I am working on a simple FM radio transmitter and I was looking for a variable capacitor to help tune the circuit. I rummaged around and took apart an old Sony radio that had this component in it. Something tells me that it is a variable capacitor, but I wanted to ask this question to verify it:

The top white disk is rotate-able.

I tried the bottom terminals with a multimeter's capacitance function, but it appeared to have no capacitance. Has anyone ever seen this device?

Another quick question: I heard that transistors can act as voltage-controlled capacitors. Is this true? Is this why people use transistors to control the oscillations on their radios?

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    \$\begingroup\$ A reverse-biased diode is a variable capacitor. More reverse voltage == less capacitance. These are called "varicap diodes" but some hams use ordinary 1n400x rectifiers. zl2pd.com/Varicaps.html \$\endgroup\$
    – markrages
    Apr 22, 2013 at 18:52
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    \$\begingroup\$ AKA varactor or varactor diode. \$\endgroup\$ Apr 22, 2013 at 19:56
  • \$\begingroup\$ @markrages Thanks for the comment! I have a 2N3094 transistor that I think acts like a varicap? (I'm using the plans from this website: lucidscience.com/pro-basic%20spy%20transmitter-5.aspx) I think it varies the capacitance in the LC circuit which changes the frequency of the transmitting signal in an FM format. \$\endgroup\$
    – masoug
    Apr 23, 2013 at 0:42

1 Answer 1


That unit is actually four variable capacitors in one package, plus four trimmer capacitors, one for each section (the four screws on the back). This is built for the typical AM/FM portable radio, in which there are two separate RF front ends, one for the AM band and one for the FM band. Each superheterodyne front end has a tunable preselector in the RF amplifier, plus a tunable oscillator to do the down-conversion.

The unit appears to have at least six terminals, and two of those should be shorted together — these will be the common (ground) connection for all four sections. You should be able to measure a variable capacitance between these and each of the remaining terminals, on the order of 10s to 100s of pF.

Long ago, it was almost a universal standard that an AM receiver used a 240 uH inductor (which also functioned as the antenna) along with a 40-365 pF variable capacitor.

  • \$\begingroup\$ Thanks a lot! I'll be experimenting around to see which terminals does what! \$\endgroup\$
    – masoug
    Apr 23, 2013 at 0:26

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