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Troubleshooting of LG 42" power board
Instruction of system and time sequence control
The whole system is composed of EMI, rectify, PFC, DC/DC and MCU. Therein MCU is the control
core of the whole system. On the one hand, it controls ON/OFF time sequence of each voltage and
the main, AC voltage test. On the other hand, it performs the standby CPU function with IIC bus to
the main CPU.
Time sequence
The system has two working modes: AUTO and NORMAL, AUTO mode is used for adjustment and
service, while NORMAL mode is used for TV at normal time. At AUTO mode(short X610), connect
5VCTRL with load of about 2A, turn on power, then output several low-voltage power supplies,
delay 1.5s or so and output Va, Vs. First stop the output of Va, Vs at power-down, and active
discharge to output-capacitor of Va,Vs. on condition that the voltage Va,Vs each is lower than 10
percent (if unsatisfied, then wait), then delay 0.5s and stop all the low-voltage power supplies
except for 5VSB into Standby mode.
At NORMAL mode, MCU receives power-on control of IR or button board, outputs several
low-voltage power supplies, after main CPU sending Va,Vs start-control through IIC bus, outputs
Va,Vs. at the status of turn on, MCU receives power-down control from main CPU and performs
power-down time sequence (power-down time sequence is the same at AUTO mode).
At the two modes, when detect the lower input AC voltage, it will not perform power-on. At the status
of turn on, when detect the lower input AC voltage or 5VCTRL,Va,Vs beyond normal scope value, it
will perform the power-down time sequence above and enter into Standby mode.
Trouble diagnosis
The voltage 5VSB is normal, while the voltages of other circuits are zero, in this case, it may switch
power on at AUTO mode and check if 5VCTRL delay 1.5s or so, if NO, check 5VCTRL and its
fore-circuit; if YES, check LED (LED601), if LED lights, check the circuit of Va, jf not, check the
circuit of Vs (note: premise N608 and its periphery circuit are normal). If the voltages of other circuits
are abnormal, check the corresponding circuits.
Trouble phenomenon (premise the voltage 5VSB is normal)
The main voltage of power board is abnormal, which is commonly detected to enter into Standby
mode, the phenomenon is that switch power off immediately after switching power on.
AC detect circuit or IR, button circuit is abnormal, which will lead to unable to switch power on.
The low-voltage power supplies are normal, while the voltage Va, Vs is zero, it maybe
communication abnormal of IIC bus between MCU and main CPU.
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