Icom IC-21 Instructions Manual page 14

Receiver Audio Amplifier, Squelch, and Speech Amplifier (Module U—4) (В-008)
The output of the discriminator drives two circuits: the audio amplifier, and the squelch.
The audio signal, after being adjsuted by the volume control R-1, is then fed to ІСІ
(TH9004AP), followed by the audio power amplifier stage Q8 and Q9 (2SD235s), which
. gives an audio output of 1.5 Watts. The squelch circuit is driven by the noise present in
the output of the discriminator when no carrier exists. The noise is amplified by
transistors Q5 and Q6 (2SC371s), and rectified by the diodes D3 and D4 (1N60s). Тһе
resulting DC voltage is used to forward bias Q7 (2SC373), which becomes conductive,
reducing the voltage on pin 3 of the IC preamplifier, resulting in silencing of the audio
amplifier. When a carrier is present in the antenna the noise in the 455 KHz IF decreases
due to the quieting action of the limiter, in turn reducing the output from the squelch
detector, and causing Q7 (2SC373) to stop conducting, thereby allowing the IC
preamplifier and audio power amplifier to perform normally. The squelch is adjusted to
choose the amount of DC required to cut off 07.
The signal from the microphone is amplified by Q1 and Q2 (25С3715), fed to the
deviation limiter, which consists of D1 and D2 (1N60s), and the active audio filter Q3
(28C371), and further amplified by 04 (2SK30) before being fed to the modulation
circuit in the transmitter oscillator unit.
Power Supply Circuit (Module U-7) (B—007)
This circuit provides a source of regulated voltage for the receiver circuits and control of
output power during antenna mismatch conditions, as described herein.
The receiver voltage is regulated to about 8 volts by the series pass transistor 05
(2SC620) which has its base set at 9 volts by the zener diode D1 (1N757). Capacitor C5
acts to eliminate voltage transients and noise from the output of the regulator.
The action of transistors Q1, 02 (2$С371$), ОЗ (CDC—9000) and 04 (250235) is
described in the APC circuit. Resistor R-3, connected to the resistor network, consisting
of resistors R1 and R2 is used to provide an adjustment in output power when the
Hi-Low Power switch is set in the low power position. Adjustment of this resistor
controls the base drive to the regulator transistors Q2, 3, and 4. This adjustment in effect
controls the actual voltage supplied to the driver transistor Q1 (U—10) and the power
output transistor Q2 (U—10) when the Hi-Low Power switch is in the low power position.
The output power can be set to any value from tenths of a watt to a few watts. This
feature is useful when using the transceiver as an exciter driving a linear amplifier.
Exciter (Module U—6) (B—004)
The modulated signal from Q6 (0—9) is multiplied eight times to a final frequency of
144 MHz by transistors Q1, 2, (2SC373s) and ОЗ (25С773). Each of these transistors
act as frequency doublers. Double-tuned circuits are used between stages to minimize
spurious radiation and to achieve an even response over the 144 to 148 МН? range. Тһе
144 MHz signal is amplified by Q4 (28C741) and Q5 (28C730) to a pongi level of one
watt. This signal is fed into the power amplifier.
Transmitter Power Amplifier (Module U—10) (B—010A)
The signal from the multiplier and pre-driver unit is amplified to 10 watts by transistor
01 (2SC550) and 02 (2SC552). Transistor Q2 is the power output stage and is coupled
to the antenna through the spurious filter.
Spurious Filter and SWR Coupler (Module U—11) (В-014 А 4 В-015А)
The spurious filter consists of L1 (LA-8), L2 (LA-9), and L3 (LA-10). The SWR coupler
consists of diodes D1—D4 (1N60s) The SWR Coupler drives the SWR meter. The
reflected SWR is fed from the coupler into the APC (U—7).
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