Micromax A27 User manual

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A27 Service Manual

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Table of Contents
1. Circuit diagram instruction ....................................................................................3
1.1 Main board explored view..............................................................................3
1.2 Detailed Description of Circuit Chart...........................................................4
1.3 RF circuit .........................................................................................................4
1.4 Base band circuit.............................................................................................5
1.5 Function circuit...............................................................................................5
2．Fault maintenance..................................................................................................6
3．Trouble shooting and flow chart...........................................................................8
3.1 Unable to power on the handset ....................................................................8
3.2 No ring tone.....................................................................................................9
3.3 LCD display fault..........................................................................................10
3.4 Charging fault ...............................................................................................11
3.5 Camera fault..................................................................................................12
3.6 Fail to identify SIM card..............................................................................13
3.7 MIC fault .......................................................................................................14
3.8 Keypad fault..................................................................................................15
3.9 Receiver fault.................................................................................................16
3.10 Vibrate fault.................................................................................................17
3.11 FM fault........................................................................................................18
3.12 Bluetooth fault.............................................................................................19
3.14 No signal.......................................................................................................20
3.15 Failure to identify T-flash card..................................................................21
3.16 Earphone fault.............................................................................................22
3.17 Unable to use USB.......................................................................................23

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1. Circuit diagram instruction
.1 Main board explored view
LCD PAD
Earphone
connector
WIFI/BT IC
direction sensor
四方向传感器
T-card connector
Battery connector
receiving filter
接收滤波器
SIM connector
SC6820
FLASH
TP connector
USB
connector
RF amplifier
射频放大器
RF transceiver
26M crystal
Crystal IC
32.768

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1.2 Detailed Description of Circuit Chart
1.3 RF circuit
This phone is designed on SC6820 platform. RF main chip integration SR1019,
PA use RDA6163A, TCXO uses 26MHz crystal oscillator.
(1) Receiving circuit
The GSM or DCS signal received from a base station via antenna is inputted from
the pin 27 of U101, send U104/U105 to pin A20/B20/A19/B19. This chip integrates
LNA and Frequency mixer. The signals are amplified by their own low-noise
amplifier (LNA). The amplified signal is sent to a down converter, where the signal is
mixed with local oscillation (LO) output from RX VCO to generate zero intermediate
frequency (ZIF) signals. The zero-IF signal passes through a filter and is amplified by
programmable gain amplifier (PGA), then the signal is outputted to I and Q
demodulator to generate I/Q base band signals.
(2) Transmitting circuit
A transmitter consists of I/Q modulation circuit, up-conversion mixer circuit,
RF PA
(U112)
SIM card 1
SIM card 2
BT/WIFI
Key pad
T-card
Camera
Rf
transceiver
U114
Rf
transceiver
U103

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power amplifier and duplex switch. The I/Q signal is inputted to be performed signal
modulation as the modulation signal. The modulated TXIF signals restrain its third
harmonic by harmonic suppression filter. The follow-up TXIF signal will be sent to a
discriminator on the offset frequency phase lock loop (OPLL). Then perform
frequency discrimination to this signal and the mixed signal of TXVCO signal and
RFVCO output signal. The discriminator outputs the signal control RFVCO and
RFVCO phase is enabled to lock at the phase of TXIF signal. TXVCO output RF is
still decided by RFVCO frequency. The phases of RFVCO and IFVCO signals lock at
the phase of VCTXO reference signal. TXVCO output signal is sent to emission
power amplifier (PA) A17/A16 pin to amplify. The TX signal outputted by PA pin
27is transmitted to the antenna via antenna switch to launch.
1.4 Base band circuit
Base band main chip adopts SC6820 chip.
1.5 Function circuit
WIFI/BT
T card
SIM card

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Camera
2．Fault maintenance
Note:
The PCBA repairing is a very important work during the final production of cellular
phones. The speed and quality of repairing decides the yield and production efficiency.
The repair idea is very important for a good repairing technician. Make sure not to
heat a board right away once a phone reaches at hand.
BT Filter
Power

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3 Trouble shooting and flow chart
3.1 Unable to power on the handset
Check battery mounted correctly &
battery voltage >3.4V?
Failure to power on
OK
Re-mount battery or
replace battery
No
Check if Keyboard backlight
can be lighted
Solder or replace
LCD
Download software! Ok? Pay attention to
retain the calibration data
Replace new mainboard
Check if LCD is
connected correctly Yes
No
No
Yes
Yes
No
Yes
Yes

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3.2 No ring tone
Yes
Check soldering of
speaker.OK?
No ringtone
Replace speaker and
test
No
Check the speaker circuit
no short ciruit or open
circuit
Replace new
mainboard
Solder
speaker
Yes
No
OK! Mark the
failed speaker
Yes
Mark the
mainboard with
analysis result
OK
No

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3.3 LCD display fault
Check If the lcd is damaged
Abnormal LCD
Display
OK
Replace new LCDYes
Check if the lcd connector is
soldered well
No
No Solder the lcd connector
Check If the LCD is mounted
inverse
Yes
Yes Re-mount the lcd
Replace the new mainboard
No

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3.4 Charging fault
Check charger or Replace new
charger
Failure to Charge
OK
Replace new ChargerYes
Check battery is good or not
No
No Replace new battery
Check charging circuit of
mainboard for fault
Yes
Yes Mark the mainboard
with analysis result
Replace new mainboard
No
Check charger connector for
soldering defect Yes Solder charger connector
No

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3.5 Camera fault
Check camera for damage
Failure to use camera
OK
Replace new camera
Yes
Check if camera is soldered or
assembled well
No
Yes Solder camera connector
or Re-assemble camera
Check camera circuit for fault
No
Yes Mark the mainboard
with analysis result
Replace new mainboard
No

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3.6 Fail to identify SIM card
Check if SIM card is inserted correctly
According to insertion direction
Failure to identify
SIM card
OK
Reinsert or change new
SIM card
No
Check
SIM connector for soldering
defect
No
Yes Solder or replace SIM
connector
Check SIM circuit for fault Yes Mark the mainboard
with analysis result
Replace new mainboard
No
No

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3.7 MIC fault
Check mic for soldering defect
Unable to record
No sound in microphone
OK
Solder or replace new
micphone
Yes
Check if mic sound hole is
jammed
No
Yes Clean mic sound hole and
re-assemble mic
No
Replace new mainboard
No
Check mic circuit for fault Yes Mark the mainboard with
analysis result

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3.8 Keypad fault
Check : DOME is pasted and
keymat is assembled correctly
Keypad doesn’t work
correctly
OK
Paste the DOME or
assemble keymat again
Yes
Is there any dirty on keypad
surface?
No
Yes Clean keypad surface and
test
No
Replace new mainboard
No
Is sidekey soldered well?
No short circuit Yes Solder or replace sidekey

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3.9 Receiver fault
Is receiver ok ?
No broken wire
No voice in receiver
OK
Replace new receiverNo
Check receiver connector for
soldering defect
Yes
Yes Solder receiver connector
No
Replace new mainboard
No
Check receiver circuit for any
fault Yes Mark the mainboard with analysis result

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3.10 Vibrate fault
Is vibrator soldered well ?
Vibrator fault or No
vibration
OK
Solder vibrator or replace
vibrator
No
Check : Wire of vibrator
isn’t broken
No
Yes Replace vibrator
No
Replace new mainboard
Yes
Check vibrator circuit
No short circuit or open circuit No Mark the mainboard with analysis result

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3.11 FM fault
Software version is correct
Failure to open FM or FM
can’t scan any station
OK
Re-download softwareNo
Earphone can be detected and
audio output is ok
Yes
No Replace earphone
Yes
Replace new mainboard
Yes
Check FM circuit
No short circuit or open circuit No Mark the mainboard with analysis result

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3.12 Bluetooth fault
Software version is correct
Bluetooth can’t work
OK
Re-download softwareNo
Is BT antenna assembled
correctly?
Yes
No Re-assemble BT antenna
Yes
Replace new mainboard
Yes
Check BT circuit
No short circuit or open circuit No Mark the mainboard with analysis result

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3.14 No signal
Is RF antenna assembled correctly?
No RF signal
OK
Re-assemble BT anntenaNo
Is the phone set into Flying
mode?
Yes
Yes Return to Normal mode
No
Replace new mainboard
No
Check RF circuit for any fault Yes Mark the mainboard with
analysis result