Icom Ic-r75 Communications Receiver Service Manual

SERVICE
MANUAL
COMMUNICATIONS RECEIVER 

This service manual describes the latest service information
for the 
IC-R75 at the time of publication.NEVERconnect the receiver to an AC outlet or to a DC
power supply that uses more than 16 V. Such a connection
could cause a fire hazard and/or electric.
DO NOTexpose the receiver to rain, snow or any liquids.
DO NOTreverse the polarities of the power supply when
connecting the receiver.
DO NOTapply an RF signal of more than 20 dBm (100mW)
to the antenna connector. This could damage the receivers
front...

TABLE OF CONTENTS
SECTION 1 SPECIFICATIONS
SECTION 2 INSIDE VIEWS
SECTION 3 CIRCUIT DESCRIPTION
3-1 RECEIVER CIRCUITS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-1
3-2 PLL CIRCUITS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-4
3-3 LOGIC CIRCUITS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5
3-4...

1 - 1
MGENERAL
• Frequency range :
*1Specifications guaranteed for 0.1 – 29.99 MHz and 50 – 54 MHz
*2Specifications guaranteed for 0.1 – 29.99 MHz
• Mode : SSB (LSB, USB), AM, FM, CW, RTTY, S-AM
• Receive system : Triple-conversion superheterodyne
• Intermediate frequencies : 
May differ according to selected IF filter.
• Sensitivity : 0.1 MHz – 1.799 MHz (Preamplifiers are OFF)
SSB, CW, RTTY less than 2.0 µV for 10 dB S/N
AM, S-AM less than 13.0 µV for 10 dB S/N
1.8 MHz – 27.99 MHz (The preamplifier 1...

2 - 1
SECTION 2 INSIDE VIEWS
2nd IF filter
(FI761: FL-272)
3rd LO circuit
3rd mixer circuit
(IC811)
BFO circuit 2nd IF circuit PLL IC (IC21)
Noise blaker circuitVCO circuit
BFO mixer circuit
(IC1101) Bandpass filter Fuse (3A)
MAIN UNITPLL UNIT
Back up battery
(BT 1521)
3rd IF filter
(FI851)
2nd mixer circuit
1st IF filter (FI701) 1st mixer circuit 

SECTION 3 CIRCUIT DESCRIPTION
3 - 1
3-1 RECEIVER CIRCUITS
3-1-1 RF SWITCHING CIRCUIT (MAIN UNIT)
The IC-R75 has two antenna connectors. RF signals enter
either the [50 ½ANT.] or [450 ½ANT.] connector.
RF signals from the [50 ½ANT.] connector are applied to the
antenna switching circuit (RL121), and then pass through the
low-pass filter (L131, L132, C131–C136).
RF signals from the [450 ½ANT.] connector are passed
through the L101 to exchange the impeadance value, and
are then applied to the antenna...

3 - 2
3-1-3 PRE-AMPLIFIER CIRCUIT (MAIN UNIT)
The pre-amplifier circuit uses low noise junction FETs (Q381,
Q382) or wideband amplifier (IC391) to provide gain over a
wide frequency range.
When the [P.AMP] switch is turned “PREAMP 1”, the signals
from the RF filter are amplified by the junction FETs pre-
amplifier circuit (Q381, Q382).
When the [P.AMP] switch is turned “PREAMP 2”, the signals
from the RF filter are amplified by the wideband pre-amplifi-
er circuit (IC391).
When the [P.AMP] switch is...

3 - 3
When the [FIL] switch is turned “2F3K”, the filter is selected
FI761 which covering the 2.4 kHz bandwith.
When the [FIL] switch is turned “2FOP”, the filter is selected
an optional filter.
When the [FIL] switch is turned “2FTH”, the signal from the
2nd IF amplifier bypass the crystal bandpass filters through
D771 and D773.
The filtered or bypassed signal is amplified at the buffer
amplifier (Q801) and applied to the 3rd mixer circuit (IC811).
3-1-6 NOISE BLANKER CIRCUITS (MAIN UNIT)
The IC-R75 uses...

3 - 4
The FM detector outputs “FMNL” signal from IC1001, pin 14
is applied to the CPU (LOGIC unit; IC101, pin 94) to control
the noise squelch level.
(3) AM DEMODULATOR CIRCUIT
The AM demodulater circuit (IC2001) has the envelope
detect function and the synchronous detect function.
An AM detector (IC2001) demodulates the AM signal into an
AF signal. The 3rd IF signal from the IF amplifier (Q911) is
amplified at the buffer amplifier (Q1031), and is then applied
to the AM demodulater circuit (IC2001)to...

3 - 5
The oscillated signal at the VCO1 and VCO2 circuit is ampli-
fied at the LO-amplifier (PLL unit; Q91), and passes through
the low-pass filter (PLL unit; Q92, D91, D92, L91, L93,
C96–C100, C102, C103) to supperss high harmonic compo-
nents. The low-pass filter controlls the cut-off frequency of
less than 29.999 MHz and more than 30.000 MHz by switch-
ing C102 and C103 “ON” and “OFF” respectively.
The filtered signal is applied to the LO-amplifier (MAIN unit;
Q411), and is then applied to the 1st...