Friday, August 16, 2019

Troubleshooting dead LCD-LED television


You are sitting in your shop and an LCD/LED television is brought in by the customer, where do you start?
First before the customer leave it is better to connect the television to the power source and confirm what he or she has said.

Also it is important to get a brief history of how the problem developed, this sometimes comes in handy when it comes to diagnosis but please don’t take any advice from the customer when they suggest what they think is the problem They are not professionals and therefore will always lead you to the wrong place.

After you connect the power source, apply the power via the power switch, and observe the power ON led at the front.

1.     Television is dead, Power led is ON.
If the power ON led is red, press the power ON button on the remote control or if you don’t have the remote press the front buttons one by one. One of them will kick start the set if all is well.

If the power ON led is present, this means the television standby voltage is present, the main board is also getting power, and keyboard/IR board is also getting power (3.3 v standby)
Tip
3.3Vdc is used to power the system control unit,
 power ON led, and the IR receiver. This means whenever you see power ON led is on means the standby circuit is okay and the 3.3 Vdc regulator is okay and therefore don’t waste time trying to test them and therefore you should concentrate on other outlets
So even without opening the set you can be sure the standby voltage is okay.

If when you press the power ON button from the remote control and the power ON led does not turn blue means there is something blocking other power to start and therefore you can now open the television and continue with your troubleshooting.

Open the television slowly and carefully because LCD-LED television are fragile not like the CRT televisions especially the screen.

Turn it upside down and use a soft cloth under the screen to avoid scratches. After opening the many screws and removing the cover, do a quick scan of the board component wise looking for any suspicious component like I said before, component with bulged tops are considered bad, burnt marks on components and circuits' board are considered as stress and you should pay close attention to them.

Any suspect components should be subjected to further tests but with capacitors like I said once a capacitor top is swollen even slightly it is considered faulty.


This is because a bulged capacitor is usually short and if you check it with an ESR meter it will read very low ohms and as you know low ESR reading means the capacitor is okay and this will not be true since the meter will be seeing low because of the short and not the actual ESR reading.

With those remarks, it is time to pull your meter and do actual voltage testing, Voltage testing is one of the finest troubleshooting methods because sometimes you may find that the components are all okay but the track/PCB is broken and therefore blocking voltage from reaching other components, also this will help pick any open components quite easily.

Also voltage testing can help you narrow the problem to the affected block

To do voltage testing on the primary side of the S.M.P.S use the main capacitor as your ground (HOT) and for the secondary side of the power supply use the COLD ground (Television body is usually cold ground)


Please note the reason we started with the voltage testing is because we had power ON led lit otherwise if it was full dead symptoms, no signs of power whatever we could have started testing any safety components like fuses open on the board.

Once you clip your meter negative probe on the HOT ground which in our case is the main capacitor negative pin, now it is time to probe every point of interest on the primary side.




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