Skyworth LCD TV 42E300: The unit is not powering on, and the power indicator is not lit at all. It appears to have no power. After a quick inspection, I noticed that the circuit design is a bit unique—there's no secondary power supply. Instead, the STB5V power supply 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 two voltages.
When measuring the voltages, I found that 24V fluctuates around 3V, and 12V is also lower than expected. However, the main filter capacitor shows 300V across its terminals. Based on this, I considered two possible causes: either a load overcurrent or an active protection mechanism.
After checking the load for any obvious short circuits, I confirmed that there’s no immediate issue. The power IC used is TEA1753, and the voltage at pin 10 (overcurrent detection) was normal, so I ruled out overcurrent as the cause.
Further inspection revealed an overvoltage protection circuit on the secondary side. Since I couldn’t find the schematic, I reconstructed the relevant part based on the actual board. From the diagram, it looks like there's an overvoltage protection circuit for both 12V and 24V. Q5 and Q13 form a kind of thyristor-like structure. When either 12V or 24V exceeds the threshold, ZD4 or ZD5 turns on, causing Q5 and Q13 to saturate. This increases the current through the optocoupler U7, which in turn lowers the voltage at pin 3 of the TEA1753, putting the chip into intermittent operation.
I measured the B-E junctions of Q5 and Q13 and found a positive bias. When I shorted the B-E junction of Q5, the output returned to normal, suggesting that the system might be mis-triggering the protection. After further testing, I discovered that Q13 had a C-E leakage. Replacing it restored the unit to full functionality.
One thing I want to note is that this model operates in a low-power standby mode, where the PFC (Power Factor Correction) circuit is inactive. In this state, 12V and 24V are normally output, but the load capacity is reduced. Once the TV boots up, the PFC activates. During standby, you might hear a slight “click†sound, especially when a dummy load is connected. This is actually a normal behavior.
Power Board
Relevant Part
[attach]415662[/attach]
Related Circuit
Used on systems above 1000 volts, protect equipment from lightning and switching surges.
Lightning Arrester,Distribution Pole,Composite Lightning Arrester,Lightning Rod
Jilin Nengxing Electrical Equipment Co. Ltd. , https://www.nengxingelectric.com