Icom IC-M21, IC-M2A service manual

SERVICE
MANUAL

VHF MARINE TRANSCEIVER

INTRODUCTION

DANGER

ORDERING PARTS

REPAIR NOTES

This service manual describes the latest service information
for the

IC-M2A/IC-M21 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 10 V. This will ruin the
transceiver.

DO NOT expose the transceiver to rain, snow or any liquids.
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 (100mW)

to the antenna connector. This could damage the transceiv­er’s front end.

Be sure to include the following four points when ordering
replacement parts:

1. 10-digit order numbers

2. Component part number and name

3. Equipment model name and unit name

4. Quantity required
<SAMPLE ORDER>

1130007020 S.IC TC7S66FU IC-M2A MAIN UNIT 1 piece
8810004860 Screw PH M 2 x 6 ZK IC-M21 Chassis 10 pieces

Addresses are provided on the inside back cover for your
convenience.

1. Make sure a problem is internal before disassembling the transceiver.

2. DO NOT open the transceiver until the transceiver is disconnected 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 turning tool MUST be used for all adjustments.

5. DO NOT keep power ON for a long time when the transceiver is defective.

6. DO NOT transmit power into a signal generator or a sweep generator.

7. ALWAYS connect a 40 dB to 50 dB attenuator between the transceiver and a deviation meter or spectrum analyzer when
using such test equipment.

8. READ the instructions of test equipment thoroughly before connecting equipment to the transceiver.

To upgrade quality, all electrical or mechanical parts and
internal circuits are subject to change without notice or oblig­ation.

IC-M2A IC-M21

Model

IC-M2A

IC-M21

Version

U.S.A.

S.E.Asia

Europe

United Kingdom

Germany

Symbol

[USA]
[SEA]
[EUR]

[UK]

[FRG]

AC adapter

BC-147A
BC-147E

BC-147E

TX high power

5W

1W

TABLE OF CONTENTS

SECTION 1 SPECIFICATIONS

SECTION 2 INSIDE VIEWS

SECTION 3 DISASSEMBLY INSTRUCTIONS

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-1

4-2 TRANSMITTER CIRCUITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-2

4-3 PLL CIRCUIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-3

4-4 POWER SUPPLY CIRCUITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-3

4-5 CPU PORT ALLOCATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-4

SECTION 5 ADJUSTMENT PROCEDURES

5-1 PREPARATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-1

5-2 PLL AND TRANSMITTER ADJUSTMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-4

5-3 RECEIVER ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-5

SECTION 6 PARTS LIST

SECTION 7 MECHANICAL PARTS AND DISASSEMBLY

SECTION 8 SEMI-CONDUCTOR INFORMATION

SECTION 9 BOARD LAYOUTS

9-1 CHARGE UNIT (BC-150) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1

9-2 MAIN UNIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-2

9-3 VR UNIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-4

SECTION 10 BLOCK DIAGRAM

SECTION 11 VOLTAGE DIAGRAM

11-1 MAIN UNIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1

11-2 VR UNIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2

1 - 1

All stated specifications are subject to change without notice or obligation.

Frequencycoverage
Mode

Power supply requirement
Usable temperature
Frequency stability

at 5 W
at 3 W
at 1 W
at 0.5 W
at maximum audio

at stand-by
Antenna impedance
Dimensions

(Projections not included)
Weight

(with antenna and BP-224)

RF output power (at 7.2 V DC)
Modulation system

Max. frequency deviation
Frequency error
Spurious emissions
Adjacent channel power
Residual modulation
Audio harmonic distortion
Audio frequency response
Microphone impedance
Receive system
Intermediate frequencies
Sensitivity
Squelch sensitivity
Adjacent channel selectivity
Spurious response
Intermodulation
Hum and noise

Audio output power ( at 7.4 V)
Audio frequency response

IC-M2A IC-M21

RECEIVER TRANSMITTER GENERAL

Tx: 156.025 MHz–157.425 MHz Tx: 156.000 MHz–161.450 MHz
Rx: 156.025 MHz–163.275 MHz Rx: 156.000 MHz–163.425 MHz

16K0G3E (FM)

BP-224 (7.2 V DC)

–20 ˚C to +60 ˚C (–4˚F to +140˚F) –15 ˚C to +55 ˚C (+5˚F to +131˚F)

± 10 ppm ± 1.5 kHz

1.5 A

1.2 A

0.7 A

0.6 A [FRG] only

200 mA

20 mA

Type STUD / 50 Ω

61(W)

× 135H) × 41(D) mm; 213⁄32(W) × 55⁄16(H) × 15⁄8(D) inch (with BP-224)

360 g; 12.7 oz (approx.)

Hi power; 5 W 5 W / 3 W / 1 W (High / Mid / Low) [OTHER]
Hi power; 1 W 1 W / 0.5 W (High / Low) [FRG]

Variable reactance frequency modulation

± 5.0 kHz

± 10 ppm ± 1.5 kHz

–65 dBc 250 nW

60 dB 70 dB

40 dB

Less than 10 % at 60 % deviation

+ 1 kHz to –3 dB of 6 dB octave from 300 Hz to 3000 Hz

2 kΩ

Double-conversion super heterodyne

1st: 21.70 MHz, 2nd: 450 kHz

0.25 µV at 12 dB SINAD 0.79 µV emf at 12 dB SINAD (typical)

0.35 µV 1.0 µV emf (typical)
70 dB (typical) 70 dB
70 dB (typical) 70 dB
70 dB (typical) 68 dB

40 dB

350 mW typical at 10 % distortion 200 mW at 10 % distortion

with an 8 Ω load with an 8 Ω load

–3 dB to +1 dB of –6 dB octave from 300 Hz to 3000 Hz

Current drain

(with BP-224)

Tx

Rx

SECTION 1 SPECIFICATIONS

Measurements made in accordance with TIA/EIA-603 or EN-301-178-2.

1 - 2

* Low power only

NOTE: Channels 3, 21, 23, 61, 64, 81, 82 and 83 CANNOT be used by the general public in USA waters.

CAN

21A

22A

23

24
25
26
27
28
60

61A

62A

64

64A

65A

66A*

67
68
69
70
71
72

CAN

73
74

77*

78A

79A

80A

81A

82A

83

83A

84

85

86

87

88

21b
25b
28b
83b

CAN

01

02
03

04A

05A

06

07A

08
09
10
11
12

13*

14

15*

16

17*

18A

19A

20*

21

‘‘

VHF MARINE CHANNEL LIST (For IC-M2A)

USA

01A

03A

05A

06

07A

08
09
10
11
12

13*

14

15*

16

17*

18A

19A

20

20A

INT

01

02
03

04

05

06
07

08
09
10
11
12
13
14

15*

16
17
18

19

20

21

Transmit

156.050

156.050

156.100

156.150

156.150

156.200

156.200

156.250

156.250

156.300

156.350

156.350

156.400

156.450

156.500

156.550

156.600

156.650

156.700

156.750

156.800

156.850

156.900

156.900

156.950

156.950

157.000

157.000

157.050

Receive

160.650

156.050

160.700

160.750

156.150

160.800

156.200

160.850

156.250

156.300

160.950

156.350

156.400

156.450

156.500

156.550

156.600

156.650

156.700

156.750

156.800

156.850

161.500

156.900

161.550

156.950

161.600

157.000

161.650

USA

21A

22A

23A

24
25
26
27
28

61A

63A

64A

65A

66A

67*

68
69
70
71
72

INT

22

23

24
25
26
27
28
60
61

62

63

64

65

65A

66

66A

67
68
69
70
71
72

Transmit

157.050

157.100

157.100

157.150

157.150

157.200

157.250

157.300

157.350

157.400

156.025

156.075

156.075

156.125

156.125

156.175

156.175

156.225

156.225

156.275

156.275

156.325

156.325

156.375

156.425

156.475
Rx only

156.575

156.625

Receive

157.050

161.700

157.100

161.750

157.150

161.800

161.850

161.900

161.950

162.000

160.625

160.675

156.075

160.725

156.125

160.775

156.175

160.825

156.225

160.875

156.275

160.925

156.325

156.375

156.425

156.475

156.525

156.575

156.625

USA

73
74

77*

78A

79A

80A

81A

82A

83A

84

84A

85

85A

86

86A

87

87A

88

88A

INT

73
74
77
78

79

80

81

82

83

84

85

86

87

88

Transmit

156.675

156.725

156.875

156.925

156.925

156.975

156.975

157.025

157.025

157.075

157.075

157.125

157.125

157.175

157.175

157.225

157.225

157.275

157.275

157.325

157.325

157.375

157.375

157.425

157.425
Rx only
Rx only
Rx only
Rx only

Receive

156.675

156.725

156.875

161.525

156.925

161.575

156.975

161.625

157.025

161.675

157.075

161.725

157.125

161.775

157.175

161.825

157.225

161.875

157.275

161.925

157.325

161.975

157.375

162.025

157.425

161.650

161.850

162.000

161.775

Channel No. Frequency (MHz) Channel No. Frequency (MHz) Channel No. Frequency (MHz)

WX01
WX02
WX03
WX04
WX05

Transmit

Receive only
Receive only
Receive only
Receive only
Receive only

Receive

162.550

162.400

162.475

162.425

162.450

WX06
WX07
WX08
WX09
WX10

Transmit

Receive only
Receive only
Receive only
Receive only
Receive only

Receive

162.500

162.525

161.650

161.775

163.275

Frequency (MHz) Frequency (MHz)

Weather

channel

Weather

channel

‘‘

WX CHANNEL LIST ([IC-M2A] only)

1 - 3

* Low power only

‘‘

VHF MARINE CHANNEL LIST (For IC-M21)

USA

01A

03A

05A

06

07A

08
09
10
11
12

13*

14

15*

16

17*

18A

19A

20

20A

21A

INT

01

02
03

04
05

06
07

08
09
10
11
12
13
14

15*

16
17
18

19

20

21

Transmit

156.050

156.050

156.100

156.150

156.150

156.200

156.250

156.250

156.300

156.350

156.350

156.400

156.450

156.500

156.550

156.600

156.650

156.700

156.750

156.800

156.850

156.900

156.900

156.950

156.950

157.000

157.000

157.050

157.050

Receive

160.650

156.050

160.700

160.750

156.150

160.800

160.850

156.250

156.300

160.950

156.350

156.400

156.450

156.500

156.550

156.600

156.650

156.700

156.750

156.800

156.850

161.500

156.900

161.550

156.950

161.600

157.000

161.650

157.050

USA

22A

23A

24
25
26
27
28

37A

61A

63A

64A

65A

66A

67*

68
69
70
71
72
73

INT

22

23

24
25
26
27
28

60
61

62
63

64

65

65A

66

66A

67
68
69
70
71
72
73

Transmit

157.100

157.100

157.150

157.150

157.200

157.250

157.300

157.350

157.400

157.850

156.025

156.075

156.075

156.125

156.175

156.175

156.225

156.225

156.275

156.275

156.325

156.325

156.375

156.425

156.475
Rx only

156.575

156.625

156.675

Receive

161.700

157.100

161.750

157.150

161.800

161.850

161.900

161.950

162.000

157.850

160.625

160.675

156.075

160.725

160.775

156.175

160.825

156.225

160.875

156.275

160.925

156.325

156.375

156.425

156.475

156.525

156.575

156.625

156.675

USA

74

77*

78A

79A

80A

81A

82A

83A

84

84A

85

85A

86

86A

87

87A

88

88A

INT

74
77
78

79

80

81

82

83

84

85

86

87

88

Transmit

156.725

156.875

156.925

156.925

156.975

156.975

157.025

157.025

157.075

157.075

157.125

157.125

157.175

157.175

157.225

157.225

157.275

157.275

157.325

157.325

157.375

157.375

157.425

157.425

Receive

156.725

156.875

161.525

156.925

161.575

156.975

161.625

157.025

161.675

157.075

161.725

157.125

161.775

157.175

161.825

157.225

161.875

157.275

161.925

157.325

161.975

157.375

162.025

157.425

Channel No. Frequency (MHz) Channel No. Frequency (MHz) Channel No. Frequency (MHz)

2 - 1

SECTION 2 INSIDE VIEWS

• MAIN UNIT

TOP VIEW BOTTOM VIEW

Antenna switching circuit
(D52: 1SV307)

Power amplifier
(Q54: 2SK2974)

APC
(IC50: NJM2125F)

IF amplifier
(Q151: 2SC2714 Y)

TX/RX switch
(D50, D51: MA77)

Antenna switching circuit
D90: 1SV307
D91: 1SS375

()

1st mixer
(Q150: 3SK272)

VCO circuit

IF DETECTOR IC
(IC170: TA31136FN)

PLL IC
(IC1: µPD3140GS)

D/A converter
(IC190: M62363FP-650C)

CPU
(IC360: HD6433844RA60H)

EEPROM
(IC340: HN58X2416TI)

Mic amplifier and
low pass filter circuits
(IC200: NJM2902V)

MAIN unit

Chassis

Earth plate

H

H

H

I

I

H

I

I

3 - 1

SECTION 3 DISASSEMBLY INSTRUCTIONS

• Removing the Rear panel

• Removing the Chassis unit

Rear panel

A

B

C

C

Rubber

Front panel



Front panel

Chassis unit

VR unit

E

G

D

F (Speaker connector)

¡ Unscrew 1 screw, A (2 x 10 mm), and remove 1 washer, B.
™ Unscrew 3 screws, C (2 x 8 mm).
£ Remove the rear panel and rubber from the front panel.

¡ Unscrew 1 nut, D.
™ Unscrew 2 screws, E (2 x 6 mm).
£ Take off the chassis unit in the direction of the arrow.
¢ Unplug, F, to separate the front panel, J251, and the

chassis unit.

∞ Remove 1 washer, G.

• Removing the MAIN unit

¡ Unscrew 9 screws, H (2 x 4 mm).
™ Unsolder 4 points, I, and remove earth plate.
£ Separate the MAIN unit and the chassis.

4 - 1

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT

(MAIN UNIT)

The antenna switching circuit functions as a low-pass filter
while receiving and as resonator circuit while transmitting.
The circuit does not allow transmit signals to enter receiver
circuits.

Received signals from the antenna connector pass through
the low-pass fileter (L80–L82, C80–C86) and antenna
switching circuit (D52, D90, D91). The filtered signals are
then applied to the RF amplifier circuit (Q90).

4-1-2 RF AND 1ST MIXER CIRCUITS (MAIN UNIT)

The 1st mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal with a PLL output frequency.
By changing the PLL frequency, only the desired frequency
will be passed through a pair of crystal filters at the next
stage of the 1st mixer.

The signals from the antenna switching circuit are passed
through the tunable bandpass filter (D92, D93) and amplified
at the RF amplifier (Q90). The amplified signals are passed
through another tunable bandpass fileter (D130), and then
applied to the 1st mixer circuit (Q150).

The filtered signals are mixed at the 1st mixer (Q150) with a
1st LO signal coming from the PLL circuit to produce a 21.7
MHz 1st IF signal. The 1st IF signal is passed through two
crystal filters (FI150, FI151) and is then amplified at the IF
amplifier (Q151).

4-1-3 2ND IF AND DEMODULATOR CIRCUITS

(MAIN UNIT)

The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. Adouble convertion superheterodyne system (which
converts receive signal twice) improves the image rejection
and obtain stable receiver gain.

The 1st IF signal is applied to a 2nd mixer section of the FM
IF IC (IC170, pin 16). The signal is then mixed with a 2nd LO
signal for conversion into a 450 kHz 2nd IF signal.

IC170 contains the 2nd mixer, limiter amplifier, quadrature
detector and active filter circuits. A21.25 MHz 2nd LO signal
is produced at the PLL circuit using the reference frequency.

The 2nd IF signal from the 2nd mixer (IC170, pin 3) passes
through ceramic filters (FI170) to remove unwanted hetero­dyned frequencies. It is then amplified at the limiter amplifi­er section (IC170, pin 5) and applied to the quadrature
detector section (IC170, pins 10 and 11) to demodulate the
2nd IF signal into AF signals.

4-1-4 AF CIRCUIT (MAIN UNIT)

AF signals from the FM IF IC (IC170, pin 9) are fed to the
analog switch (IC260).

The AF signals (detected signals) are passed through the
analog switch (IC260, pins 2 and 1) and are then applied to
the active low-pass filter (IC200c, pin 9).

The filtered AF signals are applied to and adjusted audio
level at the [VOL] control (VR unit; R500). The level con­trolled signals are passed through the AF mute switch
(Q280) which is controlled by “IVOLM” signal from the CPU
(IC9, pin 31). The passed signals are applied to the AF
power amplifier (IC280, pin 4), and then output to the inter­nal speaker or [EXT SP] jack after being passed through the
de-emphasis circuit (R286, C285) to obtain the –6 dB/octave
frequency characteristics.

Mixer

16

IF amp.

2nd IF filter
450 kHz

PLL IC

IC1

X1

21.25 MHz

RSSI

IC170 TA31136FN(D)

14

"IF" (1st IF signal: 21.7 MHz)
from Q151

"RSSIV" signal to the CPU (pin 94)

11

10

9

87 5 3

2

17

16

Active
filter

FI170

Noise

detector

FM

detector

Noise
comp.

"NOISV" signal to the CPU (pin 90)

12

C171

C174

R172

C176

C175

C180

C179

"SQLO" signal to the D/A
convertor IC (IC190, pin 15)

"SQLI" signal to the D/A
convertor IC (IC190, pin 16)

R175

R176

R174

AF signal "DET"

2nd

R171

X170

450 kHz

R5V

R170

SECTION 4 CIRCUIT DESCRIPTION

• 2ND IF AND DEMODULATOR CIRCUITS

4 - 2

4-1-5 SQUELCH CIRCUIT (MAIN UNIT)

The noise squelch circuit cuts out AF signals when no RF
signals are received. By detecting noise components in the
AF signals, the squelch circuit switches the AF mute switch.

A portion of the AF signals divided by C175 from the FM IF
IC (IC170, pin 9) are applied to the D/Aconvertor (IC190, pin

16) to adjust amplitude. The signals from the D/A convertor
(IC190, pin 15) are applied to the active filter section (IC170,
pin 8). The active filter section amplifies and filters noise
components. The filtered signals are applied to the noise
detector section and output from IC170 (pin 14) as the
“NOISV” signal.

The “NOISV” signal from IC170 (pin 14) is applied to the
CPU (IC360, pin 90). The CPU compares the set squelch
level voltage and “NOISV” signal voltage to control the
squelch output.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT

(MAIN UNIT)

The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis characteristics from the micro­phone to a level needed for the modulation circuit.

The AF signals from the microphone are passed through the
pre-emphasis circuit (R253, C254) and are then applied to
the microphone amplifier (IC200b). The amplified AF signals
are applied to analog switch (IC260, pin 5).

The AF signals are amplified again at the limiter-amplifier
(IC200a) and then passed through the low-pass filter
(IC200d, pins 10 and 8). The filtered audio is applied to the
modulation circuit as the “MOD” signal.

4-2-2 MODULATION CIRCUIT (MAIN UNIT)

The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.

The audio signals “MOD” change the reactance of D22 and
D23 to modulate an oscillated signal at the VCO circuit
(Q21, Q22, D22, D23). The oscillated signal is amplified at
the buffer-amplifiers (Q23, Q24).

4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS

(MAIN UNIT)

The signal from the VCO circuit passes through the trans­mit/receive switching circuit (D50, D51) and is applied to the
buffer-amplifier (Q50). The amplified signal is amplified by
the pre-driver (Q53) and the power amplifier (Q54) to obtain
5 W of RF power (at 7.2 V). The amplified signal passes
through the antenna switching circuit (D52), and low-pass fil­ter (L80–L82, C80–C86) and is then applied to the antenna
connector.

The bias current of the pre-driver (Q53) and the power
amplifier (Q54) is controlled by the APC circuit to stabilize
the output power.

4-2-4 APC CIRCUIT (MAIN UNIT)

The APC (Automatic Power Control) circuit provides stable
output power from the power amplifier even when the input
voltage or temperature changes, and selects HIGH, MID­DLE or LOW (1W type selects HIGH or LOW) output power.
The APC circuit consists of a power detector and APC con­trol circuits.

• POWER DETECTOR CIRCUIT (MAIN UNIT)

The power detector circuit (D80, C87) detects the transmit
output power level and converts it to DC voltage as the
“TDETV” signal. The detected signal is applied to the APC
control circuit.

• APC CONTROL CIRCUIT (MAIN UNIT)

The “TDETV” signal from the power detector circuit is
applied to the CPU (IC360, pin 92) to control the input volt­age of the pre-driver (Q53) and the power amplifier (Q54).
When the output power changes, the CPU (IC360) outputs
APC control signal to the D/A converter (IC190). And then
“PCON” signal from the D/Aconverter controls the APC con­troller (IC50) to provide stable output power.

I. SENS

Q53

IC50

Q51, Q52

APC ctrl

To the 1st IF circuit

PWR amp.

Pre-Drive

D80, C87

Q54

D52

D91

Ant SW

Ant

D90

+

"PCON" signal
from IC190 (pin 3)

"TDETV" signal
to CPU (pin 92)

The amplified signal from the
buffer amplifier (Q50)

"VCC"
"T5V"

PWR det.

TX

RX

• APC CIRCUIT

4 - 3

4-3 PLL CIRCUIT (RF UNIT)

A PLL circuit provides stable oscillation of the transmit fre­quency and receive 1st LO frequency. The PLL output com­pares the phase of the divided VCO frequency to the refer­ence frequency. The PLL output frequency is controlled by
the divided ratio (N-data) of a programmable divider.

The PLL circuit contains a VCO (Q21, Q22, D22, D23). The
oscillated signal is amplified at the buffer-amplifiers (Q23,
Q25) and then applied to the PLL IC (IC1, pin 19).

The PLL IC contains the prescalers, programmable counter,
programmable divider, phase selector and etc. The entered
signal is divided at the prescaler and programmable counter
sections by the N-data ratio from the CPU. The divided sig­nal is detected on phase at the phase detector using the ref­erence frequency.

If the oscillated signal drifts, the phase of its frequency
changes from the reference frequency, causing a lock volt­age changes to compensate for the drift in the oscillated fre­quency.

A portion of the VCO signal is amplified at buffer-amplifiers
(Q23, Q24) and is then applied to the receive 1st mixer
(Q150) or transmit buffer amplifier (Q50) via the TX/RX
switching diode (D50, D51).

Shift register

Prescaler

Phase
detector

Loop

filter

Programmable
counter

Programmable
divider

X1

21.25 MHz

"2nd LO" signal (21.25 MHz)
to the FM IF IC (IC170)

Q21, Q22, D22, D23

VCO

Buffer

Buffer

Buffer

Q24

D50

D51

Q25

Q23

3
4
5

PLST

IC1 (PLL IC)

SCK
SO

to transmitter circuit

to the 1st mixer circuit

17

16

13

19

LINE

HV

VCC

CPU5V

5V

R5V

V5V

T5V

DESCRIPTION

The voltage from the attached battery pack.
The same voltage as the HV line (battery volt-

age) which is controlled by the power switch (RF
unit; [OFF/VOL] control).

Common 5 V converted from the VCC line by the
CPU5V regulator circuit (IC220).
The output voltage is applied to the CPU (MAIN
unit; IC360), REG5V regulator, etc.

Common 5V converted from the VCC line by the
5V regulator circuit (Q223–Q225).
The output voltage is applied to the D/A convert­er (IC190) and PLL IC (IC1), etc.

Receive 5V converted from the 5V line by the R5
regulator circuit (Q221).
The regulated voltage is applied to the MOD
MUTE circuit (Q20, D20, D21) and receiver cir­cuit.

Common 5V converted from the 5V line by the
V5 regulator circuit (Q220).
The regulated voltage is applied to the VCO cir­cuit

Transmit 5V converted from the 5V line by the T5
regulator circuit (Q222).
The regulated voltage is applied to the transmit­ter circuit.

4-4 POWER SUPPLY CIRCUITS

VOLTAGE LINES

• PLL CIRCUIT