Icom IC-M710RT, Service Manual

Page: 9 / 73

3 - 4
(3) A1A AND FSK MODES
The CW8 or FSK8 voltage are applied to the balanced mod-
ulator (IC9, pin 4) to upset the balance and create a carrier
signal.
In A1A mode, the CW keying circuit (IC18a) controls the bias
voltage of the IF amplifiers (Q18, Q19) and T/R switching
diode (D19) to switch the carrier transmission.
In FSK mode, BFO frequency is shifted in the PLL unit to
create the mark and space frequencies.
3-2-3 1ST MIXER CIRCUIT (MAIN UNIT)
The amplified signal from the IF amplifier (Q17) is mixed
with a 60 MHz LO signal at the 1st mixer circuit (D52) to pro-
duce a 69.0115 MHz IF signal. The mixer is commonly used
with the receiver 2nd mixer.
The 69.0115 MHz IF signal passes through the filter (FI1b)
and is then applied to the 2nd mixer circuit.
3-2-4 2ND MIXER CIRCUIT (MAIN UNIT)
The filtered signal is mixed with a PLL output frequency
(1LO: 69.5155–99.0155 MHz) at the 2nd mixer circuit (Q3,
Q4) to produce an RF signal which is the same frequency as
the displayed one.
3-2-5 RF FILTER CIRCUIT (MAIN UNIT)
The RF signal passes through the low-pass filter (L55, L56,
C89–C93, C620, C628) and is then amplified at the RF
amplifier (Q2).
The amplified signal is applied to one of nine RF filters.
These RF filters are commonly used with the receiver circuit
which consists of eight high-pass filters and one low-pass fil-
ter. The filtered signal is amplified at the RF amplifier (Q1)
and is then applied to the PA150W board via J1.
3-2-6 POWER AMPLIFIER CIRCUIT
(PA150W BOARD)
This circuit provides a stable 150 W (at 13.6 V DC) of output
power. The RF signal from the MAIN unit is amplified at the
pre-driver (Q8), driver (Q1, Q2), and power amplifier (Q3,
Q4).
The driver and power amplifiers form class AB push-pull cir-
cuits. Bias voltage to these transistors is produced by diodes
(D1–D3) which have temperature junctions with the transis-
tors.
The amplified signal is then applied to one of eight low-pass
filters to suppress high harmonic components. The filtered
signal passes through the power detector circuit (FILTER
board; L41) and transmit/receive switching relay (FILTER
board; RL17) and is then applied to the antenna connector.
• LOW-PASS FILTERS USED (FILTER BOAED)
3-2-7 ALC CIRCUIT
The transceiver has two ALC (Auto Level Control) loops for
constant output power over all marine bands and for high
power setting.
(1) IF ALC CIRCUIT (MAIN UNIT)
A portion of the IF signals from the IF amplifier (Q17) is
applied to the IF ALC circuit. The signal is amplified at Q126
and then detected at the ALC detector (D46). The detected
signal is amplified at the ALC amplifier (IC17b) and is then
applied to the comparator (IC17a).
Q8 Q1, Q2 Q3, Q4
D9 D10
REF FOR
L41
Q3, Q4
Q19
Q126
IC17b
IC17a
Q18 Q17
D52
IF ALC CIRCUIT
Q2
Q1
MAIN UNIT PA BOARD FILTER BOARD
Current APC control (Q111)
Low power during tune (Q21, Q22)
High power set (R184)
Low power during tune (Q28)
20 W low power set (Q132)
8 V
IC16b
IC16a
–
+
–
+
Q23
RF ALC CIRCUIT
–5 V
Current APC
–
+
• ALC CIRCUIT
Pre-driver Driver PA
Frequency
(MHz)
0.5–1.999
2–2.999
3–4.999
5–6.999
7–9.999
Frequency
(MHz)
10–13.999
14–17.999
18–19.999
20–21.999
22–23.999
24–29.999
Control
signal
L5
L6
L7
Control
signal
L0
L1
L2
L3
L4
Entrance
coil
RL1
RL3
RL5
RL7
RL9
Entrance
coil
RL11
RL13
RL15