Konka LCD TV LED32E330C sound black screen troubleshooting example - Appliances

LED light-emitting diode

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A 32-inch Konka LCD TV was reported with a black screen. After disassembling the unit and checking the power supply, it was found that the backlight was not lighting up. Further testing revealed that the voltage across the LED boost filter capacitor (250V / 22μF) was 98V, which matched its input voltage. This indicated that the BOOST circuit was not functioning properly, and the issue was unrelated to the LED strip itself.

The LED driver block N701OZ9902CGN (a 16-pin IC) was tested. Pin 1 (UVLS) showed about 3.8V, which is the undervoltage protection detection pin. Pin 2 (VCC) had 12V, and Pin 3 (ENA) had around 4V, both within normal ranges. The main chip had sent a backlight-on command, so the problem wasn't there. After turning off the power, the BOOST circuit was inspected, and it was quickly discovered that the LED boost diode VD751SB2200 was shorted. Replacing this diode resolved the issue, and the TV worked properly after testing.

Upon reviewing similar cases, it appears this is a common failure point. Below is a partial schematic of the actual LED boost circuit. The BOOST circuit includes components like V701, L701, L703, VD751, C753, R715, R714, and others. It functions similarly to the PFC circuit in a switching power supply, where L701 and L703 act as energy storage inductors.

When the PWM signal from the 15th pin of the LED driver IC N701 is high, the switching transistor V7014N25 turns on. This allows current to flow through L701, L703, V701, R714/R715, and back to ground. At this moment, a voltage is induced across L701 and L703, and VD751 remains off, allowing energy to be stored in the inductors.

When the PWM signal goes low, V701 turns off. Since the current through an inductor cannot change abruptly, the polarity of the induced voltage in L701 and L703 reverses, causing VD751 to conduct. This allows C753 to charge, while L701 and L703 release their stored energy. The measured voltage across the 250V/22μF capacitor C753 was approximately 156V.