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

Probe domestic switch needle KG-300K, with a head diameter of 3.0mm, normally open switch needle

1.5PF SOT23-6 Low Capacitance 5V Breakdown Voltage 6V SRV05

Filter 18.432M

Changhong Jingxian Wang back cast color TV does not boot and shows a black screen. This article provides repair guidance for the Changhong HP Jingxian Wang series rear projection color TV that uses the PDT-3 movement. The "no boot / black screen" issue is a common problem in this model. The following maintenance steps are shared for reference. First, check if the power board is functioning properly by measuring the input and output voltages. If it's not normal, check the 380V voltage across C810 (main filter capacitor). If this is abnormal, examine the PFC circuit and its preceding components. If the voltage is normal, test whether NQ821 (STR6656) is operating correctly and verify the secondary side voltages of the switching transformer: 8V, 5V, 22V, 12V, and +B. Since this model uses dual power supplies, the failure of one group will not affect the other unless there's a short circuit. A loss of 5V will prevent the TV from powering on. A drop in 8V may lead to a black screen, while an issue with 12V can cause the main circuit to malfunction. The 22V voltage is the line scan start voltage, and without it, the TV cannot power on. Similarly, the absence of +B voltage will also prevent booting. During repairs, it is crucial not to remove the 8V power plug while the TV is powered on. Doing so could damage the projector when it is turned on again. If the +B voltage reads 0V, check the ground resistance at the +B output. In most cases, this indicates a damaged output tube. The PDT-3 movement uses a dual scanning system, so you can test W034 to determine if the transistors V404 (C5588) and V481 (C5143) are faulty. In practice, C5588 has a high failure rate. If both the switching power supply and the line output tube are working properly, measure the line excitation and oscillation signals. An issue with the row drive signal can cause the row output tube to fail or burn out. The line oscillation signal is closely related to the main board’s NZ401 (TDA9111). First, check the 5V and 12V power supplies for the IC, then confirm if the horizontal and vertical signals are being output correctly. Also, check the bus voltage (SDA, SCL). The bus signal for the PDT-3 movement is generated by the CPU board. When troubleshooting, first check if the 8MHz crystal oscillator is working. Then verify the 5V power supply, and finally test the U4 (W27C010-70) IC. Additionally, the 24C64 chip on the CPU board has a high failure rate. Abnormalities in the DPTV board's bus signal can directly impact its performance. If the bus signal is unstable and the 8V power is also problematic, a black screen may occur. Moreover, issues with the DPTV board can also lead to component failures. N20 (KA2500) is the pre-view IC. If it fails or lacks 12V power, a black screen may result. The NQ911 (TDA6120Q) is the final video display IC, and any malfunction here can also cause a black screen. Finally, during the repair process, if the power indicator doesn’t light up and no degaussing sound is heard, start by checking the power and scan boards. If the indicator lights up but the TV won’t turn on, focus on the CPU board. If the screen is black but there is sound, the DPTV board should be inspected.