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, it was found that the backlight wasn't lighting up. The voltage across the LED boost filter capacitor (250V / 22UF) was measured at 98V, which matched its supply voltage. This indicated that the BOOST circuit wasn't functioning properly, and the issue wasn't related to the LED strip itself. Further testing of the LED driver block N701OZ9902CGN (16-pin) showed normal readings: pin 1 (UVLS) had about 3.8V, pin 2 (VCC) had 12V, and pin 3 (ENA) had around 4V, which was within the expected range. The main chip had also sent a backlight-on command. After turning off the power, the BOOST circuit was checked, 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.

After examining several similar cases, it appears this is a common failure point. Below is a partial schematic of the actual LED boost circuit. The BOOST circuit primarily consists of components like V701, L701, L703, VD751, C753, R715, R714, and others. It operates similarly to a 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 backlight 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. During this phase, a voltage is induced across L701 and L703, causing VD751 to turn off while energy is stored in the inductors. When the PWM signal goes low, V701 turns off, and due to the nature of inductors, the polarity reverses. This causes VD751 to conduct, allowing the stored energy from L701 and L703 to charge C753. The measured voltage across the 250V/22UF capacitor C753 was approximately 156V.