TCL T728 Service Manual
TCL Mobile Communication
CO., Ltd
第
28
页
共
32
页
circuit. If the trouble occurs, should check these relative parts, under the control of the
signal outputted by U400.
3
、
Maintaining steps
：
3
．
1 General speaking, if there is only one Led do not light, there probably has the
trouble only with that LED. In addition, check if there is an abnormal of the value
of the resistance serially connected with the LED. If it is ok, check if the 41st pin
of U400 is a low level, otherwise check the input and output circuit parts of U400
；
3
．
2 If the 41st pin of U400 is normal
，
please if there is a 4.2v at the anode end of the
key-LED. This voltage is supplied directly by battery
，
if it is abnormal, please
check the power supply circuit part of battery.
3
．
3 The backlight of LCD is emitted by two radiation diode. The controlling signal
(LCD-BL) comes from the 38
th
pin of U400.The diode will emit light while the
level of this signal is low. if this signal is abnormal, please check the input and
output circuit parts of U400
；
3
．
4 If the signal from the 38
th
pin of U400 is right, measuring the voltage on the
resistance serially linked the LED. This voltage is supplied by battery directly,
and should be 4.2V or so. If it is abnormal, please check the power supply
circuit part of battery.
Circuit for backlight for keypad:
文件使用
"pdfFactory Pro"
试用版本创建
