How To Fix LCD/LED TVs Black Screen - Molkobain I/O

Ever had your TV showing nothing but a black screen even if the audio was working? Unfortunately, that’s a common issue with low/middle-end LCD/LED TVs these days… Even more frustrating, this issue often comes from a rather tiny and cheap component that can be easily replaced. Most common issues are:

• Defective capacitor on the power supply board
• Defective LED in the backlight system

In this post, we’ll take a look at the latter and at one way to fix it.

One of my relatives had this exact symptom happening all of a sudden. This problem on low-end TVs often occurs within the first couple years. As the repair costs for that kind of TV is pretty low, considering repairing it yourself might be a good idea!

Diagnosis

The first step into repair is to find the root cause of the issue. As backlight failure is a very common issue, this is the first thing to test. To do so, the easiest way is to power on your screen, put a flashlight very close to it and check if you can see the image through. The image would be very dark, like turning the brightness of the screen very very low.

Now that we now the image itself is fine, this means the main board is probably fine too, so we are going to test the backlight system itself.

That implies disassembling the TV to access the backlight which is between the LCD screen in the front and the boards in the rear. In my case, with a Samsung F5000, I had to process as follows:

Disassembly

First we have to remove the back housing to reveal the boards (from left to right: main board, T-CON, power supply) and disconnect the LCD panel from the T-CON board.

Turn the TV around to remove the front housing and the LCD panel. Be very careful with the panel as it is extremely fragile!

Now that LCD panel is removed, we can remove the bezel that maintain the backlight diffuser and access the backlight itself.

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Here we can see that the backlight system is made of LEDs which is pretty simple to replace when being the cause of the failure.

Note: Older TVs have neon tubes for backlight, which is thicker and less exposed to this kind of failure. LED backlight is the most common thing these days, but do not mistake an LED TV with an OLED TV. The first one is a classic LCD panel with a LED backlight, whereas the second is an OLED panel that doesn’t need any backlight as it is integrated in each pixels (making the spare parts much more expensive by the way).

So, let’s remove that white cover which is part of the light diffuser to have a good look at the backlight.

As we can see, the backlight system is made of 5 LED strips. First thing to do is look for burnt LEDs. Most LED backlight systems have strips set in series, meaning that if one of the them fails, all the system goes dark…

Note: If you look closely, you’ll see that one looks different than the others! 3rd strip from the top, 6th LED from the left. We’ll test it soon!

Testing

Using a multimeter, we can confirm that the strips are indeed set in series, so now we have to test each strip individually. Professionals use LED testers such as this one (about 40$ on amazon) but as I didn’t had one at the time, I decided to make one, McGyver style! 🤓

One LED like those ones typically needs between 2.5-3.6v input voltage to light up. By looking up this model online, I found out that the ones used on those strips need 3.6v; so as there are 9 LEDs per strip: 3.6 x 9 = 32.4v input voltage required to light a single strip. That’s the maximum voltage we do NOT want to exceed, otherwise LEDs could be damaged during testing.

So, I took 3 9v batteries that were laying in a drawer, put them in series to make a virtual 27v battery (3 x 9v). It’s less then the optimal 32.4v required but not that much lower, it might be able to light the strip a bit so we can identify which is not working. Here is a look at the set-up:

The thing with 9v batteries is that they are made to be plugged with one another! So you just need to put some cables on both ends and there you go.

Now we simply have to test each strips individually to see if they light up or not. For each that doesn’t completly, it will mean it has at least one defective LED.

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After repeating this operation on all strips, I found only 1 defective LED, the same we thought looked burnt when we first had a look at the backlight (3rd strip from the top, 6th LED from the left). For a better understanding at what a burnt LED looks like here are 2 pictures of a burnt one and an OK one. Mind the roasted color compared to the regular one.

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Repair

Now that we have identify what seems to be the issue, we have 3 options:

• Replace the whole strip with a new one
• Replace the LED with a new one
• By-pass the LED and cross our fingers that the backlight will still be homogeneous enough

For starters I’ll go with the third one, just to make sure there is no other issues with the TV, but afterwards it’s better to replace the LED with a new one, otherwise you might notice a darker spot on the image.

Usually we need a heater to remove the LED properly but I had none. So after a few try with a hairdryer, I went messy and soldered a wire below it. 🙈

Once we have by-passed the LED, we can power the TV on. Careful! High voltage (200-300v) runs through the TV when plug, so be very careful how you handle it so you don’t electrify yourself!

And Voilà! all the backlight should light up again.

Now we just need to unplug the TV, replace the LED with a new one and put everything back together. Just to be sure, we should power the TV back on and check that everything is fine.

Conclusion

There might be a lot of other root causes for similar symptoms, a black screen often looks like something very serious and therefore expensive to repair, but this case is the perfect example that taking some time to look for the root cause can sometime lead to a good surprise: here a 1$ fix!

And most of all, repairing is always better than throwing away! 😊