Icom IC-RX7, Service Manual Addendum

4 - 3

AF CIRCUITS

Other than WFM mode operation, and when the AF fi lter is 
activated, the demodulated AF signals are passed through 
the AF fi lter (Q20) and applied to Q18.

In WFM mode operation, and when the AF filter is 
inactivated, the demodulated AF signals are passed through 
Q23 and applied to Q18.

The amplifi ed AF signals from Q18 are applied to the tone 
controller (IC19). The tone controller adjusts the frequency 
characteristic of the AF signals. The output signals of tone 
controller are then applied to the electric VR IC (IC10) and 
level-adjusted.

AF AMP

The level-adjusted AF signals are power-amplified by the 
speaker AMP (IC9), then applied to the external/internal 
speaker.

CHARGING CONTROL CIRCUIT

IC18 is a charge control IC that contains battery connect 
detection, full charge detection, charging current control, 
temperature detection, etc. The remaining battery capacity 
and type of batter y is detected by the voltage to be 
applied to pin 2, and the IC interpret whether pre-charge is 
necessary or not.

The charging period is set by resisters connected to the pins 
2 and 6, and the temperature is ministered by the voltage to 
be applied to pin 8 with external thermostat.

While charging, pin 3 outputs “L” level to the CPU for charging 
indication on the LCD. Only when an external power supply 
is connected and chargeable batteries are detected through 
“BDET” terminal, the IC permits to start charging.
 
 

4-4 LOGIC UNIT

The LOGIC UNIT has CPU, D/A converter (DAC), AF circuits 
and power supply circuits. The CPU and DAC control the 
whole operation of the receiver (RX frequency setting, BPF 
tuning etc.), and the AF circuits filter, amplify and tone-
control the demodulated AF signals. The power supply 
circuits supply the voltage for each circuits.

CPU

IC17 is a 16 bits CPU that integrated ROM, RAM, the intrude 
controller, real time pulse unit, serial interface and A/D 
converter on the chip, and the CPU and external EEPROM 
(IC14 and IC15) compose the main control system of 
receiver.
 

RESET CIRCUIT

RESET CIRCUIT power-detects of ON/OFF with IC6, and 
resets the CPU when the receiver is turned ON/OFF.

CLOCK CIRCUIT

The clock frequency (around 12.288 MHz) of the CPU is 
oscillated by X1, C19 and 22. The clock frequency is able to 
be shifted to prevent spurious interference.

DAC

IC16 is a 3-ch 8-bit D/A converter. The DAC controls the LO 
frequencies, gain of RF AMP, tuning voltage, etc. according 
to the serial data from the CPU. 

DC UPCONVERTER

The clock signal from the CPU is upconverted to 12 V DC 
by the full-wave rectification circuit (IC11, D24 to 27). The 
upconverted voltage is passed through the ripple fi lter (Q24, 
C158, 195) then applied to the VP terminal of PLL IC as the 
charge pump voltage.

POWER SUPPLY CIRCUITS

IC3 is a 3.2 V 3-terminal regulator and supplies voltage to 
CPU, EEPROM and RESET IC. While the receiver’s power 
is ON, +3 V DC is provided by Q2.

IC4 is also 3.2 V 3-terminal regulator and supplies 3 V (as 
R3V) in power save mode.

BAR ANTENNA TUNING CIRCUIT

The internal bar antenna for AM broadcast band RX is tuned 
to the frequency to receive by varying the capacitance of 
D19 using the voltage from DC-DC upconverter via Q21 and 
D37.

DECODE CIRCUIT

The demodulated AF signals “DET” from each detector 
circuit are passed through the tone fi lter (1/2 of IC12) and 
applied to the CPU for tone signal interpretation.

Another 1/2 of IC12 composes the LPF for VSC function, 
and cut the AF line when unwanted continuous audible 
signals is out there.

The demodulated AF signals from detector circuit are also 
applied to the CPU for other tone signals interpretation (WX/
MSK) via some LPFs.