Icom IC-7800, Service Manual Addendum

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3 - 11
• SCOPE UNIT SYSTEM CLOCK CIRCUIT
The 40 MHz reference signal is amplified at the buffer ampli-
fier (Q601) via the “40M” line, and then passes through the
low-pass filter (L602, L603, C609–C613). The filtered signal
is applied to the buffer amplifier (SCOPE unit; Q701) as the
40 MHz system clock via the J601.
3-4 ANTENNA TUNER CIRCUITS
3-4-1 MATCHING CIRCUIT (NETWORK BOARD)
The matching circuit is a T-network. Using 2 tuning motors,
the matching circuit obtains rapid overall tuning speed.
Using relays (RL50, RL60, RL70, RL80, RL90, RL100,
RL110, RL120), the relay control signals from the antenna
tuner CPU (CTRL board; IC1) via the buffer amplifier
(IC402) ground one of the taps of L50, L60, L70, L80, L90,
L100, L110, L120. After selecting the coils, 2 motors
(TUNER unit; MF1, MF2) adjust C1 and C2 using the anten-
na tuner CPU (CTRL board; IC1) and the motor controller
(CTRL board; Q3–Q10, D3–D6) to obtain a low SWR
(Standing Wave Ratio).
3-4-2 DETECTOR CIRCUIT (CTRL BOARD)
(1) SWR DETECTOR
Forward and reflected power are picked up by a current
transformer (L101), detected by D101 and D102, and then
amplified at IC101, pin 3 and pin 5, respectively. The ampli-
fied voltages are applied to the antenna tuner CPU (IC1,
pins 2, 3) via “AN0” and “AN1” lines. The tuner CPU detects
the SWR.
(2) REACTANCE COMPONENTS DETECTOR
Reactance components are picked up by comparing the
phases of the RF current and RF voltage. The RF current is
detected by L151 and R151–R154 and buffer-amplified at
IC151, pin 13 and IC154, pin 1 and then applied to the
phase comparator (IC156, pin 1). RF voltages are detected
by C151–C153, C158 and then applied to the phase com-
parator (IC156, pin 13) after being amplified at the buffer
amplifiers (IC151, IC153). The output signal from the phase
comparator (IC156, pin 6 for RF current, IC156 pin 7 for RF
voltage) is rectified at D154 and D153 for conversion into
DC voltage. The rectified voltage signals are combined, then
amplified at the differential amplifier (IC201, pin 2), then
applied to the antenna tuner CPU (IC1, pin 64) via the “AN3”
line.
A C-MOS IC is used for the buffer amplifier (IC151) to
improve functionable sensitivity; the differential amplifier
(IC201) is very responsive even with a low signal level input.
Together, these ensure quick and stable signal detection
even at low RF signal level input.
(3) RESISTANCE COMPONENTS DETECTOR
Resistance components are picked up by L201, and detect-
ed by D201, D202 and Q201. The detected resistance com-
ponents are amplified at the differential amplifier (IC201, pin
6), and then applied to the antenna tuner CPU (IC1, pin 1)
via the “AN2” line.
3-4-3 MOTOR CONTROL CIRCUIT
(CTRL BOARD AND TUNER UNIT)
The control circuit of the internal antenna tuner consists of
the CPU, EEPROM (Electronically-Erasable Programmable
Read Only Memory), tuning motors and tuning relays.
(1) CPU AND EEPROM (CTRL BOARD)
The antenna tuner CPU (IC1) controls the tuning motors via
the motor controller (Q3–Q10, D3–D6) and tuning relays,
and memorizes the best preset position in 100 kHz steps.
The memory contents are stored in the EEPROM (IC2) with-
out a backup battery.
(2) TUNING MOTORS (CTRL BOARD)
A motor controller (Q3–Q10 D3–D6) rotates the tuning
motors (TUNER unit; MF1, MF2) to obtain a low SWR.
(3) TUNING RELAYS (TUNER UNIT)
According to the operating frequency band and antenna
condition, tuning relays select the capacitors and coils.
3-4-4 ANTENNA TUNER CPU PORT ALLOCATION
(CTRL BOARD; IC1)
1
2
13
15
17
21
22, 23
26
27–32
34–40
41–48
64
Input port for the resistance com-
ponents detection voltage.
Input port for the reflected RF
power voltage.
Outputs tuner data signal.
Input port for the serial signal.
Input port for [TUNER] ON/OFF
signal.
Input port for the TX/RX switching
signal.
Input port for the antenna tunner
CPU system clock.
Outputs the coil selection signal.
High : While 46–60 MHz band is
displayed.
Output the coil selection signal.
Output the capacitor selection sig-
nal.
Output pulse-type control signals
for the tuning motors (MF1, MF2).
Input port for the reactance compo-
nents detection voltage.
AN2
AN1
IKEY
IKEYB
THRU
TRC
#CL1, CL2
VHF
L24M, L18M,
L14M, L10M,
L7M, L3.5M
CO3, CO2,
CO1, CI3,
CI2, CI1
AZ, AY, AX,
AW, PZ, PY,
PX, PW
AN3
Pin Port
Description
number name