When repairing the switching power supply of a color TV, it's common to encounter situations where components are damaged upon turning on the device, making it difficult to identify the issue without powering it up. In such cases, you can disconnect the AC input and apply a DC voltage to the primary filter capacitor (rectifier output, usually around 100μF/400V to 330μF/400V). Normally, this voltage is about 300V, so using a regulated DC power supply with around 30V can help test the circuit safely. The original power supply won't start, but you can reduce the starting resistor by 10 times to check if the power supply can be activated.
If the secondary output shows a voltage, like +B reaching around 10V, it suggests that the switching power supply has started oscillating, and the fault may lie in the load circuit. At this point, you can restore the original starting resistor for further testing. However, if there's no output voltage, it means the problem is likely in the power supply itself, and further troubleshooting is needed.
The image above shows the schematic of the Yellow River 54FS power supply. R616 (330k/2W) is the starting resistor. After replacing it with a 33kΩ resistor, the measured voltage was 12V, and the voltage across C617 was 2.3V. If the output voltage is 0V, it indicates an issue with the power supply. This method allows safe testing of whether the switching power supply is functioning without risking damage to the circuit or exposing yourself to the main power grid. I often use this technique when repairing switching power supplies, and it has proven very effective. Readers are encouraged to try it as well.
This approach not only helps avoid unnecessary damage during diagnosis but also provides a clearer path to identifying the root cause of the failure. It’s especially useful when dealing with complex or sensitive circuits where direct power-up could lead to further complications. By carefully controlling the input voltage and monitoring the output, technicians can make informed decisions before proceeding with more invasive repairs.
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