Lenovo T420 backlight/fuse problem

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hackoff

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Dec 9, 2013
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Hi guys,
is here someone, who can advise me where exactly is a fuse on motherboard responsible for display backlight? I got one Lenovo T420 with black screen.
When i plug external monitor, it works ok. when i shine on the display i recognize very dark desktop when laptop is running, so it has to be only backlight problem.
when i replaced only display and its cable with other display, which i know works ok, it had the same problem. backlight didnt work. when i place that display from broken T420 into my T430 laptop it works ok. Do i now know that problem is not in display but somewhere on motherboard of T420. so it could be broken fuse of backlight. But where it is located. or at least which number it is - F? ?
I will not take it to service. it must be cheap solution so i think i will just do soldering bridge (just make short) in the place of that fuse and *** on it :D
Is there at least some manual for motherboard of T420 where are described all parts (fuses)?
thaks for any advices
 

hackoff

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Dec 9, 2013
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i could wait for an answer till the end of the universe. so i took my multimeter a found all fuses near display connector and started with measuring resistance. there were fuses F2, F10 which had some mili Ohms resistance. so they were OK. after that i found one fuse on the reverse side of the board exactly on the place of the connector on the front side. my ohm meter said it has infinite resistance so it means that fuse is already broken. i think it was F7 fuse.
when i knew which fuse is broken i took solder and went to work. i didnt try to remove it because that board has more layers so i could demage connections if i would try to remove it. i just simply made a soldering bridge/short over that fuse.
i made a power on test and lcd backlight was shining like it was in the time of manufacturing.
i hope this helps a little for others with similar problem. but this solution is dangerous if you will once more make a mistake to work on display with power/battery on. next time you will propably demage something on motherboard.
 

wavelets

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Aug 20, 2015
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Hello

Please could elaborate on this repair? I have a T420 motherboard in same state and would like to attempt repair. What tools do I need?

Thanks
Gareth





 

hackoff

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Dec 9, 2013
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i needed only solder and multimeter (lets say only ohm meter). i just only mesured fuses nearby lcd connector and i found one (maybe there could be more than one) fuse which i measured. ohm meter should say something nearby Zero 0 Ohms. when it says some big number limitting to infinity it is wrong for a fuse and you should make a short over it. or replace the fuse but making short over it is much more easier. if you have T420 you should loook exactly on that fuse numbers which i have written above. maybe F7 fuse is that you shoul look at. i am not native english speaker so if you didnt understand anything, you are welcome to ask anything
 

egiles14

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Nov 4, 2015
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hackoff....I just wanted to reply and say thank you for the information you posted. I have a T420 that the backlight recently went out, and swapping in another screen didn't fix the issue. I knew it was a fuse, but your answer helped me to locate fuse F2. Sure enough, when testing with an ohmmeter the fuse showed to be open. I soldered a tiny bit of solder so that it would bridge the connection, and the backlight now works perfectly.
 
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