Icom IC-M302 Service Manual - page 11
4 - 4
4-3-4 VCO CIRCUIT (MAIN UNIT)
The VCO outputs from Q13, Q14 are buffer-amplified at the
buffer amplifiers (Q15 and Q16), and are then sent to the
Tx/Rx switch (D16 and D17). The receive LO signal is
applied to the 1st mixer circuit (Q2) through a low-pass filter,
and the transmit signal is applied to the pre-drive amplifier
(Q17). A portion of the VCO output is reapplied to the PLL IC
(IC8, pin 2) via the buffer amplifier (Q26).
4-4 DSC CIRCUITS
4-4-1 DSC ENCODE CIRCUIT
(LOGIC BOARD AND MAIN UNIT)
The DSC signal is created at the CPU (IC1, pin 9). The sig-
nal passes through the buffer amplifier (IC6A, pins 1, 3), and
is then applied to the D/A convertor (R176–R179) to convert
into the analog signal. The analog signal passes through the
low-pass filter (MAIN unit; IC2D, pins 12, 14), and is then
applied to the VCO circuit to modulate the DSC signal.
4-4-2 DSC DECODE CIRCUIT
(MAIN UNIT AND LOGIC BOARD)
The AF signal from the FM IF IC (IC1, pin 9) are filtered at
the low-pass filter (IC2A, pins 1, 3) to suppress unwanted
higher noise signals. The filtered signals are converted ana-
log signals into digital signals at the DSC decoder IC (IC3),
and are then applied to the CPU (LOGIC board; IC1, pin 22)
via the “DSCDEC” signal.
4-5 LOGIC CIRCUITS (LOGIC BOARD)
• CPU
IC1 is an 8 bit single chip micro-computer and contains LCD
driver, serial I/O, timer, A/D converter, programmable I/O,
ROM and RAM. The CPU controls to display characters on
the LCD too.
• SYSTEM CLOCK CIRCUIT
X1 is a ceramic oscillator and oscillate 7.9872 MHz system
clock for the CPU (IC1).
• RESET CIRCUIT
IC5 which is a reset IC outputs a reset signal (“HIGH” pulse
signal) to the CPU (IC1, pin 33) from pin 1 when the trans-
ceiver turns power ON.
• LOW BATTERY DETECTOR
“HVS” voltage is divided by R14 and R15, and is applied to
the low battery detector section in the CPU (IC1, pin 1).
• DIMMER CIRCUIT
CPU (IC1), Q2, Q3 and Q4 compose dimmer circuit. The cir-
cuit controls 4 steps LCD backlight (DS2–DS7).
• UART PORT
IC-M302 has the UART port which is used for cloning data
and NMEA interface. The signals pass through the photo
coupler circuit (IC2), and are then applied to the buffer
amplifier (IC3, pin 2). The amplified signals are applied to
the CPU (IC1, pin 20).
4-6 POWER SUPPLY CIRCUITS
LINE
HV
HVS
5V
T5
R5
R8
DESCRIPTION
The voltage from the connected DC power sup-
ply.
Same voltage as the HV line which is passed
through the [PWR] switch (VR-A board; R1).
Common 5 V converted from the HVS line at the
5V regulator circuit (MAIN unit; IC9).
Transmit 5 V controlled by the T5 control circuit
(MAIN unit; Q31, Q32) using the SEND signal
from CPU (LOGIC board; IC1). The controlled
voltage is applied to the transmitter circuit.
Receive 5 V controlled by the R5 control circuit
(MAIN unit; Q33, Q34) using the RCV signal
from CPU (LOGIC board; IC1). The controlled
voltage is applied to the receiver circuit.
Receive 8 V converted from the HVS and R5
lines at the R8 regulator circuit (MAIN unit; Q35,
Q36).