Scramble phone

Posted on Mar 6, 2013

IC-1 and the associated circuitry form a stable audio tone generator that feeds a buffer amplifier, Ql and Q2. The tone output is taken from the emitters of the transistor pair to supply a carrier voltage for a balanced modulator made up of four diodes—Dl through D4—and Tl and T2. If the two transformers and the four diodes are perfectly matched (which is almost impossible to achieve and not necessary in any case) no carrier will appear at the input or output of Tl or T2.

Scramble phone
Click here to download the full size of the above Circuit.

In a practical circuit, a small amount of unbalance will occur and produce a low-level carrier tone at the input and output of the balanced modulator. A telephone carbon mike and earpiece are connected to the low impedance winding of 1, with a three volt battery supplying the necessary mike current. Trim potentiometer R4 is used to make a fine frequency adjustment of the oscillator so that two scrambler units may be synchronized to the same carrier frequency. Rg limits line current to 25 mA.

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