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Vacuum cap drive Ö. How is it supposed to be put together?




 
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Author Topic: Vacuum cap drive Ö. How is it supposed to be put together?  (Read 485 times)
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K8DI
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« on: December 26, 2021, 09:31:34 PM »

Iíve got a Jennings 15kV vacuum cap that came as part of the package of two RCA transmitters Iíve been working on. Today I was trying some brackets I made to mount it the way I want. My intent is to remotely turn it with a servomotor.

So Iím at my bench. Iím turning the shaft by hand, two fingers, verifying the max and min capacitance. Turn it till it stops, get the minimum of 25 pF. Turn it easily back towards maximum and it doesnít stop. The shaft unscrews, the cone on the end falls away. After examining it thereís a threaded shaft inside thatís an adjustable stop but it does no good as it is on the cone that falls off.

Whatís supposed to retain this cone? The only thing that seems to hold it is the shaft.

Where does the stop go? On the surface or in one of the two shallow holes it could line up with?  Iíll attach photos later to clarify this, but if anyoneís taken these apart before and can tell me how to put it together, Iíd appreciate it!

On a slightly related topic, is there any workable lubricant one can use on a roller inductor?

Ed


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W1RKW
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« Reply #1 on: December 27, 2021, 06:41:19 AM »

a dry lube like graphite (from a pencil) for the roller inductor shaft would be most appropriate.  anything that is greasy or oily will attract dust and dirt.
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K8DI
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« Reply #2 on: December 27, 2021, 09:47:12 AM »

Photos. Sorry, phone doesnít like taking closeups.

Ed


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Patrick J. / KD5OEI
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« Reply #3 on: December 27, 2021, 08:33:29 PM »

It should be at max C now with the plates meshed?

doesn't it go back in reverse as you put the black cone on, then screw the driver in, the drive thrust washer meets the front of the cone and it starts pulling the brass cylindrical thing out, though the pin in the cone must fit into the hole of the brass cylinder so that it can not rotate.  ?

and at some point it is back at min capacitance?
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K8DI
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« Reply #4 on: December 27, 2021, 08:54:03 PM »

It should be at max C now with the plates meshed?

doesn't it go back in reverse as you put the black cone on, then screw the driver in, the drive thrust washer meets the front of the cone and it starts pulling the brass cylindrical thing out, though the pin in the cone must fit into the hole of the brass cylinder so that it can not rotate.  ?

and at some point it is back at min capacitance?

It all goes back together and appears to operate normally. Thereís no anti-rotation feature in that the pin/threaded rod in the cone doesnít touch the brass bit until itís at or near minimum capacitance. If you rotate the cone the pin either hits the top of the brass thing or goes inside a shallow hole then hits it. But you can see daylight between the fixed and moving plates before it bottoms in the hole.

There doesnít seem to be anything missing. Iíve zero prior hands on time with vacuum caps; it just seems like the max capacity has been reached point shouldnít be indicated by the thing coming apartÖ. 

Would I be crazy to put a little epoxy on it? Or something else?

At least in my usage it will never be near the maximum. Itís a 700pF cap being used as the tune/input side of the pi network; it may never even be half meshed.

Ed
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« Reply #5 on: December 27, 2021, 09:26:02 PM »

Most vacuum variable caps are like that. You can unscrew them to change the nose housing and lube/replace the bearings.
There are motor assemblies that have turns limiters. Some of the ones Iíve had use a 10 turn potentiometer, along with a belt, clutch and rotation limiters. If your going to use a stepper motor and the micro controller of your choice, I believe all of the limits can be set in the programming..
Old way is how I mentioned. I might have a complete assembly. But the transmitter they came from didnít utilize the whole range of the variable. So youíd only have to 10 full rotations, not your typical 20 end to end.
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