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I am modifying the insides of a vintage radio. I wish to avoid damaging it or changing it as much as practical.

Here is the insides of the radio in question: enter image description here

A primary goal is to measure the position of the tuner when the user turns the tuning dial.

Vintage radios tend to use tuning capacitors like this:

enter image description here

You can see one in the radio picture above.

So, as the user turns the tuning dial, I need a value - any value really to indicate where in the turning range the tuner is.

I've done alot of research and trial and error, and it is quite hard to get an electronic value from the variable capacitor. I could replace the variable capacitor with a potentiometer, but that can be quite hard too - these old radios usually have the variable capacitor connected up to strings and pulleys which pull an indicator across the front panel to tell the user which station they are tuned in to. Replacing the variable capacitor with a potentiometer risks breaking the tuning dial mechanism that the user sees.

So I started wondering, maybe there is a way to "indirectly" measure the rotational position of the existing variable capacitor. What I mean by this, is maybe there is a way to connect a potentiometer to the axel of the tuning capacitor? So I could have my potentiometer above or to the side, and through some mechanism like pulleys or something, mechanically turn a potentiometer as the variable capacitor is turned. Hopefully that makes sense. So effectively, piggybacking a potentionmeter onto the existing mechanism.

This way I could leave the tuning mechanism intact and somehow "add" a potentiometer rather than having to rip it all apart.

Is there any precedent for doing such a thing in electronics? Any existing mechanisms/parts that enable measuring the rotation of an axel?

Or maybe there is some other electronic component that could physically measure the position of all the blades of the variable capacitor? I'm really guessing here for something left of field.

Hopefully my question makes sense. Thanks.

UPDATE:

Thanks to the comments I have been able to find new directions to research. The one I am most hopeful about is the possibility of using a through hole potentiometer and mounting it onto the shaft of the existing tuning capacitor. Here's an example of what I found:

enter image description here

https://au.element14.com/bourns/3382h-1-103/pot-rotary-10kohm-22mm-30/dp/2469483

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  • \$\begingroup\$ Basically think servo; a motor and a potentiometer. \$\endgroup\$
    – Andy aka
    Commented Nov 20, 2022 at 21:14
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    \$\begingroup\$ Duke, I'd just get and attach an optical quadrature shaft encoder with added home pulse. Very easy to set this up. You will want to leave it snug at first and just display the home pulse as you rotate the encoder with respect to the shaft position until you get it 'just right'. Then use a dot of epoxy to nail it onto the shaft, permanently. Calibrate once and use forever. Part of my work on medical infusion pumps (life critical by the way) involved a crafted process almost like I just described except that I had to test the encoder over a wide range of rotation rates. \$\endgroup\$
    – jonk
    Commented Nov 20, 2022 at 21:19
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    \$\begingroup\$ How to add a potentiometer so that an existing mechanism also moves the potentiometer is a mechanichal question. You could have the axle or the string to move an optical system that can be electronically measured. \$\endgroup\$
    – Justme
    Commented Nov 20, 2022 at 21:22
  • \$\begingroup\$ @jonk thanks this gives me a direction to research in. \$\endgroup\$
    – Duke Dougal
    Commented Nov 20, 2022 at 21:43
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    \$\begingroup\$ Is this a working radio? If so, one of the two gangs of plates goes to an oscillator, whose frequency can be measured with a frequency counter. The frequency is a measure of to shaft rotation. \$\endgroup\$
    – glen_geek
    Commented Nov 20, 2022 at 22:42

1 Answer 1

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A 180° dial / pointer may be attached to the variable capacitor shaft and graduated / calibrated using a signal generator.

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