Jonathan Lam

EE/CS @ The Cooper Union


Blog

On replacing a broken laptop screen backlight

On 9/9/2021, 12:36:11 AM

Return to blog


Update 12/20/21: I picked up another laptop with a broken screen from the computer center. This time the LCD was broken. In the process of ripping it apart to see the model number (in this case, the screen was glued to the front glass of the monitor assembly), I shattered a lot of glass. Newer and thinner notebooks are much scarier to take apart :(

I picked up an old Dell Latitude E6420 from our computer center, which was about to be thrown out because its monitor was broken. As I mentioned in previous posts, I already own a Latitude E5420 (basically product line but one generation older, also picked up from the computer center) and love the solidness of the business laptop feel. The performance of both Latitudes are roughly on par with my main laptop, a 2017 Acer Spin 3, to the point where I don't really feel a difference.

This is the second time I'm "servicing" a Latitude, the first being when I repaired the keyboard on my previous Latitude1.

So I took the Latitude to the lab to see what exactly was broken. When turned on, I could faintly see the text. Interestingly, when someone turned off the lights in the lab, I couldn't see the text anymore! I thought at that point that it was a transient issue, that maybe it only showed text for a brief moment when turned on. But when my friend R came over and I asked about it, he took his phone flashlight and shone it one the screen, and voila -- the text showed up. The backlight was broken, but the LCD was working.

I didn't know anything about screen technology up until now, but it turns out that the LCD displays the image but do not emit light on their own, in the same way that Kindles' so-called "e-paper"2 cannot be read in the dark.

LCD displays require a "backlight" that evenly illuminates the whole screen. In lieu of covering the entire area with individual LEDs, the backlight is usually implemented as a light-emitting strip (older screens used CCFL lamps, which have been replaced with LED strips) on one edge of the screen, which is diffused evenly throughout the screen via several special purpose layers.

Diagram of the LCD structure

If you look up backlight repair on YouTube, you will most likely get videos of replacing older CCFL lamp backlights on TV's. I couldn't find a tutorial for the exact screen model3 (and didn't know I was supposed to be looking for a LED backlight as opposed to a CCFL one), so I basically went straight to disassembling the screen.

I mean to have a lot more here and photo documentation of the process. However, I was hasty and didn't have the diligence to follow through on that, and ended up destroying the screen in the process. You're probably better off following tutorials on YouTube for LED strip backlights, but here are some of the things learnt:

TODO: add some photos

At this point, I had bent the part of the frame that held the backlight and cracked two corners of the LCD. Luckily, in my case I had gotten the laptop for free and a screen replacement for this older screen is only about $35 (laptop screen replacements can easily run over $100), so it was well worth the learning experience.


Footnotes

1. And I may have to service the new laptop once again in the near future, since it idles at 60deg C and goes up to 100deg C. The thermal paste has probably degraded.

2. Or regular paper, for that matter.

3. In my case, MJ154 LTN140KT04.


© Copyright 2021 Jonathan Lam