Icom IC-211 Instruction Manual page 31

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10.
When there are no audio signals, rectified DC voltage from D44 and D46 is applied to
the base of Q52, turning it ON. Since the collector of Q52 is connected to the base of
AF amplifier Q49, base voltage of O49 falls and O49 is turned OFF, thus squelch action
is applied, and no audio is heard from the speaker. At this time, signal lamp control
transistor О51 does not conduct, and so the signal lamp goes OFF.
When incoming signals are received, noise is suppressed, the base voltage of Q52 falls,
and Q52 is turned OFF.
Therefore, normal voltage is applied to the base of O49, the
squelch circuit is opened, and audio signals are heard from the speaker. О51 also is
turned ON, and the signal lamp lights up. The point at which squelch becomes operative
(squelch threshold) is adjusted by R3.
During transmit, positive voltage is supplied through R215 to the base of Q52 and the
squelch circuit is operative, so squelch action is started the moment there is switchover
from transmit to receive, and no loud cracking or similar noise is heard.
AUDIO CIRCUIT
In the FM mode, the audio signals are passed through the de-emphasis circuit consisting
of R196 and C206, and amplified by O49, and fed to the low-pass filter Q50.
In the SSB and CW modes, the detected and amplified andio signals from Q11 are also
fed to the low-pass filter. High frequency components are cut by а low-pass filter О50,
in order to improve the signal-to-noise ratio. Then the audio signals are adjusted to a
suitable level by the volume control R2, amplified by AF power amplifier IC5 to
1.5W or more, and fed to the speaker.
During transmission, positive voltage is applied via D54 to pin No. 8 of ІС5, and so it
does not function, and there is no risk of transmission signals being supplied to the
receiver circuit.
TRANSMITTER CIRCUITS
1. MICROPHONE
AMPLIFIER CIRCUIT
Voice signals from the microphone are amplified by 018 and 017. These output signals
are fed to the MIC GAIN control R-4 and the VOX circuit. The signals from the MIC
GAIN control are kept below a set value of instantaneous frequency deviation by an IDC
circuit consisting of Q16, Q15 and O14.
013 is an active low-pass filter which cuts out harmonics produced in the IDC circuit,
and prevents spread of the sidebands. Output is taken out at the emitter of Q13, and
fed to FM and SSB modulator circuits. R60 is a thermistor which minimizes modulation
deviation caused by temperature variations.
. BALANCED MODULATOR CIRCUIT
IC4 is a balanced modulator where the BFO signal, fed through Pin 10, and the voice
signal from the microphone amplifier, is amplified by Q66 and fed through Pin 4, result
in a double side band signal output from Pin 6. When the mode switch is in the CW
mode, power for the MIC AMP circuit is turned OFF and therefore, the voice signal is
not applied to ICA. Instead, a voltage from 049 is applied to Pin 4, 1С4 loses balance,