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X220 keeps blowing the backlight fuse

X200/X201/X220 (including equivalent tablet models) and X300/X301 Series
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Andrea9001
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Joined: Mon Jun 20, 2022 12:56 pm
Location: Bergamo, Italy

X220 keeps blowing the backlight fuse

#1 Post by Andrea9001 » Mon Jun 20, 2022 2:53 pm

Hello everyone!
Some time ago I had a little incident with my X220 (i5 model).
My AC charger, already known to be a bit faulty, started making some loud noises while I was using the laptop, which suddently turned off. When I restarted it, the screen backlight was dead, even in the BIOS menu. Everything else was functional though, including the display (I could see images on it using a strong flashlight).
Following the advices on this forum, I tested the fuses. F1 was blown, while F2 and F6 were still functional.

So I grabbed my soldering iron and replaced the F1 fuse. I turned the laptop on with the screen disconnected, and the fuse "survived". But after booting with the screen connected, and noticing that the backlight was still dead, I found out that the F1 fuse was blown again!

Following this thread in particular, I tried to diagnose the problem. Please note that I didn't bridge the F1 fuse like others did (I'm too scared to do that :? )
Here's some measurements I took with the laptop ON and the screen connected:
  • PANEL_BKLT_CTRL (pin 5) = 3.39V
  • BACKLIGHT_ON (pin 4) = 3.36V
  • VGA_BLON (measured on U39) = 3.39V
And with the laptop OFF and the screen disconnected
  • Resistance from ground to BACKLIGHT_ON = 53 kohm (a little less than the optimal 64 kohm, according to the linked thread)
What I'd like to know is - what might be causing this problem? Is there some component I need to replace? At the very least, I have to buy a new mobo or a new screen?

Thank you all in advance.

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