Icom IC-725 Service Manual page 11

4·1·12 SQUELCH CIRCUIT (MAIN UNIT)
The squelch circuit mutes the audio output when the S-
meter signal is lower than the [SOL] control setting level.
The S-meter signal from IC4 pin 7 is applied to the
comparator (IC4 pin 2) to be compared to a threshold
level controlled by the [SOL] control. The squelch control
signal is applied to control terminals of the AF input mode
selector switch (IC8).
When the S-meter signal is lower than the threshold level,
the comparator turns "HIGH" and then 032 turns OFF to
deactivate the AF input mode selector switch. This
signal is applied to 034, turning OFF the [RX] indicator
and is also applied to the [MIC] connector pin 4.
4·2 TRANSMITTER CIRCUITS
4-2·1 MIC AMPLIFIER (MAIN UNln
Audio signals from the [MIC] connector are applied to the
[MIC] control and amplified at the mic amplifier (045).
External modulation input from the [ACC(1)] socket pin 4
is also applied to 045 via R255.
The AF signals from 045 or CW keying signal is applied
to the balanced modulator (IC6). 044 cuts the signals
from 045 in CW or receiving.
4-2·2 BALANCED MODULATOR (MAIN UNln
Output signals from the mic amplifier or CW keying signal
are applied to the balanced modulator circuit (IC6) to be
converted to a 9 MHz IF signal using a BFO signal. The
BFO signal, buffer-amplified at 042, is applied to IC6 pin 7
as a carrier signal. IC6 outputs a double sideband signal
and passes through a 9 MHz filter to create an SSB
signal.
R177 and R179 adjust the balance level of IC6 for
maximum carrier suppression. In CW mode, the CW
keying signal upsets the balance to create a carrier signal.
4·2·3 IF CIRCUITS (MAIN UNln
The 9 MHz IF signal passes through one of the three
9 MHz filters where unwanted sideband or out-of-band
signals are removed. The filters are selected with mode
selecting signals and the "T8" voltage line. The optional
CW narrow filter is not used in transmitting.
The resulting signal is amplified at 022, and is then mixed
with the 2nd LO signal to be converted to a 70.45 MHz IF
signal at IC1. IC1 is used in receiving and transmitting.
The FM signal from the optional AM · FM UNIT is amplified
at 022 and is then applied to IC1.
The 70.45 MHz IF signal is amplified at the IF amplifier
(07) and is then converted to the displayed frequency at
the balanced mixer (02, 03).
4-4
The gates of the IF amplifiers (07, 022) are controlled
by ALC bias voltage from the ALC circuit. R89, con-
nected to the gate of 022, improves the temperature
characteristics of the transmitter gain. R85 adjusts the
transmitter gain.
4·2·4 RF CIRCUITS (MAIN AND PA UNITS)
The converted signal from 02 and 03 is applied to the
bandpass filter where the unwanted LO signal emission is
reduced. The converted signal is amplified at 01. and
is then applied to the PA UNIT via J11.
Incoming signals from the MAIN UNIT are amplified at the
predrive amplifier (01), drive amplifier (02, 03) and power
amplifier (05, 06) to obtain stable 100 W RF output
power. The predrive amplifier is a class A amplifier with a
Vee of 13.8 V. The drive and power amplifiers are class
AB push-pull amplifiers with a Vee of 13.8 V. A stable bias
voltage is applied to these amplifiers. 01 controls a bias
voltage to the drive amplifier. 04, 02 and 03 supply a
bias voltage to the power amplifier.
A 0.012
n
resistor (R26), inserted in the 13.8 V Vee line, is
provided for the le APC circuit. A voltage generated at
both terminals of R26 is applied to the MAIN UNIT via
the "ICH" and "ICL" signal line.
Thermal switch S1 and thermistor R32 detect the
temperature of 06 and 05 respectively, and control the
cooling fan speed.
TEMPERATURE °C (°F)
Below50 50-90
(122) (122-194)
THERMAL SWITCH (S1) OFF OFF
RESISTANCE OF R32 HIGH LOW
COOLING
RECEIVE STOP LOW
FAN SPEED
TRANSMIT LOW
COOLING FAN CONTROL CIRCUIT
THERMAL SWITCH
S1
THERMISTOR
R32
Fig. 4
D6
Above90
(194)
ON
LOW
HIGH
HIGH
PATS
(8 Vin Tx)