Power Supply Circuits; Control Unit Voltage Line - Icom IC-2720H Service Manual

(1) NORMAL OSCILLATING SIGNAL CIRCUIT
The signal from the VCO swtich (D7) passes through the
attenuator (R33, R37, R38, C41) and low-pass filter (L5, L9,
C45, C47, C53, C63, C64), and then applied to the VCO
swtich (D10) which is controlled by the "L_VR5" signal.
(2) TWICE OSCILLATING SIGNAL CIRCUIT
The signal from the VCO switch (D9) passes through the
high-pass (L6, C46, C48, C49), low-pass (L8, C52, C54,
C57) and high-pass (L11, C58, C65) filters to obtain twice
oscillating signal. The signals is applied to the VCO switch
(D11) which is controlled by the "L_UR5" signal.
(3) HARF OSCILLATING SIGNAL CIRCUIT
The signal from the VCO switch (D15) is applied to the pre-
scaler circuit (IC3, pin 2) to divide harf oscillating signal. The
divided signal is applied to the VCO switch (D16).
The reglator circuit provides the pre-scaler's power supply.
The circuit is controlled by the "L_LO_SW" signal.
The signal from the each VCO switch is applied to the 1st
mixer circuit for right side display (MAIN unit; IC1005, pin 4)
as the 1st LO signal.
A portion of the signal from the buffer amplifier (Q7) is ampli-
fied at the buffer amplifier (Q1), and is then fed back to the
PLL IC (IC1, pin 8) as the comparison signal.
• PLL CIRCUIT FOR LEFT SIDE DISPLAY
VCO for
left side display
Loop
Q2, Q3,
filter
D2
15
1
X1
12.8 MHz
VCO UNIT
Buff.
Q7
Buff.
Q6,
D3—D5
Buff.
IC1 (PLL IC for left side display)
Phase Programmable
detector divider
Programmable
Shift register
reference divider

4-4 POWER SUPPLY CIRCUITS

4-4-1 CONTROL UNIT VOLTAGE LINE

Line
The 13.8V external DC power from the power
connector (MAIN unit; J2000). The voltage is
HV
supplied
(DS1–DS12), etc.
Common 8 V converted from the HV line at the
+8 regulator circuit (Q1, Q3, D1). The output
voltage is applied to the microphone amplifier
8V
regulator circuit (Q16), key back light circuit
(DS13–DS18).
Common 5 V converted from the HV line by the
+5 regulator circuit (CONTROL unit; IC3). The
output voltage is applied to the buffer amplifier
CPU5
(CONTROL unit; Q2) and reset circuit (CON-
TROL unit; IC2), control unit CPU (IC4) and PTT
detector (Q5, D3, D4).
D12, D13
Q8
VCO
SW
D12, D13
VCO
Q1
SW
8
Prescaler
9
PLLCK
10
PLLDATA
4 - 8
Description
to the LCD back
D77, D78
to TX amplifier
TX
LPF ATT
circuit
SW
from the PLL circuit
for right side display
1st LO signal to the 1st mixer
circuit for right side display
MAIN UNIT
Q1009,
Q1012
38.4 MHz 2nd LO signal
to the ...FM IF IC for left side display
3
(IC1001, pin 2)
light circuit