Changhong Jingxian Wang rear projection TV does not boot black screen fault repair example - Home Appliances

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Changhong Jingxian Wang back cast color TV does not boot, black screen repair Changhong HP Jingxian Wang series rear projection color TV using PDT-3 movement. "No boot / black screen" is a common fault of this series of color TVs. This article discusses its maintenance ideas for reference.
First, judge whether the power board is normal (measured by the road and the out-of-way voltage). If it is not normal, check whether the 380V voltage across C810 (main filter capacitor) is normal. If it is not normal, check the PFC circuit and the previous circuit; if it is normal, check whether NQ821 (STR6656) is working, and whether the 8V, 5V, 22V, 12V and +B voltages of the secondary of the switching transformer are normal or not (the movement adopts Dual power supply, as long as the other group of power supply is not short-circuited, its operation will not affect the boot). If the 5V voltage is lost, the whole machine cannot be turned on normally; if the 8V voltage is lowered, the black screen will be generated; if the 12V voltage is abnormal, the main circuit will work abnormally and will not be turned on; 22V is the line scan starting voltage, if not, it is impossible. Power on; without +B voltage, it will not be able to boot.
During the repair process, it should be noted that in the power-on state, never remove the 8V power plug. Otherwise, the projector will be burned when it is turned on again.
If the measured +B voltage is 0V, check the ground resistance of the +B output. At this time, the output tube is mostly damaged. The movement uses a double scanning circuit. You can pick W034 to judge whether the two tubes V404 (C5588) and V481 (C5143) are damaged. (In actual repair, C5588 has a high damage rate. ). If the switching power supply and the line output tube are both normal, the line excitation signal and the line oscillation signal can be measured. If the row pushes the work, it will cause the row output tube to work abnormally and cause burnout.
The normality of the line oscillating signal is closely related to the NZ401 (TDA9111) of the main board. The 5V and 12V power supply of the integrated block can be measured first. Whether the line, field signal is output normally from the integrated block, and the voltage of the bus (SDA, SCL).
The bus of the PDT-3 movement is output by the CPU board. When checking, first check whether the 8MHz crystal oscillator works normally or not; check whether the 5V power supply is normal; finally check whether the U4 (W27C010-70) integrated circuit is normal. In addition, the 24C64 failure rate of the CPU board of this machine is also very high.
Whether the bus signal of the DPTV board is normal or not will directly affect its working status. If it is abnormal and the 8V power supply is abnormal, it may cause a black screen. In addition, abnormalities in the DPTV board may also cause burnout.
N20 (KA2500) is a pre-viewing integrated block. When it works abnormally, if there is no 12V power supply, there will be a black screen failure. Of course, the integrated block itself will also cause this phenomenon. The NQ911 (TDA6120Q) is a final-level video display integrated block, and its operation is abnormal, and a black screen will be generated.
Finally, during the repair process, if the power-on indicator does not light up and the degaussing sound is not heard, the power board and the scan board should be checked first; the indicator light is on, but it cannot be converted. The CPU board should be checked first; if the black screen has sound, it should be Check the DPTV board.