Service manual
SERVICE MANUAL LS07 chassis
24
2 The example of service
Example 1 Can’t turn on the TV
Troubleshoot phenomenon: after connect power supply and turn on the
power supply ,the “changhong “logo will not appear on screen. Press the key or
use the remote control, the TV has no reflect, no display and black screen.
The process of maintenance : remove the plug of adapter output, measure
adapter output voltage is +12V,the adapter works normally. check the
connectiom with key-press panelremote control panel and mainboard. if it is
ok, the problem must be MCU not work. First check supply power of U2 chip.
Connect power supply, then measure the voltage of the 24 pin of
U13NTMS10P02 is 0.2V,and over-heat , the reason must be short circuit of
power supply backendYou should remove the plug quickly, otherwise the
chip must be bad. Use the multimeter to measure confirm it is short circuit to
ground. the places which connect U2 with 3.3V voltage are so many, you
should disconnect and measure it one by one. when you disconnect the C68,if it
is normal,C68 grounding Short circuit.
Example 2 No picture in VGA mode
Troubleshooting phenomenon: in TV/AV mode ,the image and sound is ok,
PC (in VGA mode) is no picture, but have state icon.
The process of maintenance: first check the signal source, signal socket
plug are ok or not. Because during switch the mode, the image display
normally in non-PC mode ,it is proved that the backend image public
processing channel is ok, namely U3MST518 and backend image signal
processing circuit work normally. In PC mode, measure the arbitrary pin of
4,7,12 pin ,image date terminal of U6P15V330A by oscillograph. there are
no waveform, change U6P15V330A and the Troubleshooting phenomenon
disappear.
Example 3 No picture in VGA mode and the “signal beyond boundary” is
appeared on screen.
Troubleshooting phenomenon: no picture in VGA mode and the “signal
beyond bound” is appeared on screen. in TV/AV mode, imagesound is ok.
The process of maintenance: first check the signal output from the signal
source is not standard or beyond the resolution which is supported by TFT.
except of the two cases, the phenomenon comes from the TFT display. Under
general circumstance it is caused by the abnormal of the horizon sync
impulse ,vertical sync impulse of VGA signal. Inspect the 38th39th pin of U3
by oscillograph, find the magnitude of horizon sync impulse is below the
normal, measure the waveform of 13
th
pin of VGA J1 ,if it is ok, so maybe
resistance of the R23 will be largen, measure the resistance of R23 is 10K
,but the rating resistance is 1K , so it is obvious that the magnitude of
horizon sync. impulse is attenuated too large caused by value of couple