Icom IC-751A Service Manual page 21

4-1-10 S-METER CIRCUIT
When operating in FM mode, output signals from Q45 (an
FM IF signal amplifier) are detected by D114 and D115, and
are then applied to the S-METER. D112 is a compression
circuit which maintains the S-METER dynamic range at
approximately 40dB.
When operating in modes other than FM, AGC voltage is
amplified by a differential amplifier which consists of
Q26~Q28 and IC7(h). AGC voltage is then supplied to the
S-METER. Offset in the differential circuit is adjusted by
R132 and sets the S-METER at 0. An AGC reference voltage
is adjusted by R442 to obtain excellent meter linearity. When
the [AGC] SWITCH is in the OFF position, AGC voltage is
fixed and the S-METER will not operate.
4-1-1411 SQUELCH CIRCUIT
(a) NOISE SQUELCH (FM MODE)
A portion of the output signals from the FM detector circuit
pass through the [SQUELCH] CONTROL on the front panel.
They are then applied to the noise amplifier circuit on the
MAIN UNIT. The [SQUELCH] CONTROL is a double
variable resistor, so it can be used to squelch noise and the
S-METER.
The noise amplifier circuit consists of an active filter (Q22)
and noise amplifier (Q23). Q22 detects 10~20kHz noise
components which are determined by L9 and C55. Noise
components amplified at Q23 are detected by D55 and D56,
and are then applied to comparator !C7(g).
When operating in FM mode, pin 2 on iC7(g) is at OV, as
determined by Q83, and the S-METER squelch control
voltage is grounded by Q25. When no detected voltage is
supplied from D56, pin 3 on 1C7(g) is at a negative voltage, as
determined by R117. The output of IC7(g) is therefore
"LOW" and thus the squelch switching circuit (consisting of
Q74 and Q75) is turned OFF (squelch opens). Q24 receives
the output of IC7(g) through D57, turning ON and OFF the
RECEIVE INDICATOR.
When a detected voltage is supplied from D56 (no signal
being received), pin 3 on IC7(g) is greater than OV and the
output of IC7(g) is "HIGH". Q74 and Q75 then turn ON,
cutting the audio signal.
(b} S-METER SQUELCH
When operating in a mode other than FM, IC7(g) receives
S-METER voltage at pin 2 and reference voltage from the
{SQUELCH] CONTROL at pin 3. When the [SQUELCH]
CONTROL. is turned CCW to its maximum position, the
reference voltage is approximately —0.5V; when turned
maximum CW the reference voltage is approximately +2.5V.
When the S-METER voltage is lower than the reference
voltage, Q74 and Q75 are turned ON, closing the squelch.
4-2 TRANSMITTER CIRCUITS
4-2-1 MIC AMPLIFIER CIRCUIT
An audio signal from the MIC CONNECTOR is amplified at
Q2, the mic preamplifier circuit in the AF VR UNIT, and then
passes through the [MIC GAIN] CONTROL. The signal is
then applied to the mic amplifier circuit which consists of
Q34 and Q35, and passes through the [TONE] CONTROL
which consists of R154, R155, C86, and C87. Frequency
characteristics of the audio signal are controlled by R155.
The MOD terminal on the ACC CONNECTOR socket is
connected to the base of Q35 through R148 and C83.
1C-751A MIC CONNECTOR (Front View)
@ AF OUTPUT
@ GND (microphone
ground)
@® MIC INPUT
@® +8V DC OUTPUT @® GND (PTT ground)
@ FREQ UP/DOWN
@NC
Fig. 4
HM-36 SCHEMATIC DIAGRAM
Ri: 2kQ
tren an) R2: 4702
C1: 4700pF
C2: 4700pF
C3: 10uF
C4: 0.33uF
4-2-2 BALANCED MODULATOR CIRCUIT
When operating in SSB or AM mode, an audio signal from
the mic amplifier circuit is applied to balanced mixer IC9
where it is mixed with a BFO signal. A double sideband
(DSB) signal is then output from ICQ. In SSB mode, the bias
voltage of IC9 is adjusted by R158 and R161, reducing the
carrier signal. In AM mode, R159 and D75 add voltage to pin
5 on ICQ, disrupting the balance and generating an AM
signal.
4-2-3 SSB AND AM TRANSMITTER IF CIRCUITS
A DSB or AM signal generated at ICQ is applied to the 9MHz
filter as in the receiver circuits. 1n SSB mode, the DSB signal
passes through FI2, creating an SSB signal. In AM mode, an
AM signal passes through the circuit. After passing through
the filter, a signal is buffer-amplified at Q84 and fed into
mixer 1C10 for conversion into a 455kHz 2nd IF frequency
signal. This 2nd IF frequency signal is applied to Q38 directly
when the [COMP] SWITCH is OFF or indirectly through the
compressor circuit when the [COMP] SWITCH is ON.
A 455kHz filter, buffer amplifier Q42, and mixer 1C11 are
commonly used with the receive circuit, so 455kHz 2nd IF
frequency signals are re-converted to SMHz 3rd IF
frequency signals and are applied to Q1, the transmitter IF
amplifier.
4-2-4 COMPRESSOR CIRCUIT
The compressor circuit increases the average power needed
to limit amplitude when in SSB mode. Since amplitude is
limited with an IF frequency signal, signal distortion is slight.
SSB signals from IC10 are amplified at Q36 and are
amplitude limited by diode limiters D92 and D93. Parts of the
4—4