Icom IC-751A Service Manual page 24

4-2-12 MONITOR CIRCUIT
The transmitter monitor circuit is not simply a modulation
monitor: it also receives signals at the point where ALC is
applied, allowing accurate monitoring. After a portion of the
transmitting signal is amplified by Q49, itis detected by IC15
and fed into IC19(a), the monitor preamplifier. The monitor
circuit is turned ON and OFF by the power source of Q49.
4-2-13 MONITOR PREAMPLIFIER CIRCUIT
iC19(a) controls the monitor preamplifier circuit and
amplifies output from the monitor circuit, CW sidetone
circuit, and EX-310, an optional voice synthesizer unit.
Output from IC19(a) is applied to 1C18, the AF power
amplifier, through the [AF GAIN] CONTROL. Monitor gain
can be controlled by the [MONITOR] SWITCH on the top
panel of the transceiver.
4-2-14 ALC CIRCUIT
Detected forward voltage from the SWR detector circuit is
fed into the negative (—) input of IC5(c). The positive (+)
input of IC5(c) receives the reference voltage, so when the
forward voltage is higher than the reference voltage, output
from IC5(c) is at a negative voltage level. The resulting gain
of Q1 on the MAIN UNIT and Q11 on the RF UNIT will be
reduced. The reference voitage for IC5(c) is controlled by
the [RF POWER] CONTROL to keep peak power
continuously between 10 and 100W.
The time constant of the ALC is for fast attack except in AM
mode. Slow release occurs in SSB and CW modes and fast
release occurs in FM and RTTY modes. The fast release time
is fixed by C43 and R70. In FM or RTTY modes, R67
combines with Q18 and Q19 to create slow release times. In
AM mode, the time constant is made by Q16 and Q17 to
make an average ALC. If an external linear amplifier is used,
Q82 controls the ALC line and the input voltage level is O~
—eVv.
4-2-15 APC CIRCUIT
The APC circuit will operate when the antenna impedance is
high or when an excessive amount of current flows through
the final transistor. Output from IC6(e) (the SWR METER
amplifier) or IC6(f) (the IC METER circuit) is applied to the
negative (—) input of |C4(b). The positive (++) input receives
the reference voltage which is fixed by R75 and R76. The
resulting output from 1C4(d) controls the output power. The
input voltage is adjusted by R95 (SWR) and R99 (Transistor
current).
4-2-16 TRANSMITTER METER CIRCUITS
(a) Po METER:
Detected forward voltage from the SWR detector circuit
is amplified at !C5(d) and is then applied to the Po
METER. D51 and C50 are connected to the output of
IC5(d), creating peak conditions on the voltmeter.
(b) SWR METER:
Detected reflection voltage from the SWR detector
circuit is amplified at |C6(e) and is then applied to the
SWR METER.
(c) ALC METER:
This meter indicates the ALC level. Tne ALC circuit
begins to function when the RF output power reaches a
preset level. The detected ALC voltage is applied to
operational amplifier |C4(a) for amplification and the
ALC METER indicates the output voltage.
(d) Ic METER:
This meter indicates the collector current of the final
transistors in the PA UNIT. A very low resistance
resistor is connected in the 13.8V line of the PA UNIT in
series. This resistor creates voltage when the collector
current is flowing. The voltage between both leads of
the resistor is applied to a DC differential amplifier
IC6(f) for indicating the Ic METER current level.
(e) Vc METER:
This meter indicates the power supply voltage. The
power supply voltage is divided by resistors on the
FRONT UNIT and a portion of the divided voltage
creates the Vc indicator. R106 on the MAIN UNIT is for
voltage indicator calibrations.
(f) COMP METER:
This meter indicates the compression level when the
speech compressor is in use.
4-2-17 VOX AND ANTI-VOX CIRCUITS
A portion of output signals from the mic amplifier in the
FRONT UNIT passes through the [VOX] GAIN CONTROL
and is applied to the VOX circuit on the MAIN UNIT. Input
audio signals are applied to a comparator IC chip, I1C16(a),
through a variable attenuator for anti-vox control. This
attenuator consists of R340, R341, Q59, and amplifier Q60.
The comparator outputs 8V normally and Q61 is turned OFF.
When an audio signal is applied to the comparator and the
peak of the audio signal is over the threshold level, the
comparator output voltage decreases. Therefore, Q61 is
turned ON and C227, C228 and C229 are charged so that the
SEND line is grounded, creating the transmit mode via Q62
and Q63.
The charged voltage of Q61 is discharged by the [VOX
DELAY] CONTROL on the front panel through R355. The
transceiver then returns to receive mode. Delay time of the
VOX operation is adjusted by the [VOX DELAY] CONTROL.
in CW mode, a voltage from the keying circuit is charged into
C227 and C228 and the VOX circuit has the same function as
described above. However, the voltage is not charged in
C297 since D133 is connected to the collector of Q61. The
delay time in CW mode is therefore shorter than in the other
modes. Also, C228 is separated from the circuit so that the
[VOX DELAY] CONTROL is set for a minimum value and the
delay time is negligible when operating with CW full
break-in.
The ANTI-VOX circuit prevents the VOX circuit from
malfunctioning due to noise from the speaker. A portion of