Skyworth LCD TV 42E300: The unit is not powering on, and the power indicator is not lit. It appears to have no power at all. After examining the circuit, I noticed it has a unique design without a secondary power supply. Instead, the STB5V power board is empty, and 12V and 24V are directly sent to the motherboard. I suspect that the standby power is supplied by one of these voltages.
Measuring the voltage, 24V fluctuates around 3V, and 12V is also lower than expected. However, the main filter capacitor shows 300V across its terminals. Based on this, there are two possible causes: [1] Overcurrent in the load or [2] A protection mechanism being triggered.
After checking the load for any obvious short circuits, I found that the power IC used is TEA1753. The overcurrent detection pin [10] shows a normal voltage, so overcurrent is ruled out. Further inspection revealed an overvoltage protection circuit on the secondary side. Since I couldn't find the schematic, I reconstructed the circuit based on the physical layout.
From the diagram, it looks like there's an overvoltage protection system for both 12V and 24V. Q5 and Q13 form an analog thyristor. If either voltage becomes too high, ZD4 or ZD5 turns on, causing Q5 and Q13 to saturate. This increases the current through the optocoupler U7, which lowers the voltage at pin [3] of the TEA1753, putting it into intermittent operation.
When measuring the B-E junctions of Q5 and Q13, they showed a positive bias. I tried shorting the B-E junction of Q5, and the power output returned to normal. This led me to suspect that the system was triggering a false protection signal.
After further testing, I discovered that Q13 had a C-E leakage. Replacing it restored the unit to full functionality.
One thing I’d like to note is that this TV operates in a low-power standby mode, where the PFC (Power Factor Correction) is not active. In this state, 12V and 24V are normally output, but the load capacity is reduced. Once the TV boots up, the PFC engages. During standby, you might hear a slight “click†sound, especially when a dummy load is connected. This is a normal behavior and not a sign of malfunction.
Power Board
Relevant Part
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Related Circuit
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