Abell A720T User manual

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A720T (VHF) Service Manual
ABELL INDUSTRIES CO., LTD

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CONTENT
CHAPTER 1 INTRODUCTION..............................................................................................................................................4
1.1 SCOPE OF APPLICATION...............................................................................................................................................4
1.2 SECURITY ISSUES............................................................................................................................................................4
CHAPTER 2 STRUCTURE AND KEY FUNCTIONS......................................................................................................... 5
2.1 STRUCTURE...................................................................................................................................................................... 5
CHAPTER 3 COMPOSITE MODE........................................................................................................................................7
3.1 MODE...................................................................................................................................................................................7
3.2 HOW TO ENTER EACH MODE..................................................................................................................................... 7
3.3 PC MODE............................................................................................................................................................................ 7
3.3.1 PREFACE..........................................................................................................................................................................7
3.3.2 CONNECTION STEPS........................................................................................................................................................7
CHAPTER 4 DISASSEMBLED PARTS IN MAINTENANCE PROCESS........................................................................ 9
4.1 DISASSEMBLE THE RADIO...........................................................................................................................................9
4.2 EXPLOSIVE VIEW.......................................................................................................................................................... 10
CHAPTER 5 UNIT CIRCUIT............................................................................................................................................... 11
5.1 DMR DIGITAL CIRCUIT SIGNAL BLOCK DIAGRAM........................................................................................... 11
5.2 ANALOG CIRCUIT STRUCTURE................................................................................................................................ 11
5.3 RECEIVER........................................................................................................................................................................ 12
5.3.1 FRONT-END RADIO-FREQUENCY AMPLIFIER.................................................................................................................. 12
5.3.2 FIRST FREQUENCY MIXER............................................................................................................................................. 13
5.3.3 INTERMEDIATE FREQUENCY AMPLIFIER (IF AMP).......................................................................................................13
5.3.4 AF AMPLIFIER...............................................................................................................................................................13
5.3.5 NOISE SUPPRESSION CIRCUIT....................................................................................................................................... 13
5.4 PHASE LOCKED FREQUENCY SYNTHESIZE.........................................................................................................14
5.4.1 PLL CIRCUIT.................................................................................................................................................................14
5.4.2 UNLOCKED DETECTOR................................................................................................................................................. 15
5.5 TRANSMITTER............................................................................................................................................................... 15
5.5.1 RF AUDIO..................................................................................................................................................................... 15
5.5.2 QT/DQT/4FSK CODING...............................................................................................................................................15
5.5.3 WIDE/NARROW BAND SWITCH.....................................................................................................................................15
5.5.4 VOLTAGE-CONTROLLED OSCILLATOR(VCO) AND RF AMPLIFIER................................................................................16
5.5.5 ANTENNA TRANSFER SWITCH AND LPF........................................................................................................................16
5.5.6 POWER CONTROL..........................................................................................................................................................16
5.5.7 TEMPERATURE PROTECTION CIRCUIT........................................................................................................................... 16
5.6 POWER SUPPLY..............................................................................................................................................................17

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5.7 CPU CONTROL SYSTEM.............................................................................................................................................. 18
CHARTER 6 FUNCTIONS AND CONFIGURATIONS.................................................................................................... 19
6.1 FUNCTION DEFINITION AND PARAMETER CONFIGURATION...................................................................... 19
6.1.1 CHANNEL...................................................................................................................................................................... 19
6.1.2 SCAN.............................................................................................................................................................................19
6.1.3 POWER/BATTERY SAVE................................................................................................................................................. 19
6.1.4 VOX............................................................................................................................................................................. 20
6.1.5 TOT.............................................................................................................................................................................. 20
6.1.6 HIGH/LOW POWER SWITCH.......................................................................................................................................... 20
6.1.7 SQUELCH LEVEL........................................................................................................................................................... 20
6.1.8 RECEIVE AND MAKE CALL...........................................................................................................................................20
6.1.9 RADIO CHANNEL, RADIO ID AND GROUP ID............................................................................................................... 20
6.1.10 LOW BATTERY WARNING............................................................................................................................................ 22
6.2 CONFIGURATIONS........................................................................................................................................................ 23
6.2.1 SQUELCH LEVEL........................................................................................................................................................... 23
6.2.2 HIGH/LOW POWER SWITCH.......................................................................................................................................... 23
6.2.3 VOX............................................................................................................................................................................. 23
6.2.4 PROMPT TONE TURN ON/OFF....................................................................................................................................... 23
6.2.5 CHECK BATTERY CHARGING STATUS............................................................................................................................23
6.2.6 CTCSS..........................................................................................................................................................................24
6.2.7 CDCSS......................................................................................................................................................................... 24
CHAPTER 7 RADIO DEBUGGING.................................................................................................................................... 25
7.1 THE REQUIRED TEST EQUIPMENT......................................................................................................................... 25
7.2 DEBUGGING INTRODUCTION................................................................................................................................... 27
CHAPTER 8 PARAMETERS................................................................................................................................................32
BILL OF MATERIALS (ELECTRONICS)..........................................................................................................................34
FIG.1 A720T BIT MAP...........................................................................................................................................................49
TOP SIDE：.............................................................................................................................................................................49
BOT SIDE：............................................................................................................................................................................ 50
FIG. 2 A720T SCHEMATIC DIAGRAM.......................................................................................................................... 51
FIG. 3 A720T BLOCK DIAGRAM.................................................................................................................................... 55
FIG. 4 A720T PACKING..................................................................................................................................................... 56

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Chapter 1 Introduction
1.1 Scope of application
This manual is written for technicians who well know communication profession and have maintenance experience. It
includes all information and existing published data required by the equipment maintenance. The change may happen
after its publication, if required, you can use repair bulletin and manual revised edition as supplement.
Before repair the equipment, please read this manual first.
1.2 Security Issues
Safe power radiation of two way radio
Two way radios can produce and emit electromagnetic energy to outside, and electromagnetic transmission safety
design of ABELL two way radio completely conforms with national and international standard. In order to ensure the
best communication effect of two way radio and the electromagnetic transmission safety for human, when you use
two way radios for communication, please keep the two way radio perpendicular to the ground, and your month away
from microphone 2.5-5 cm.
Electromagnetic interference
In order to avoid electromagnetic interference, please turn off two way radio when you are in the places where there is
clear indication to turn off the two way radio (such as hospital, airport)
Explosive environment
You'd better turn off two way radio when in explosive environment.
Please turn off two way radio when in gas station or parking there.
Please turn it off when near electric fuse.
It is strictly prohibited to replace or recharge battery in the place where there is potentially explosive air.
Damaged antenna
When antenna is damaged, you better not to use two way radios. When the damaged antenna touches human skin, it
may cause mild burn.
Parts replacement
When start to replace any parts, please note the type, don't randomly use the components which differ from the
information provided by the company.
Maintenance service
In order to repair the equipment, we set up a completed service system, and provide the material including schematic
diagram, printed substrate as your reference.

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Chapter 2 Structure and key functions
2.1 Structure
○
1○ 3○ 4 Programmable function key:These keys are programmable function keys; Long press (key-press
time should reach the time we set for long press key) and short press (normal key), which can be set for different
functions. Press it to enable programmable auxiliary functions.
Your dealers can can program key 1, 3 and 4 to add hotkeys and preset channel/group:
Short press: quick press and then release
Long press: Keep pressing for several seconds(preset time between 0.25s to 3.75s )
No function: If you preset this key as no function, it will warn you when pressing it.
○
2PTT key:Press this switch, and then call the opposite by microphone. Release the switch to listen.
○
5 Antenna
○
6 Knob: Channel switch
Rotate it from channel 1 to channel 32. (note: there are totally 32 channels in two areas of A720T)
○
7 Knob: Power on/off and Volume control
Clockwise rotate it to turn on the two way radio power, you will hear a sound of "click", then continuously rotate it to

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adjust volume. When you want to turn off two way radio, counter-clockwise rotate the knob until hear a sound of
"click".
Note: if you turn off the power on sound when programming, you will not hear the power on sound.Check the battery
when power on failure to make sure battery installed properly. If you still cannot power on the radio, please contact
the dealer. Clockwise to higher the volume
○
8 Luminous diode Indicator
It shows red when transmitting; It shows green when power on, receiving or detecting signal; Red flashes when low
battery.
○
9Speaker
○
10 Dust cover for earpieces and programming

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Chapter 3 Composite Mode
3.1 Mode
Mode type
Function
User mode
This mode is used for general operation
PC mode
By this mode, use external programming cable to make various settings
Debugging mode
Adjust parameter of machine
3.2 How to enter each mode
Mode type
Operation steps
user mode
Boot whilst automatically come into user mode
PC mode
Use programming cable to connect with PC
Debugging mode
Through programming software, come into debugging mode.
3.3 PC mode
3.3.1 Preface
Use programming cable to connect A720T and PC, operate programming software (ABELL A720T programming
software) on PC to make programming setting for communication equipment. Figure 1 shows the process of
programming setting on PC.
3.3.2 Connection Steps
1. Use programming cable to connect A720T and PC
2. Operate programming software (ABELL A720T programming software).
3. Set the communication port.
4. Conduct various programming operation on two way radio.
5. When the radio transmit, red light is blinking. When the radio receives, green light is blinking.
User mode
Debugging mode
PC mode

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Notice：
When data is written into EEPROM, the data stored in PC must conform with model of radio. Turn on power of
radio only on condition that the software must run and cable must be connected.
In the course of transmitting or receiving data, do not press [PTT] key.
Programming cable (optional):
Connect A720T with PC using programming cable.
Programming S/W
ABELL A720T programming software (installed) allow users to conduct programming setting on A720T through
programming interface cable.
Program on PC
If use PC with ABELL two way radio programming for transmitting data to radio, the setting destination data (basic
communication parameters) can all be modified. as the value is automatically generated in setting frequency
range(frequency type), generally it is not necessary to change destination data. the value can be changed only in
necessary condition. The data in form of RS-232C can be input into EEPROM, in this mode, the function of PTT line
can be served for sending and receiving data.

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Chapter 4 Disassembled parts in maintenance process
4.1 Disassemble the radio
1. Remove two screws and take away the belt clip
2. Rotate to remove antenna
3. Pull out volume and channel knob
4. Remove waterproof ring
5. Remove antenna base
6. Remove waterproof screw nut
7. Remove 4 screws
8. Pull out the logo sticker
9. Remove 2 screws
10. Pull housing
11. Use the soldering iron to melt the tin on speaker
wire. Then you can separate up case and aluminum
housing part.
12. Remove 6 screws and 2 copper screw nuts.
13. Up warp two BTB
14. Against battery connector from back then lift
PCB part

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4.2 Explosive View