slightly higher standby voltage in smps board in LED TV

Thread Starter

electron_jj

Joined Aug 28, 2021
14
Hello all, I have been trying to troubleshoot a smps board in a LED TV that would not turn on. There seem to be standby voltage as there is a red light blinking when the power button is switched on but just no sound or display. According to the service manual there should be a 5.8V standby voltage but when I checked it with DMM, the voltage reads 6.15V. Could this 0.35 voltage difference cause the LED TV not to turn on? Or such difference should be fine.

The 3 fuses are all okay and I have checked the various diodes using DMM diode modes and they all seems okay as well. Also there are no physical signs of any capacitors or resistors blown or enlarged.

Does anyone knows what would be the likely cause of the problem? I have attached a schematic of the power supply board for your reference. Thank you for any suggestions!
 

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AlbertHall

Joined Jun 4, 2014
11,610
According to the service manual there should be a 5.8V standby voltage but when I checked it with DMM, the voltage reads 6.15V. Could this 0.35 voltage difference cause the LED TV not to turn on? Or such difference should be fine.
I doubt the voltage difference is the problem.
Did the LED flash or was it continuously lit before this fault appeared?
SMPS often have protection circuitry which shuts down the supply if a fault is detected. Then the supply tries again. This can lead to it pulsing as may be inducated by the flashing LED.
 

Thread Starter

electron_jj

Joined Aug 28, 2021
14
I doubt the voltage difference is the problem.
Did the LED flash or was it continuously lit before this fault appeared?
SMPS often have protection circuitry which shuts down the supply if a fault is detected. Then the supply tries again. This can lead to it pulsing as may be inducated by the flashing LED.
Thanks. The LED should be normally be lit but instead it is now flashing 4 times and pause for a second or so and then repeat flashing 4 times. I have had a look at the technical support guide for this LED TV as attached but there is very little guidance as to what flashing 4 times means as there seem to be many possibilities. According to the guide, flashing 4 times indicates a F12 15V SOS. But I am unable to measure any voltage at the 12V at the DC output. Is this because the the protection circuit has shut down the 12V output circuitry? and the only working circuitry now is the standby voltage only? From the schematics, it seems that the 12V is been produced by another separate transformer from the one that produces the 5.8V standby voltage.

Another strange thing I noticed is that although there is a reading of 6.15V standby voltage, there is almost no voltage measured at the AC inputs or the DC outputs in the bridge rectifier (D102 in the schematic), ie. only around 0.5volts or less. I dont understand how standby voltage of 6.15V can be sustained if there is so little voltage at the bridge rectifier.
 

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AlbertHall

Joined Jun 4, 2014
11,610
Yes, I think the system has shut down the supply, possibly by not activating S101 and S102. The standby voltage is produced by the D105-D108 bridge rectifier.
 

Thread Starter

electron_jj

Joined Aug 28, 2021
14
Yes, I think the system has shut down the supply, possibly by not activating S101 and S102. The standby voltage is produced by the D105-D108 bridge rectifier.
Thanks! Are there any common types of faults that would cause this sort of shut down? Is it most likely to be caused by bad resistors, capacitors and diodes? Or the problem is most likely one of the control chips or SMD components? The power board's flip side contains loads of SMD components and various IC chips. The main side of the power board seems easier to check as they are mostly comprise of the capacitors, resistors, inductors, transformers, transistors and a few photocoupliers.
 
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