Icom IC-725 Service Manual page 10

BFO FREQUENCY IN EACH MODE
MODE
FREQUENCY (MHz)
USB
9.0130
CW (Tx) 9.0106
LSB
9.0100
CW (Rx)
9.0098
AM NO OUTPUT
4·1·8 DEMODULATOR CIRCUITS
(MAIN UNIT)
The IC-725 has 2 detector circuits, a product detector and
a diode detector to demodulate the SSB, CW signal and
AM signal respectively.
In SSB or CW mode, the 2nd IF signal from the IF amplifier
(029) is mixed with the BFO signal at the product detector
(IC5) to demodulate the 2nd IF signal into an AF signal.
The detected signal passes through the AF input mode
selector switch (IC8).
In AM mode, the 2nd IF signal from 029 passed through
C121 is detected at D62 and passes through the AF input
mode selector switch (IC8).
4-1·9 AF INPUT MODE SELECTOR SWITCH
(MAIN UNIT)
The AF signal from a detector circuit or the optional AM
· FM UNIT is applied to the AF input mode selector switch
(IC8). IC8 consists of 4 analog switches and they are
selected with a mode signal from IC15 and the squelch
control signal. The AF signal is applied to the AF amp
circuit.
IC8 AF INPUT MODE SELECTOR SWITCH
MODE
ACTIVATING CONTROL
PIN NUMBERS
PIN NUMBER
USB, CW 2-1 13
AM 3-4 5
FM 10-11 12
ANY MODES
9-8 6
(for S-meter)
AGC, S-METER AND SQUELCH CIRCUIT
Squelch signal
av
S.meter
signal
FM S·meter
signal
SQLS
to [RX)
indicator
Fig. 3
4-3
4·1·10 AF AMP CIRCUIT (MAIN UNIT)
The AF signal from the AF input mode selector switch is
applied to the AF preamplifier (035, 036). The CW
sidetone signal is applied to 036.
The output from the AF preamplifier is applied to the [AF]
control (FRONT UNIT, R1 b) and the 2.8 kHz cut-off active
low-pass filter (037). The AF signal is power-amplified at
IC9 to drive the speaker.
4·1·11 AGC AND S·METER CIRCUIT
(MAIN UNIT)
The receiver gain is determined by the voltage on the AGC
line (030, collector). When strong signals are received,
the AGC circuit decreases the voltage on this line.
The IF signal from the IF amplifier (029) passes through
C117, is detected at D59 and 060, and applied to the base
of 030. A time constant (C113, R120) is connected to
the AGC line that determines the AGC release time.
The time constant is controlled by the [AGC] switch. When
the [AGC] switch is pushed OUT, C112 and R119 are
connected in parallel with the AGC line to obtain a slow
AGC release time.
The AGC bias voltage is applied to the differential amplifier
(IC4, pin 6) where the difference between the bias and
reference voltages is detected. The resulting S-meter
signal passes through the meter switching circuit (IC8)
and is then applied to the meter on the front panel. The
reference voltage is adjusted with R116. IC8 pins 8 and
9 are connected inside the IC in receiving.
The FM S-meter signal from the optional AM · FM UNIT is
applied to the meter switching circuit (IC8) via 057. The
signal is also applied to the squelch circuit (IC4 pin 2).