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Author Topic: Johnson variable air capacitors What value ?  (Read 5355 times)
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« on: November 18, 2009, 03:31:13 PM »

Interested in building a "1929 QSO Party transmitter" and have two Johnson variable air capacitors but cant find values on them or on-line, etc.  Nr 1. has a Johnson number:  154-12. with 6 fixed  plates...  Nr 2.  appears to be  number 153-1. (may not be correct number, decal almost unreadable.. ) and has 25-260 (value Huh)  hand written on base and 11 fixed plates.
 Both are a about 2-1/2 x 2-3/4" or so....   Any ideas or where I can find a source. 
Thanks in advance..   John
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« Reply #1 on: November 18, 2009, 03:37:55 PM »

If you cant find the exact info, compairing them to known caps of similar size and spacing gets close, and some DMM's have a capacitance measuring function built in, I have one of those that reads capacitance and inductance.

I can look at a cap and get a close idea of the values, and most designs are not critical at all.

Is this for the output network of a transmitter?
In 1929, they did not use pie net outputs, so I suppose its some sort of balanced output....


Pete, WA2CWA

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« Reply #2 on: November 18, 2009, 03:39:30 PM »

154-12 info here:

Look around the site and compare your units with the pictures and info:

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
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« Reply #3 on: November 18, 2009, 04:17:10 PM »

If you don't have an instrument for measuring capacitance you can measure the reactance range of the cap using a signal generator and variable resistor. 

Wire the variable resistor in series with the variable cap and run an AC signal into the circuit.  Adjust the variable resistor so that the voltage drop across the variable resistor is equal to the voltage drop across the variable capacitor.  Then pull the variable resistor out of the circuit and measure its resistance with an ohmmeter. The  resistance of the pot. will be equal to the reactance of the variable capacitor.  Using the capacitor reactance formula  (C = 1 /2πf Xc) you can calculate the capacitance of the cap.  Do 2 measurements for the cap with plates fully meshed and fully unmeshed and you'll have the range. 

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