Power Amplifier Circuit; Alc Circuit; Low-Pass Filters Used (Filter Board); Rf Alc Circuit (Filter Board) - Icom IC-M700PRO Servise Manual

3-2-6 POWER AMPLIFIER CIRCUIT

(PA150W BOARD)
This circuit provides a stable 150 W (at 13.6 V) of output
power. The RF signal from the MAIN unit is amplified at the
pre-driver (Q4008), drivers (Q4001, Q4002), and power
amplifiers (Q4003, Q4004).
The driver and power amplifiers form class AB push-pull
circuits. Bias voltage to these transistors is produced by
diodes (D4001–D4003) which have temperature junctions
with the transistors.
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; L4341) and transmit/receive switching relay
(FILTER board; RL4317) and is then applied to the antenna
connector.

• LOW-PASS FILTERS USED (FILTER BOARD)

Control Entrance
Frequency
signal Relay
(MHz)
L0 RL4301
0.5–1.999
2–2.999 L1 RL4303
L2 RL4305
3–4.999
5–6.999 L3 RL4307
L4 RL4309
7–9.999

• ALC CIRCUIT

Q19
IF ALC CIRCUIT
IC17a IC17b
High power set (R184)
Low power during tune (Q28)
20 W low power set (Q132)
Current APC
– 5 V
Control Entrance
Frequency
signal Relay
(MHz)
L5 RL4311
10–13.999
14–17.999
L6 RL4313
18–19.999
20–21.999
L7 RL4315
22–23.999
24–29.999
MAIN UNIT
D52 Q3, Q4
Q18 Q17
Q126
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).
The reference voltage for the comparator is set by R184.
The antenna tuning control voltage (TUN8) and low power
set signal (POC1) are also affected by the reference volt-
age to decrease the IF signal level.
The comparator output controls the gate bias of the IF
amplifier (Q19), so that the IF signal level is determined by
the reference voltage of the comparator (IC17a).

(2) RF ALC CIRCUIT (FILTER BOARD)

The RF output power level is detected at D4309 of the
power detector circuit (FILTER board; L4341, D4309,
D4310). The detected signal ("FOR" signal) is applied to
the RF ALC amplifier (MAIN unit; IC16a).
The amplified signal enters the transmit gain controller
(IC16b) which functions as an inversion amplifier. The gain
controller decrease the gain of the IF amplifier (MAIN unit;
Q2) to constant output power from differential amplifier
gains which are occurred by their frequency characteristics.
The bias voltage of the RF ALC amplifier (IC16a) is con-
trolled by the low power control signal (POC1 for 20 W,
POC2 for 60 W) and APC signal.
Q2
Q1
Q4008
8 V
IC16b
Current APC control (Q111)
Low power during tune (Q21, Q22)
3 - 4
PA UNIT FILTER UNIT
Q4001 Q4003
Q4002 Q4004
L4313
IC16a
Q23
60 W power set (Q24)
20 W power set (Q25)
RF ALC CIRCUIT