Components Description - Kenwood TM-281A Service Manual

■ D/A Converter
The D/A converter (IC705) is used to adjust the MO mod-
ulation, AF volume, TV voltage, FC reference voltage, and
PC POWER CONTROL voltage level.
Adjustment values are sent from the MCU as serial data.
The D/A converter has a resolution of 256 and the following
relationship is valid:
D/A output = (Vin – VDAref) / 256 x n + VDAref
Vin: Analog input
VDAref: D/A reference voltage (Approx. 2.2V)
n: Serial data value from the microprocessor (MCU)
Power Supply Circuit
When the power switch on the display unit is pressed,
the power port on the display unit which is connected to
port 17 (POWER), goes low, then port 82 (SBC) goes high,
Q713 turns on, SB SW (Q714) turns on and power (SB) is
supplied to the radio.
When the DC power is supplied to the radio, the volt-
age regulator IC (IC713) is supplied to the MCU VDD and
reset voltage detect IC (IC714). IC714 will generate a signal
(RESET) into the reset terminal of the MCU (IC707) to carry
out a power ON reset. If DC power is less than approxi-
mately 9.5V, the radio is unable to power on.
When the DC power voltage deceases from the normal
voltage, the INT voltage detector IC (IC712) will set to high
TX-RX UNIT (X57-8130-XX)
Ref. No. Use / Function
IC1 PLL IC
IC2 Divider
IC302 Power module
IC303 APC controller
IC501 FM IC
IC703 OP amplifi er
IC704 OP amplifi er
Digital
IC705
potentiometer
CIRCUIT DESCRIPTION

COMPONENTS DESCRIPTION

Operation / Condition / Compatibility
Frequency control
Frequency divider
TX fi nal amplifi er
TX power control
FM detector/ 2nd mixer/ RSSI/
Noise amplifi er
MIC amplifi er/ IDC
Splatter fi lter
Frequency adjustment
Modulation balance adjustment
AF level adjustment
Backlight control
Modulation limit adjustment
TX power/ BPF control
RX signal level control
on MCU port 18 (INT) if B line becomes less than approxi-
mately 9.5V. The MCU will then send the backup data to the
EEPROM (IC708) and enter stop mode.
This circuit has an overvoltage protection circuit. If a DC
voltage of 18V or higher is applied to the base of Q707, this
voltage turns Q707 on and will set to high on MCU port 18
(INT). The MCU will then send the backup data to the EE-
PROM (IC708) and enter stop mode. (See Fig. 16.)
SB
Q714
SW
Q713
SW
SBC
IC707
MCU
5M
POWER
IC714
RST
POWER
SW
Fig. 16 Power supply circuit
Ref. No. Use / Function
IC706 OP amplifi er
IC707 MCU
IC708 EEPROM
IC709 OP amplifi er
IC710 5V AVR
IC711 8V AVR
IC712 Voltage detection
IC713 5V AVR
IC714 Voltage detection
IC715 Audio amplifi er
IC901 LCD driver
Q1,2 Switching
Q3 RF amplifi er
TM-281A
B
D710 D712
Q707
SW
5M
IC713
AVR
IC712
INT
Operation / Condition / Compatibility
Buffer amplifi er/ SUM amplifi er/
DC amplifi er/ 1/2 VCC
Tuning data/ Memory data
Audio fi lter
5C
8C
MCU INT voltage detection
5M
MCU reset voltage detection
Audio fi nal amplifi er
Display control
Charge pump
PLL Fin
9