Icom IC-7400 Service Manual page 13

The operating (transmitting) frequency is produced at the 1st
IF mixer circuit (RF unit; D271) by mixing the 1st IF and 1st
LO signals. The mixed signal is then applied to the RF cir-
cuit.
4-2-6 RF CIRCUIT (RF AND PA UNITS)
The RF circuit amplifies operating (transmitting) frequency
to obtain 100 W of RF output.
The signal from the 1st IF mixer is passed through one of the
low-pass or bndpass filters (Refer to page 4-1 for bandpass
filters used) and high-pass filters after passing through the
low-pass filter (RF unit; L285, L286, C286–C289, C291).
The filtered RF signal is amplified at the wide-band YGR
amplifier (RF unit; IC151), and is then applied to the PA unit
via J151.
The signal applied from the RF unit is amplified at the 2 pre-
drive (PA unit; Q1, Q2) and drive (DRV board; Q1, Q2) am-
plifiers. These amplifiers cover all HF, 50 MHz and 144 MHz
band amplifying operation.
When operating on the HF or 50 MHz band, the signal from
the drive amplifier is power amplified at the HF/50 MHz
power amplifier (PA unit; Q5, Q6) via the band switching relay
(PA unit; RL2). However, the 144 MHz signal is power am-
plified at the 144 MHz power amplifier (PA unit; Q201, Q202)
to obtain a stable 100 W of RF output power.
The amplified signal is applied to the desired antenna con-
nector via one of 8 low-pass filters.
4-2-7 ALC CIRCUIT (MAIN UNIT)
The ALC (Automatic Level Control) circuit controls the gain of
IF amplifiers in order for the transceiver to output a constant
RF power set by the [RF PWR] control even when the sup-
plied voltage shifts, etc.
The RF power level is detected at the SWR detector circuit
(HF/50 MHz bands: CTRL unit; D2, 144 MHz band: PA unit;
D602) to be converted into DC voltage and applied to the
MAIN unit as the FOR (HF/50 MHz) or VFOR (144 MHz) sig-
nal.
The FOR or VFOR signal is applied to the comparator
(IC300, pin 6). The POCV signal, controlled by the [RF PWR]
control via the I/O expander (IC502, pin 4), is also applied to
the other input (pin 5) for reference. The compared signal is
output from pin 7 and applied to the IF amplifiers in the MAIN
(Q330) and RF (Q271) units to control amplifying gain.
• DSP receiver circuit
IC601b/a
AF
DTAF
Differential
signal
converter
MAIN unit DSP unit
DSP IC
IC501 CODEC IC
A/D Level
converter converter
When the FOR or VFOR signal exceeds the POCV voltage,
ALC bias voltage from the comparator controls the IF ampli-
fiers. This adjusts the output power to a specified level from
the [RF PWR] control until the FOR (VFOR) and POCV volt-
ages are equalized.
In AM mode, the comparator operates as an averaging ALC
amplifier. Q304 turns ON and the POCV voltage is shifted for
40 W AM output power (maximum) through R320.
The ALC bias voltage is also applied to the ALC meter am-
plifier (IC300, pin 2) to obtain an ALC meter signal (ALCL).
The amplified signal is passed through the analog switch (IC
1900, pins 5, 4) and applied to the main CPU (IC1500, pin
107) to drive the S/RF meter via the sub CPU (IC1001) on
the DISPLAY board.
An external ALC input from the [ALC] jack or [ACC] sockets
is applied to the buffer amplifier (Q307). External ALC oper-
ation is identical to that of the internal ALC.
The FOR or VFOR signal is also applied to the power meter
amplifier (IC360, pin 3). The amplified signal is applied to the
main CPU (IC1500) after being switched by the analog
switch (IC1900, pins 12, 14) as an FORL signal to drive the
S/RF meter via the sub CPU (DISPALY board; IC1001).
4-2-8 APC CIRCUIT (MAIN UNIT)
The APC (Automatic Power Control) circuit protects the
power amplifiers on the PA unit from high SWR and exces-
sive current.
The reflected wave signal appears and increases when the
connected antenna is mismatched to 50 Ω. The SWR detec-
tor circuit (HF/50 MHz bands; CTRL unit; D1, 144 MHz band;
PA unit; D603) detects the reflected signal, and applies it to
the APC circuit (IC300, pin 9) as a REF (HF/50 MHz) or
VREF (144 MHz) signal.
When the REF or VREF signal level increases, the APC cir-
cuit decreases the ALC voltage to activate the APC.
For the current APC, the power transistor current is obtained
by detecting the voltages (ICH and ICL) which appear at both
terminals of the current detector (PA unit; R28). The detected
voltages are applied to the differential amplifier (IC300, pins
12, 13). When the current of transistors is increased, the am-
plifier controls the ALC line to prevent excessive current flow.
A portion of the REF or VREF signal is applied to the SWR
meter amplifier (IC360, pin 5). The amplified signal is applied
to the main CPU (IC1500) after being switched by the analog
switch (IC1900, pins 2, 15) as an REFL signal to drive the
SWR meter.
IC301
D/A Level
converter converter
4 - 5
IC701b
DTIF
Aactive
36 kHz
filter
IF signal
MAIN unit