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THE AM BULLETIN BOARD => Technical Forum => Topic started by: K8DI on December 26, 2021, 09:31:34 PM



Title: Vacuum cap drive …. How is it supposed to be put together?
Post by: K8DI on December 26, 2021, 09:31:34 PM
I’ve got a Jennings 15kV vacuum cap that came as part of the package of two RCA transmitters I’ve 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 I’m at my bench. I’m 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 doesn’t stop. The shaft unscrews, the cone on the end falls away. After examining it there’s a threaded shaft inside that’s an adjustable stop but it does no good as it is on the cone that falls off.

What’s 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?  I’ll attach photos later to clarify this, but if anyone’s taken these apart before and can tell me how to put it together, I’d appreciate it!

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

Ed


Title: Re: Vacuum cap drive …. How is it supposed to be put together?
Post by: W1RKW 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.


Title: Re: Vacuum cap drive …. How is it supposed to be put together?
Post by: K8DI on December 27, 2021, 09:47:12 AM
Photos. Sorry, phone doesn’t like taking closeups.

Ed


Title: Re: Vacuum cap drive …. How is it supposed to be put together?
Post by: Opcom 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?


Title: Re: Vacuum cap drive …. How is it supposed to be put together?
Post by: K8DI 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. There’s no anti-rotation feature in that the pin/threaded rod in the cone doesn’t touch the brass bit until it’s 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 doesn’t seem to be anything missing. I’ve zero prior hands on time with vacuum caps; it just seems like the max capacity has been reached point shouldn’t be indicated by the thing coming apart…. 

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. It’s a 700pF cap being used as the tune/input side of the pi network; it may never even be half meshed.

Ed


Title: Re: Vacuum cap drive …. How is it supposed to be put together?
Post by: KF7WWW 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 I’ve 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 didn’t utilize the whole range of the variable. So you’d only have to 10 full rotations, not your typical 20 end to end.
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