IC-2720双频FM对讲机维修手册：介绍、注意事项

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The IC-2720H is a Dual Band FM Transceiver, and the provided service manual offers comprehensive information for maintenance and repair. It warns against connecting the transceiver to power supplies exceeding 16V, exposure to liquids and adverse weather conditions, reversing power supply polarities, or applying RF signals greater than 20dBm, as these actions could result in irreversible damage to the device. The manual details the internal components and their respective functions, providing guidance on troubleshooting and repair procedures. It also outlines safety precautions and procedures for disassembly, reassembly, and adjusting circuitry. Furthermore, it includes technical specifications, schematics, and diagrams for reference during servicing. The IC-2720H service manual is an invaluable resource for technicians and amateurs alike, offering in-depth knowledge and guidance for maintaining and repairing this Dual Band FM Transceiver.

4 - 1

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 TRIPLEXER AND RX BAND SWITCHING

CIRCUITS (MAIN UNIT)

The transceiver has a triplexer (low-pass and high-pass fil-

ters) on the first stage from the antenna connector to sepa-

rate the signals into VHF and UHF signals. The RF signals

from the antenna connector are applied to the tripler or RX

band swtich circuits.

• RF SIGNALS V-V (118 MHz–180 MHz), U-V (136

MHz–174 MHz)

The V-V and U-V RF signals from the antenna connector

pass through the low-pass filter (L76, L77, L80, C205, C209,

C242, C243, C264), and then applied to the TX/RX switching

circuit (D42, D46, D52, D67, D2049). The filtered signals are

amplified at the pre-amplifier (Q33), and are applied to the

left side or right side displayed RX circuits.

• RF SIGNALS U-U, V-U2 (375 MHz–550 MHz)

The U-U and V-U2 RF signals from the antenna connector

pass through the high-pass filter (L78, L81, C206, C210,

C213, C265), and then applied to the TX/RX switching circuit

(D55, D64, D65, D2050) via the SWR detector (D50, D58).

The filtered signals are amplified at the pre-amplifier (Q24),

and are applied to the left side or right side displayed RX cir-

cuits.

• RF SIGNALS U-U3 (810 MHz–1000 MHz)

The U-U3 RF signals from the antenna connector pass

through the two low-pass filters (L76, L77, L80, C205, C209,

C242, C243, C264, L87, L88, C266–C268), and are then

applied to the RX band swtiching circuit (D2061). The filtered

signals are amplified at the RF amplifier (Q18), and are

applied to the right side displayed RX circuits.

• RF SIGNALS V220 (174 MHz–260 MHz), V-U1 (225

MHz–375 MHz)

The V220 and V-U1 RF signals from the antenna connector

are applied to the RX band swtiching circuit (Q34, D66, RL1),

and are applied to the left side displayed RX circuit.

4-1-2 RF CIRCUIT FOR LEFT SIDE DISPLAY

(MAIN UNIT)

• RF SIGNALS V-V (118 MHz–180 MHz)

The amplified signals are applied to the RF amplifier (Q29)

after being passed through the attenuator (D59) and band-

pass filter (D47, D53). The signals are applied to the RX

band switching circuit (D28) via the another bandpass filter

(D32, D39) to supress the unwanted signals.

• RF SIGNALS V220 (174 MHz–260 MHz)

The signals are applied to the RF amplifier (Q31) after being

passed through the RX band switching circuit (D62) and

bandpass filter (D51). The amplified signals are applied to

the RX band switching circuit (D28) via the another bandpass

filter (D34) to supress the unwanted signals and attenuator

(R195–R197).

• RF SIGNALS V-U1 (225 MHz–375 MHz)

The signals are applied to the RF amplifier (Q32) after being

passed through the RX band switching circuit (D63) and

bandpass filter (D49). The amplified signals are applied to

the RX band switching circuit (D31) via the attenuator

(R198–R200) and another bandpass filter (D35) to supress

the unwanted signals.

• RF SIGNALS V-U2 (375 MHz–550 MHz)

The amplified signals are applied to the RF amplifier (Q20)

after being passed through the attenuator (D25) and band-

pass filter (D23, D73). The signals are applied to the RX

band switching circuit (D9) via the another bandpass filter

(D13, D17) to supress the unwanted signals.

BPF BPF

ANTENNA

1st mixer

(IC1005)

1st LO

to 2nd mixer

circuit

ATT

PRE

BPF BPF

ATT

PRE

BPF BPF

ATT

BPF BPF

ATT

D59Q29

V-V (118 MHz—174 MHz)

V220 (174 MHz—260 MHz)

V-U1 (225 MHz—375 MHz)

V-U2 (375 MHz—550 MHz)

D28 D32, D39 D47, D53

D62Q31D30 D34 D51

D63Q32D31 D35 D49

Q33

Q34, D66,

RL1

D25Q20D9 D13, D17 D23, D73 Q24

• RF CIRCUIT FOR LEFT SIDE DISPLAY

4 - 2

The signals from the RX band swtiching circuits are then

applied to the left side displayed 1st mixer circuit (IC1005, pin

6).

4-1-3 1ST MIXER AND 1ST IF CIRCUIT FOR LEFT

SIDE DISPLAY (MAIN UNIT)

The 1st mixer circuit converts the received RF signals to a

fixed frequency of the 1st IF signal with a PLL output fre-

quency. By changing the PLL frequency, only the desired fre-

quency will pass through the bandpass filter at the next stage

of 1st mixer circuit.

The RF signals are mixed with 1st LO signals at the 1st mixer

(IC1005) to produce a 38.85 MHz 1st IF signal. The 1st IF

signal is output from pin 1, and passed through the crystal

bandpass filter (FI1003) to suppress unwanted harmonic

components. The filtered signal is amplified at the IF amplifi-

er (Q1040) after being passed through the limiter circuit

(D1021). The amplified signal is applied to the 2nd mixer cir-

cuit (IC1001).

4-1-4 2ND IF AND DEMODULATOR CIRCUITS FOR

LEFT SIDE DISPLAY (MAIN UNIT)

The 2nd mixer circuit converts the 1st IF signal to a 2nd IF

signal. A double conversion superheterodyne system (which

converts receive signal twice) improves the image rejection

ratio and obtains stable receiver gain.

The FM IC IC (IC1001) contains the 2nd mixer, limiter and

noise amplifiers, quadrature detector, S-meter detector,

active filter circuits, etc. A 2nd LO signal (38.4 MHz) is pro-

duced at the PLL circuit by dividing it’s reference frequency.

The 38.85 MHz 1st IF signal from the IF amplifier (Q1040) is

applied to the 2nd mixer section of the FM IF IC (IC1001, pin

16), and is mixed with the 2nd LO signal (38.4 MHz) to be

converted to a 450 kHz 2nd IF signal.

The 2nd IF signal is applied to the each demodulator circuits

by AM or FM mode.

• FM MODE

The 2nd IF signal is output from the FM IF IC (IC1001, pin 3)

and passes through the ceramic bandpass filter (FI1001).

The filtered signal is fed back to the IC, and amplified at the

limiter amplifier section (pin 5), then demodulated into AF sig-

nals at the quadrature detector section (pins 10, 11). The

detected AF signals are output from pin 9 and are applied to

the AF circuit via the AM/FM selector circuit (IC2015, pins 7,

1).

• AM MODE

The 2nd IF signal is output from the FM IF IC (IC1001, pin 3)

and passes through the ceramic bandpass filter (FI1000).

The filtered signal is applied to the AM detector circuit

(Q1017) to convert into AF signals, and then amplified at the

Q1014 (pins 5, 1). The amplified AF signals are applied to the

AF circuit via the AM/FM selector circuit (IC2015, pins 6, 1).

4-1-5 AF AMPLIFIER CIRCUIT FOR LEFT SIDE

DISPLAY (MAIN UNIT)

The AF amplifier circuit amplifies the demodulated AF signals

to drive a speaker.

The AF signals pass through the AF mute switch (Q1010),

and are then applied to the electric volume control circuit

(IC2011, pin 1) as “VAFO” signal after being passed through

the low-pass filter (Q1007). The level controlled AF signals

are output from pin 2, and are then applied to the AF power

amplifier (IC2012, pin 2) via the “VOUT1” signal. The power

amplified AF signals are applied to the internal speaker (SP1)

via the [EXT SP] jack (J2005).

The electronic volume control circuit controls AF gain, there-

fore, the AF output level is according to the [VOL] setting and

also the squelch conditions.

4-1-6 NOISE SQUELCH CIRCUIT FOR LEFT SIDE

DISPLAY (MAIN UNIT)

• NOISE SQUELCH

A noise squelch circuit cuts out AF signals when no RF sig-

nal is received. By detecting noise components in the AF sig-

nal, the squelch circuit switches the AF mute switch.

Limiter

amp.

2nd IF filter

450 kHz

PLL IC

IC1

12.8 MHz

38.4 MHz

RSSI

IC1001 TA31136F

1st IF (38.85 MHz)

from Q1040

"L_RSSI" signal to the CPU

(IC2013, pin 33)

VCO UNIT

Active

filter

FI1001

FI1000

Noise

detector

Noise

comp.

"L_SQL" signal to the CPU

(IC2013, pin 40)

R1124

R1291

C1108

C1100

R1108

C1078

C1085

C1084

C1097

DET.

amp.

C1069

R1092

R1109

R1114

L_R5

X1001

FM or WFM AF signal

to AM/FM selector

AM AF signal

to AM/FM selector

Mixer

2nd

Q1049

from Q1013

C1271

• 2ND IF AND DEMODULATOR CIRCUIT FOR LEFT SIDE DISPLAY

4 - 3

A portion of the AF signals from the FM IF IC (IC1001, pin 9)

are applied to the active filter section (IC1001, pin 8). The

active filter section amplifies and filters noise components.

The filtered signals are applied to the noise detector section

and output from the IC1001 (pin 14) as the “L_SQL” signal.

The “L_SQL” signal from IC1001 (pin 14) is applied to the

CPU (IC2013, pin 40). The CPU analyzes the noise condition

and outputs the “L_DET_MUTE” signal to the AF mute switch

(Q1010).

• TONE SQUELCH

The tone squelch circuit detects AF signals and opens the

squelch only when receiving a signal containing a matching

subaudible tone (CTCSS). When tone squelch is in use, and

a signal with a mismatched or no subaudible tone is

received, the tone squelch circuit mutes the AF signals even

when noise squelch is open.

A portion of the AF signals from the FM IF IC (IC1001, pin 9)

passes through the low-pass filter (Q1003) to remove AF

(voice) signals. The filtered signal is applied to the CTCSS

decoder which is inside the CPU (IC2013, pin 41) via the

“L_DTCS_IN” line to control the AF mute switch (Q1010).

4-1-7 RF CIRCUIT FOR RIGHT SIDE DISPLAY

(MAIN UNIT)

• RF SIGNALS U-V (136 MHz–174 MHz)

The amplified signals are applied to the RF amplifier (Q30)

after being passed through the attenuator (D60) and band-

pass filter (D48, D54). The signals are applied to the RX

band switching circuit (D29) via the another bandpass filter

(D33, D40) to supress the unwanted signals.

• RF SIGNALS U-U3 (810 MHz–1000 MHz)

The signals are applied to the RF amplifier (Q18) after being

passed through the RX band switching circuit (D2061). The

amplified signals pass through the attenuator (L19, C20,

C46, C51, R178–R180) and high-pass filter (L20, C278,

C279), and are then applied to the another RF amplifier

(Q35) again. The signals pass through the attenuator (L10,

C15, C280, R183–R185) and RX bamd switching circuit

(D11).

• RF SIGNALS V-U2 (375 MHz–550 MHz)

The amplified signals are applied to the RF amplifier (Q19)

after being passed through the attenuator (D24) and band-

pass filter (D22, D72). The signals are applied to the RX

band switching circuit (D8) via the another bandpass filter

(D12, D16) to supress the unwanted signals.

The signals from the RX band swtiching circuits are then

applied to the right side displayed 1st mixer circuit (IC1006,

pin 6).

4-1-8 1ST MIXER AND 1ST IF CIRCUIT FOR RIGHT

SIDE DISPLAY (MAIN UNIT)

The 1st mixer circuit converts the received RF signals to a

fixed frequency of the 1st IF signal with a PLL output fre-

quency. By changing the PLL frequency, only the desired fre-

quency will pass through the bandpass filter at the next stage

of 1st mixer circuit.

The RF signals are mixed with 1st LO signals at the 1st mixer

(IC1006) to produce a 46.05 MHz 1st IF signal. The 1st IF

signal is output from pin 1, and passed through the crystal

bandpass filter (FI1004) to suppress unwanted harmonic

components. The filtered signal is amplified at the IF amplifi-

er (Q1041) after being passed through the limiter circuit

(D1022). The amplified signal is applied to the 2nd mixer cir-

cuit (IC1004).

4-1-9 2ND IF AND DEMODULATOR CIRCUITS FOR

RIGHT SIDE DISPLAY (MAIN UNIT)

The 2nd mixer circuit converts the 1st IF signal to a 2nd IF

signal. A double conversion superheterodyne system (which

converts receive signal twice) improves the image rejection

ratio and obtains stable receiver gain.

The FM IC IC (IC1004) contains the 2nd mixer, limiter and

noise amplifiers, quadrature detector, S-meter detector,

active filter circuits, etc. A 2nd LO signal (45.595 MHz) is pro-

duced at the PLL circuit by dividing it’s reference frequency.

BPF BPF

ANTENNA

1st mixer

(IC1006)

1st LO

to 2nd mixer

circuit

ATT

PRE

BPF BPF

ATT

PRE

HPF

ATT ATT

D60Q30

U-V (136 MHz 174 MHz)

U-U3 (810 MHz 1000 MHz)

V-U2 (375 MHz 550 MHz)

D29 D33, D40 D48, D54

D2061

Q35

Q18D11

Q33

D24Q19D8 D12, D16 D22, D72 Q24

• RF CIRCUIT FOR RIGHT SIDE DISPLAY

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