Icom IC-M73 User manual

S-14919XZ-C1
April 2013
VHF MARINE TRANSCEIVER

This service manual describes the latest technical information
for the IC-M73, IC-M73EURO VHF MARINE TRANSCEiVER,
at the time of publication.
NEVER connect the transceiver to an AC outlet or to a DC
power supply that uses more than the specified voltage. This
will ruin the transceiver.
DO NOT reverse the polarities of the power supply when con-
necting the transceiver.
DO NOT apply an RF signal of more than 20 dBm (100 mW) to
the antenna connector. This could damage the transceiver’s
front-end.
To upgrade quality, any electrical or mechanical parts
and internal circuits are subject to change without notice
or obligation.
MODEL VERSION AC ADAPTER FLEXIBLE
ANTENNA
IC-M73
USA BC-123SA
FA-S64V
EXP BC-123SE
USA-10 BC-123SA
EXP-10
BC-123SE
IC-M73EURO
EUR-01 FA-S59V
EUR
FA-S64V
UK –
FRG BC-123SE
HOL
AUS BC-123SV
EUR-11 BC-123SE FA-S59V
EUR-10
FA-S64V
UK-10 –
FRG-10 BC-123SE
HOL-10
AUS-10 BC-123SV
Be sure to include the following four points when ordering
replacement parts:
1. 10-digit Icom part number
2. Component name
3. Equipment model name and unit name
4. Quantity required
<ORDER EXAMPLE>
1110003491 S.IC TA31136FNG IC-M73 MAIN UNIT 5 pieces
8820001210 Screw 2438 screw IC-M73 Top cover 10 pieces
Addresses are provided on the inside back cover for your
convenience.
ORDERING PARTS
1. Make sure that the problem is internal before dis-assem-
bling the transceiver.
2. DO NOT open the transceiver until the transceiver is dis-
connected from its power source.
3. DO NOT force any of the variable components. Turn them
slowly and smoothly.
4. DO NOT short any circuits or electronic parts. An insulated
tuning tool MUST be used for all adjustments.
5. DO NOT keep power ON for a long time when the trans-
ceiver is defective.
6. DO NOT transmit power into a Standard Signal Generator
or a Sweep Generator.
7. ALWAYS connect a 30 dB to 40 dB attenuator between the
transceiver and a Deviation Meter or Spectrum Analyzer,
when using such test equipment.
8. READ the instructions of the test equipment thoroughly
before connecting it to the transceiver.
REPAIR NOTES
INTRODUCTION CAUTION
(IC-M73EURO)
Icom, Icom Inc. and the Icom logo are registered trademarks of Icom Incorporated (Japan) in Japan, the United States, the
United Kingdom, Germany, France, Spain, Russia and/or other countries.

TABLE OF CONTENTS
SECTION 1 SPECIFICATIONS
SECTION 2 INSIDE VIEWS
SECTION 3 DISASSEMBLY INSTRUCTION
SECTION 4 CIRCUIT DESCRIPITON
4-1 RECEIVER CIRCUITS...................................................................................... 4-1
4-2 TRANSMITTER CIRCUITS............................................................................... 4-2
4-3 FREQUENCY SYNTHESIZER CIRCUITS ....................................................... 4-3
4-4 POWER SUPPLY CIRCUITS............................................................................ 4-4
4-5 PORT ALLOCATIONS ...................................................................................... 4-4
SECTION 5 ADJUSTMENT PROCEDURES
5-1 PREPARATION ................................................................................................. 5-1
5-2 FREQUENCY ADJUSTMENT .......................................................................... 5-3
5-3 RECEIVE AND TRANSMIT ADJUSTMENT ..................................................... 5-4
SECTION 6 PARTS LIST
SECTION 7 MECHANICAL PARTS
SECTION 8 BOARD LAYOUTS
SECTION 9 BLOCK DIAGRAM
SECTION 10 VOLTAGE DIAGRAM

1 - 1
SECTION 1
.
SPECIFICATIONS
GENERAL
IC-M73 IC-M73EURO IC-M73EURO
(AUS version)
• Frequency coverage TX 156.025–157.425 MHz 156.000–161.450 MHz 156.025–157.425 MHz
RX 156.050–163.275 MHz 156.000–163.425 MHz 156.300–162.025 MHz
• Type of emission 16K0G3E
• Antenna impedance 50 Ω(nominal)
• Operating temperature range –20°C to +60°C
(–4°F to +140°F) –15°C to +55°C –10°C to +55°C
• Power supply requirement 7.4 V DC nominal (negative ground)
• Current drain
(approximately) RX 0.25 A (at maximum audio, using external 8 Ω speaker)
0.45 A (at maximum audio, using internal speaker)
TX
6.0 W 1.5 A –
5.0 W – 1.4 A
3.0 W 1.2 A
1.0 W 0.7 A
0.5 W – 0.6 A –
• Dimensions (projections not included) 52.5(W) × 125(H) × 30(D) mm; 2.0(W) × 4.9(H) × 1.2(D) inches
• Weight (approximately) 280 g; 9.9 oz (Including only BP-245N)
320 g; 11.3 oz (Including BP-245N, FA-S64V and MB-103)
TRANSMITTER
• Output power High 6 W 6 W (1 W for [FRG]) 5 W
Middle 3 W
Low 1 W 1 W (0.5 W for [FRG]) 1 W
• Modulation Variable reactance frequency modulation
• Maximum frequency deviation ±5 kHz
• Frequency error ±10 ppm ±1.5 kHz ±10 ppm
• Spurious emissions –68 dBc (typical) 0.25 µW
• Adjacent channel power 70 dB
• Audio harmonic distortion 10% (at 60% deviation)
• FM hum and noise 40 dB
• Audio frequency response +1 dB to –3 dB of 6 dB octave from 300 Hz to 3000 Hz
• Microphone impedance 2 kΩ
RECEIVER
• Intermediate frequency 1st 21.7 MHz 2nd 450 kHz
• Sensitivity –13 dBµ (0.22 µV)
(at 12 dB SINAD) typical
–4 dBµ emf (at 20 dB SINAD)
typical
–5 dBµ emf (at 20 dB
SINAD) typical
• Squelch sensitivity (at threshold) –9 dBµ (0.35 µV) typical –5 dBµ emf typical –6 dBµ emf typical
• Adjacent channel selectivity 70 dB typical 70 dB 65 dB
• Spurious response 70 dB typical 70 dB 65 dB
• Intermodulation rejection ratio 70 dB typical 68 dB 65 dB
• Hum and Noise 40 dB
• Audio frequency response +1 dB to –3 dB of –6 dB octave from 300 Hz to 3000 Hz
• Audio output power
(at 10% distortion with an 8 Ωload)
0.35 W typical (External) 0.2 W (External)
0.7 W typical (Internal) 0.7 W typical (Internal) at 1 kHz
• Output impedance (audio) 8 Ω
All stated specifications are subject to change without notice or obligation.

2 - 1
SECTION 2
.
INSIDE VIEWS
• MAIN UNIT
IF IC
(IC170)
2ND IF FILTER
(FI170)
EEPROM
(IC340)
RESET IC
(IC341)
BUFFER
(IC364)
BUFFER
(Q350)
AF MUTE
(IC430)
MIC AMP
(IC200)
FLASH ROM
(IC501)
AFV LINE CONTROL
(Q231)
AFV LINE CONTROL
(Q401)
AF PWR AMP
(IC280)
AF SW
(IC260)
AF PWR AMP
(IC401)
NOISE CANCELLER
PLAYBACK/RECORD IC
(IC500)
AFV LINE CONTROL
(Q230)
CPU 5V
(IC220)
CPU
(IC360)
BACKLIT LED DRIVER
(Q240)
CPU CLOCK
(X480)
AF MUTE
(IC504)
DISCRIMINATOR
(X170)
LPF
(IC432)
+5 V REGULATOR
(Q221)
+5 V REGULATOR
(Q225)

2 - 2
VCO
• RF UNIT
(TOP VIEW)
• RF UNIT
(BOTTOM VIEW)
TX MUTE SW
(Q351)
RF AMP
(Q90)
VCC LINE
CONTROLLER
(Q480)
1st MIXER
(Q150)
PLL IC
(IC1)
+3.3 V REGULATOR
(IC2)
BUFFER
(Q3)
+5 V REGULATOR
(Q222)
REFERENCE
FREQUENCY
OSCILLATOR
(X1)
APC AMP
(IC50)
1ST IF AMP
(Q151)
BPF
(FI150)
BPF
(FI151)

3 - 1
SECTION 3
.
DISASSEMBLY INSTRUCTION
1. Removing the Front panel
1) Remove 6 screws from the rear panel.
2) Disconnect the speaker cable from the MAIN UNIT
and remove the front pannel form rear. (The lock
plate is simultaneously come off.)
J251
MAIN UNIT
REAR PANEL
Screw×4
Screw×2
J251
MAIN UNIT
MAIN UNIT
CHASSIS
Screw×5
J251
J251
MAIN UNIT
SPEAKER
CABLE
LOCK PLATE
FRONT PANEL
CHASSIS
J251
MAIN UNIT
MAIN UNIT
RF
UNIT
J300
FLAT
CABLE
CHASSIS
2. Removing the MAIN UNIT
1) Remove 5 screws from the MAIN UNIT.
2) Disconnect 2 flat cables from the MAIN UNIT.
3) Remove the MAIN UNIT from the CHASSIS.
BE CAREFUL about the flat cable and connector when
separating the MAIN UNIT from the CHASSIS.

4 - 1
J251
MAIN UNIT
RF UNIT
UNSOLDER
Solder
remover
CHASSIS
J251
MAIN UNIT
CHASSIS
Screw×3
RF UNIT
3. Removing the RF UNIT
1) Unsolder total of 7 points.
2) Remove 3 screws from the RF UNIT.
3) Remove the RF UNIT from the CHASSIS.

4 - 1
SECTION 4
.
CIRCUIT DESCRIPTION
1st IF signal
to the FM IF IC
(MAIN UNIT; IC170, pin 16)
(Q151) 21.7 MHz
1st IF signal
Received signals
from the RF circuits
1st IF amp
1st mixer
(Q150)
signals from the VCO
• 1st IF CIRCUITS
FI150, FI151
LPF
BPF
2 BPF’s
Mixer
Quadrature
detector
1st IF signal from the IF amplifier (RF UNIT: Q151)
16
Noise
detector
R5V
X170
1110
IF IC
(IC170)
Filter
amp.
Limiter
amp.
Demodulated signal
to the AF circuits
• 2ND IF AND DEMODULATOR CIRCUITS
9
(IC190)
19
18
“NOISV” signal to the CPU (IC360, pin 32)
“RSSIV” signal to the CPU (IC360, pin 33)
1412
21.25 MHz
2
17
X1
21.25 MHz
8 7 35
FI170 LPF
BPF PLL IC
(IC1)
RF UNIT
SQLO
SQLI
2NDLO
D/A
converter
4-1 RECEIVE CIRCUIT
• ANTENNA SWITCH (RF UNIT)
The received signal from the antenna connector are passed
through the low-pass filter (LPF; L81, L82, C83−C86 and
C89) and the antenna switch (D90), which toggles the re-
ceive (RX) line and transmit (TX) line.
While transmitting, the voltage on the T5V line is applied to
D52 and D90, and these are ON. Thus the TX line is con-
nected to the antenna, and RX line is connected to the
ground to prevent transmit signal entering.
While receiving, no voltage is applied to D52 and D90, and
these are OFF. Thus the TX line is disconnected from the
antenna to prevent received signal entering, and RX line is
disconnected from the ground and L90, C90 and C91 com-
pose a two-staged LPF, which guides received signal to the
RX circuits.
The received signal is applied to the RF circuits.
• RF CIRCUITS (RF UNIT)
The RF circuit amplifies received signal within the frequency
coverage. The received signal is filtered at the bandpass fil-
ter (BPF) and amplified at the RF amplifier.
The received signal from the antenna switch is passed
through the two-staged tunable BPFs (D92, L92, C95, C97
and D93, L93, C99, C100) to filter out unwanted signal. The
filtered signal is applied to the RF amplifier (Q90). The am-
plified signal is then applied to the 1st mixer (Q150), through
another two-staged tunable BPF (D130, L96, C110, C112
and D131, L97, C114, C115).
• 1st IF CIRCUITS (RF UNIT)
The received signal is converted into the 1st IF signal, and
amplified at the 1st IF circuits.
The amplified received signal from the RF circuits are ap-
plied to the 1st mixer (Q150), and mixed with the 1st local
oscillator (LO) signal from the VCO (Q21, Q22, D20−D22),
resulting in the 21.7 MHz 1st IF signal.
The converted 1st IF signal is passed through two 1st IF
filters (FI150 and FI151) to filter out unwanted signal, and
then applied to the 1st IF amplifier (Q151). The amplified 1st
IF signal is applied to the IF IC (MAIN UNIT: IC170, pin 16).
• 2nd IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 1st IF signal is converted into the 2nd IF signal, and de-
modulated.
The 1st IF signal from the 1st IF circuits is applied to the 2nd
IF mixer in the IF IC (IC170) and mixed with the 21.25 MHz
2nd LO signal from the reference oscillator (RF UNIT: IC1,
X1), resulting in the 450 kHz 2nd IF signal.
The converted 2nd IF signal is output from pin 3, and passed
through the 2nd IF filter (FI170) to suppress sideband noise.
The filtered signal is amplified by the limiter amplifier (IC170,
pin 5) and FM-demodulated by the quadrature detector with
the discriminator (X170), and then output from pin 9. The de-
modulated AF signal is applied to the AF circuits.

4 - 2
Power
amp.
APC
amp.
Buffer
amp.
+
–
VCC
• APC CIRCUIT
To the anntena
T5V
PCON
LOW
TXMS
from the TX/RX switch (D50)
Q53
Pre-drive
amp.
Q50
IC50
3
1
4
Q351
Q54
LPF
ANT
SW
Q100
D91
D52
T5V
• AF CIRCUITS (MAIN UNIT)
The demodulated AF signal from the demodulator circuits is
amplified and filtered in AF amplifier circuits.
The demodulated AF signal from the IF IC (IC170, pin 9) is
passed through the AF mute switch (IC430, pins 10, 11),
LPF (IC432) and variable register (VR UNIT: R801) which
adjusts the AF signal in level.
The level-adjusted AF signal is passed through the de-em-
phasis circuit (R286, C280 and C285) to obtain −6 dB of au-
dio characteristic, and passed through the AF switch (IC260,
pins 1, 6), and then applied to the AF power amplifier (IC401,
pin 7) to obtain 0.6 W of output power. The power-amplified
AF signal is then output from pin 1, and applied to the inter-
nal speaker, through J251.
When an external speaker-microphone or headset is at-
tached to the [SP MIC] connector (MIC UNIT: J416), the
de-emphasized AF signal is passed through the AF switch
(IC260, pins 1, 7) and applied to the AF power amplifier
(IC280, pin 4) to obtain 0.2 W of output power. The power-
amplified AF signal is output from pin 10, and then applied
to the external speaker, through the [SP MIC] connector (MIC
UNIT: J416).
• SQUELCH CIRCUIT (MAIN UNIT)
The squelch mutes the AF output signal when no RF signal
is received. By detecting noise components in the demodu-
lated AF signal, the squelch circuit toggles the AF power
amplifier ON and OFF.
A portion of the demodulated AF signal from the IF IC
(IC170, pin 9) is applied to the D/A converter (MAIN UNIT:
IC190, pin 18) for level adjustment (squelch threshold ad-
justment). The level-adjusted AF signal is output from pin 19,
and passed through the noise filter (R174−R176, C179 and
C180). The only filtered noise signal is applied to the noise
amplifier in the IF IC (IC170, pins 7, 8).
The amplified noise components are converted into the
pulse-type signal at the noise detector section, and output
from pin 19 as the “NOISV1” signal. The signal is amplified
by the Amplifier (IC432) and then applied to the CPU (IC360,
pin 32), and the CPU outputs “AFVS” signal from pin 100,
depending on the “NOISV1” signal level, to the AF power
regulator (Q230, Q231 and Q401), which turns the AF power
amplifiers (IC280 and IC401) ON or OFF.
4-2 TRANSMIT CIRCUITS
• MICROPHONE AMPLIFIER CIRCUITS
(MAIN UNIT)
The AF signal from the microphone (MIC signal) is filtered
and level-adjusted at microphone amplifier circuits.
The AF signal from the microphone is passed through the
MIC mute switch (IC430A, pins 1, 2).
While an external microphone is connected to the [SP MIC]
connector (MIC UNIT: J416), the MIC mute switch shuts out
the MIC signal from the internal microphone (MC1).
The AF signal from the MIC mute switch (IC430A, pins 1, 2)
is passed through another AF mute switch (IC430B,
pins 3, 4), and applied to the microphone amplifier (IC200B,
pins 6, 7). The amplified MIC signal is passed through AF
mute switch (IC430, pins 8, 9), and is applied to the D/A con-
verter (IC190) and adjusted in level (=deviation adjustment).
The level-adjusted MIC (MOD) signal is passed through the
pre-emphasis circuit (R201 and C450) to obtain +3 dB of
characteristic. The pre-emphasized signal is then applied to
the limiter amplifier (IC200D, pins 13, 14), which limits the
amplitude of the MIC signal to prevent over deviation.
The amplitude-limited MIC signal is then passed through
the splatter filter (IC200A, pins 1, 3), which suppresses the
3 kHz and higher audio components, and then applied to the
modulation circuit (RF UNIT: D20).
• MODULATION CIRCUIT (RF UNIT)
The modulation circuit modulates the VCO oscillating signal
using the AF signal from the microphone.
The MIC signal from the microphone amplifier circuits is ap-
plied to the D20, and modulate the VCO oscillating signal by
changing the reactance of D20. The modulated VCO output
signal is buffer-amplified by Q23 and Q24, and then applied
to transmit amplifier circuits, through the TX/RX switch (D50).
• TRANSMIT AMPLIFIERS (RF UNIT)
The VCO output signal is amplified to the transmit output
power level by the transmit amplifiers.
The transmit signal from the TX/RX switch (D50) is ampli-
fied by the buffer amplifier (Q50), and then further amplified
by the pre-driver (Q53) and power amplifier (Q54) to the
transmit output level. The power-amplified transmit signal is
passed through the TX power detector of APC circuit (D91),
antenna switch (D52) and LPF (as a harmonic filter; L81,
L82, C83−C86 and C89), before being applied to the an-
tenna connector (CHASSIS: J1).