14 March 2014

Air Variable Capacitor brackets

As a part of my RF source generator project I need 2 ganged Air Variables. In my junk box I have a number of high quality air variables from a Company called Kopt. A search on the internet turned up a number of capacitors from this company. However I could not find which country of origin they come from.

Here are the capacitor details:

PHYSICAL SIZE: 1.25 inches wide X 1.75inches high X 2.25inches deep
ENCODING SHAFT: Not geared. Protruding from the rear.
BODY: Cast aluminum
PLATES: Aluminum and very closely spaced.

Each unit contains 4 ganged capacitors as follows:

Front   C1  20pf - 400pf
Front   C2  10pf - 25pf
Rear    C3   10pf - 300pf
Rear    C4   10pf - 25pf

The issue with these capacitors is that they are not easy to mount having unusual hole locations. Not having a shop full of tools my challenge was to fashion brackets that I could make with hand tools only and which would be sturdy enough to ensure full stability of the capacitors. ie they should not vibrate when the chassis is touched.

So far I have made the brackets (see pics below) from thin galvanized iron which I obtained from an air conditioning duct cover at the local hardware store for R30. This was already in the shape of an angle piece which meant that I did not have to try to make an angle piece myself.

I first made a trace of the hole locations using a paper template. I transferred the hole locations onto the iron plate and then cut the pieces using a pair of aviation tin snips. I then drilled and filed the holes. I first made 2 master templates which were then used to mark out the rest of the brackets. While I was doing the job I went ahead and made brackets for 6 of these capacitors.

Since I did not have any bolts that were the right thread I had to 'self tap' the holes using some american standard 6-32 bolts in stock. Luckily these fitted well and the soft cast aluminum was easy to tap.

The local 4mm bolts are slightly too big for these holes. They would need to be bored out with a number drill and then self tapped. Assuming the 4mm bolts are made of hard enough steel to cut the aluminum.

Next I cut the bolts to a length where they did not protrude into the capacitor moving mechanism. This I did with a hacksaw and a pair of vise grips and a file. After tweeking the holes a bit with a small round file I bolted the brackets to the capacitors. I then soldered a cross piece in under the bracket to improve the stability. As can be seen in the pics below, the brackets are quite high. This is necessary to allow room for a large knob to be easily turned.

I then bolted the capacitors to a piece of PC board which will serve as the floor for my RF generator. To improve stability further I screwed the PC board onto a solid piece of very flat particle board.

It will be interesting to see how stable this arrangement will be in practice.

Brackets made of air conditioning duct cover that already has the angle piece. Cut with tin snips.
Underside showing a cross bracket soldered in place to improve rigidity
Ball bearings at one end of the shaft. Also includes a 2.75 turn gear and an encoding shaft out the rear
Capacitors mounted on a piece of PC board, mounted to a piece of particle board to improve rigidity. RF signal generator. project.

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