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LED-backlit LCD panel: Replaced panel, cable and inverter in a Thinkpad T400 but the screen is still dim.

Posted on 2011-02-23
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Last Modified: 2013-11-17
Hey there,

I am troubleshooting a Thinkpad T400 notebook. It uses an LED-backlit screen. The original screen was dim. I have replaced the LCD cable, "inverter" (not a true inverter but it does connect the LED power), as well as the LCD itself. The screen is still dim.

The only hunch I have right now is that if it's possible the LCD dimming switch that activates when the screen is closed is stuck, but I can't identify where this switch is in the IBM/Lenovo teardown and guide for this model.
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Question by:techasker
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Expert Comment

by:lauchangkwang
ID: 34967500
Could there be the Display Brighness setting ?? try to adjust it till maximum of the brightness

Take a look for this:
http://www-307.ibm.com/pc/support/site.wss/MIGR-57556.html
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Author Comment

by:techasker
ID: 34967572
Yes, the brightness is set to max in all settings.
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Expert Comment

by:nobus
ID: 34967925
maybe this helps ?  http://digitalchunk.com/thinkpad-t400-standby-problem.htm

anyhow, afaik for most laptops the switch can be located in the front or near the hinges
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LVL 92

Expert Comment

by:nobus
ID: 34967944
this  comment "but I notice it will disable the screen when the lid is less than about a 40° angle"  seems to indicate a location near the hinges :  http://forum.thinkpads.com/viewtopic.php?f=45&t=85643&view=previous

it also indicates lid switch problems are not uncommon
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Accepted Solution

by:
Davis McCarn earned 500 total points
ID: 34969554
I hate to say it; but, I'll bet its the circuitry on the systemboard which controls the brightness and you'll need a new systemboard to fix it.
To prove that, go into the power settings and set it to go into standby when you close the lid.  Then, close the lid.  The unit should go into standby and then reawaken when you open it.  If it does, the switch is fine.
BTW, almost all of the lid switches are near a hinge or under the hinge cover and get pressed by an arm on one hinge.  They also generally turn off the display, not dim it.
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Author Closing Comment

by:techasker
ID: 34970936
After attempting to use 4 different screens and two sets of cabling and inverter, this solution proved to be correct.
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