A .The backlight works well, when white-screened, first of all, check whether the indicator light is changing color, if it does, the main board works well.

1.Check whether the wire between main board and the screen is connected well---Using spare wire or changing the screen.

2.Check whether every working point to see whether the supply Voltage is normal.

3. Check Line-field signal and Time-clock signal with Oscillator.

B.  When the indicator light doesn’t light-up, which shows the main board goes wrong.

1. Pay attention to the voltage of EPROM(around 4.8V), Resetting Voltage( high level or low level, depend on different machine type), and Voltage of CPU. When short circuit happens, check its position carefully, and it’s possible that the short circuit is on the copper foil of PCB

2. Check whether t CPU Pins is well connected with main boards.

3. Check whether main board IC and CPU work well, and check whether Crystal Oscillator can start Oscillation.

4.when necessary, change CPU or reburning CPU