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Sharp LCD TV No Power
This sharp lcd tv model: LC32A33M was tested to be dead and unable to power up neither with a remote or “power on” button . When the back cover was removed it was quite obvious of the fault. This TV has been struck by lightning or power surge with black flashover marks on the power supply board (RUNTKA419WJN1) around the Varistor ( photo 1).
Photo 1- Burnt marks due to power surge
The two fuses were tested and found to be blown. The function of the Varistor VA7103 was to reduce the excessive current due to voltage surge. The excessive current caused it to overheating or overload and breakdown. The resistance measured on the terminals is only a few hundred ohms. The Varistor was temporary removed and a 100w bulb were connected across the blown fuse. When the power was turned on the bulb lights up at continuous full brightness indicating there were still short circuit on the power supply.
I checked on the smps power IC 7101 VSS(pin4) and found to be short circuited to ground .The IC and Varistor were replaced and retested again. The bulb lighted up brightly initially and slowing started to dim indicating there were no more short circuit, but the tv was still dead with no standby power or output voltages on the secondary. Couldn’t find any shorted components on the secondary or primary side of the smps.
It is much easier to diagnose and repair with a schematic. I could not find the schematic for RUNTKA419WJN1 but by coincidence I found another model power supply (RUNTKA396WJQZ) with the exact footprint and components layout on the board. The only difference is the smps power IC 7101 marked as TNY276PN instead of TNY274PN. Checked with the datasheet showed that it is from the same family but with different power rating (Photo 2)
Photo 2- power supply schematic for RUNTKA396WJQZ
Pin 4 voltage was found to be normal i.e 340Vdc and the smps should work unless it is in shutdown mode. I observed that pin 2 was designed to monitor the output voltages and any abnormally or high voltages will result in the shutdown mode as stated in the IC data sheet. I checked all the components connected to pin 2 and found C7124 to be defective or very low capacitance.
After changing the capacitor, the smps turn on with the standby voltage of 5.1v as shown in photo 3. The TV can now turn on as normal with the remote control or the TV side panel power button.
Photo 3-standby voltage of 5.1v
This article was prepared for you by SK Wong from Malaysia and he graduated from London with a degree in Electronics and Electrical Engineering. He has more than 40 years of experience in the consumer product manufacturing. With the passion to repair electronic products from young , he has been doing it part time until now.
Currently, he can provide turnkey solutions for consumer products including services from idea or concept to design , prototyping , finished products (including electronic,mechanical and plastic parts) and manufacturing.
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