akira LCT-20CHST, Руководство по обслуживанию

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Model No.: LCT-20CHST
Version: 1.0
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
“AKIRA” 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 connection with key-press panel
、 remote 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 2, 4 pin of U13NTMS10P02 is
0.2V, and over-heat, the reason must be short circuit of power supply backend. You 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, image, sound 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 38
th
, 39th 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 large, measure
the resistance of R23 is 10K Ω , but the rating resistance is 1K Ω , so it is obvious that the
magnitude of horizon sync impulse is attenuated too large caused by value of couple resistance
R23 become large. So the pixel clock inside of the MST518 can not vibrate, result in the A/D
converter of MST518 internal can not work normally. So appear no picture in VGA mode and
“signal beyond boundary” on TFT screen.