Low Power Broadcast Company

AM Transmitter Model RC-25B

20 watts carrier output minimum
+125% modulation capability


Note crystal for 560 kilohertz.  A trimmer capacitor was added to allow setting the carrier frequency to within 1 hertz of the assigned frequency.

The modulator has excellent audio.  However, it is best to isolate the d.c. from the modulation transformer secondary and feed the d.c. to the rf amplifier through a 30 henry 50 ohm choke and use a 16 mfd. coupling capacitor for the audio signal from the modulation transformer.  This modification results in an audio frequency response of 40-10,000 hertz at 100% modulation with less than 1% total harmonic distortion.

Another important modification is the addition of a high voltage on/off switch and pilot lamp.  This allows quickly turning the carrier off and on for adjustments while keeping the filaments heated.

With a proper broadcast antenna tower, this transmitter will serve a large town with adequate field strength.

SCHEMATIC DIAGRAM LPB RC-25B, MODIFIED FOR PROFESSIONAL BROADCAST SERVICE