Microphone Amplifier
Q15 Voltages
(transmit mode)
Collector 6.29 VDC
Emitter 1.63 VDC
Base 2.34 VDC
R92 provides DC voltage to operate an Electret type microphone, which uses a thin metalized mylar
foil capacitive transducer driving an internal FET amplifier. Audio AC voltage is amplified and
appears across R92, where it is coupled through C60 to the base of Q15. If you are not using this
type of microphone you should not install (or should remove) R92. Electret microphones are common
in inexpensive audio devices (tape recorders, etc.) or can be purchased directly from component
suppliers. Most inexpensive computer microphones are Electret types.
Audio input level for transmitter testing should be 50 mv or less. Use capacitive coupling (10 mfd) if
R92 is in place, or temporarily lift one end of R92 to avoid sending DC power into your test oscillator.
Microphones used with this transceiver should be capable of providing 40 to 50 mv of audio output. If
using a Electret microphone you will need R92 in circuit to power the microphone. Overdriving the
audio input may cause harmonics of audio frequencies to be generated in the balanced modulator.
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