a lot of work if at all.

I'm wondering if you seasoned experts could pass on any tips for how best to clean these off?

I have seen suggestions on dishwashers but I'm not so sure it would do this?

Is there a magic fluid I can leave it in that will help? (I.E. steradent?)

The third picture shows the problem, it's mainly the external vanes, the inside has a dark ring around the edge that takes a heck of a lot of rubbing to remove.
perhaps I should get the bench grinder & fluffy disc out?

It's on my list of little things to do while watching telly during the winter

Nowt but hard work, sorry. 800 or 1200 grit wet and dry then Brasso.

[Mattylad]

Or five minutes in Worcestershire sauce!

It's worked wonders, even on the spacers. (I should have Googled first)
However methinks I need some Brasso & elbow grease on the edges where they are most tarnished.

Any suggestions for screw threads? it has 2 brass screws that are tarnished in the threads too.

[paulsherwin]

Any acidic solution will remove tarnish - Worcestershire sauce, brown sauce, vinegar, cola, bathroom anti-limescale cleaner etc. It looks to me that there's lots of corrosion there though.

[Mattylad]

Well, after 15 mins sauce, some rubbing with 30 year old Brasso (the same tin I started using many years ago, I need some new stuff lol) and 15 mins on the screws in the sauce & cleaning with a toothbrush its now cleanish enough.

I don't have spanners to hand to dismantle the variable vane so just cleaned it for now.

There is a short flexi cable I need to replace that goes from the screw at the back end of the variable vanes to a little screw on the back plate, I also need to strip it and move the lug thats on the nut at the back onto the fixed vane so I can connect to it - replacing the fibre washer that is currently there - I wonder where that was supposed to go lol?

Alignment is likely to take me hours!

Any suggestions for what this may have been used on? (or can be used on?)

The wife is trembling at the thought of me finishing the tiling in the kitchen now.

[emeritus]

I clean up tarnished screw threads either by running them though a die (assuming I have the appropriate die or die nut), or else use a rotary wire brush. For this size, a fine brass or steel wire brush used in a Dremel or Minicraft drill should do the trick. They are almost certainly a BA thread.

[Mattylad]

Cheers guys.

I have now put it back in its plastic bag pending me finding another round "tuit" I think some spanners so I can align it better. Hopefully it won't be another 20 years (Might be worth something by then though).

At least I didn't have to watch Eastenders

[AC/HL]

.0003, or 300pF, was a typical value for a reaction control in the early TRF radios.

[Bazz4CQJ]

When you are happy that you've finished cleaning it, consider what you might do to stop/delay it tarnishing again. For the vanes, this might be just a light blast of WD40 or similar, or for the major external surfaces, an aerosol varnish might be usable.

B

[Boater Sam]

Sliding ever so slightly off the thread,

1) Steradent I think you will find is very mild Hydrochloric acid, it will attack Chrome plating! And brass is a zinc/copper alloy, I would expect it to pit the zinc out, not good.

2) Coca Cola (Coke) is mild phosphoric acid, will strip brass clean as a whistle.
( Other trade names are available etc.)

[Mike Phelan]

For cleaning tarnished brass, a clock cleaning fluid like Horolene is good.

Or you can make your own by mixing ammonia, washing up liquid and oleic acid; latter is optional.

[vidjoman]

Originally Posted by Bazz4CQJ

For the vanes, this might be just a light blast of WD40 or similar

That's like asking for dust and dirt to stick to it as it will leave an oily coating that will become sticky as the solvent evaporates. I would use a light spray of varnish on each vane and leave it to dry thoroughly before assembly.

[jonnybear]

Try (Bar Keepers Friend) works wonders on copper & brass. http://www.ebay.co.uk/itm/Homecare-B...eedingUtensils
John

[Bazz4CQJ]

Originally Posted by Mike Phelan

Or you can make your own by mixing ammonia, washing up liquid and oleic acid; latter is optional.

There is a risk with ammonia on brass of initiating stress corrosion cracking. Probably not an issue with machined parts, but might occur on any cast components with residual stresses, or other components subject to stressing during forming.

B

[Mattylad]

Yes varnish might be an idea, but it's back together now however stripping it should be easy enough, I have a tin of PCB lacquer.

Will this affect the capacitance or voltage characteristics though?

300pf sounds good - ISTR that's ATU territory too and my wife suggested using cola too.

Cheers all.
Matty

[vidjoman]

Haven't tried PCB lacquer but think it should be good, it is meant for protection of the boards without affecting performance. I was thinking of a fine spray coat. Make sure the contact points are making a good connection when you rebuild.

[Mattylad]

I'd put a sticky paper dot on the holes before spraying if I get a round tiut.

Cheers all.