Icom IC-2720 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.
4 - 8
Shift register
Prescaler
Phase
detector
Loop
filter
Programmable
divider
Programmable
reference divider
X1
12.8 MHz
3
38.4 MHz 2nd LO signal
to the …FM IF IC for left side display
(IC1001, pin 2)
Q1009,
Q1012
Q6,
D3—D5
VCO for
left side display
Buff.
Q1
Q7
Buff.
Q8
VCO UNIT
MAIN UNIT
D12, D13
Q2, Q3,
D2
9
10
PLLCK
IC1 (PLL IC for left side display)
PLLDATA
1
15
8
LPF ATT
Buff.
VCO
SW
D77, D78
to TX amplifier
circuit
from the PLL circuit
for right side display
1st LO signal to the 1st mixer
circuit for right side display
TX
SW
D12, D13
VCO
SW
• PLL CIRCUIT FOR LEFT SIDE DISPLAY
Description
The 13.8V external DC power from the power
connector (MAIN unit; J2000). The voltage is
supplied to the LCD back light circuit
(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
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
(CONTROL unit; Q2) and reset circuit (CON-
TROL unit; IC2), control unit CPU (IC4) and PTT
detector (Q5, D3, D4).
Line
HV
8V
CPU5
4-4 POWER SUPPLY CIRCUITS
4-4-1 CONTROL UNIT VOLTAGE LINE

Icom IC-2720 Specifications

General

Brand	Icom
Model	IC-2720
Category	Radio
Language	English