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Thinkpad T420 display backlight voltage issue

T400/T410/T420 and T500/T510/T520 Series
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nihar13
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Joined: Sun Aug 16, 2020 5:43 pm
Location: Ahmedabad, India

Thinkpad T420 display backlight voltage issue

#1 Post by nihar13 » Sun Aug 16, 2020 5:59 pm

Dear fellows,
I have a ThinkPad T420 (with intel onboard graphics and i5 2nd gen). I was using it proudly since a decade now! Recently it showed some white lines on the screen on booting, but returned back to normal after 3-5 secs. Hence I ignored it.

Last week, one fine day, the backlight refused to start. There's display, but no backlight. I got a new display panel and tried to hook it up, same thing.

I checked the datasheet of the LP140WH2 display panel used. It says that the voltages on pins 38,39,40 are used for LED backlight and should be in range of 7-24Volts. But when I measured, it was showing me only 2.1 Volts. I checked the voltages inside the panel (at the boost up circuit and VLED test pad), same 2.1 V.

I tried to check the same pin number 40 on the motherboard connector, but seems the display cable has crossed wires. Now, I don't know which pin on the motherboard is connected to pin40 of the screen.

I need the pinout diagram of the display connector on the motherboard. Better, if there are any pointers if someone had same issue and managed to resolve it.

TIA.

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Re: Thinkpad T420 display backlight voltage issue

#2 Post by RealBlackStuff » Mon Aug 17, 2020 1:41 am

Get schematics here: https://www.laboneinside.com/lenovo-thi ... c-diagram/
Also check the backlight fuse F3 near the LCD-connector (page 32 in the above).
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nihar13
Posts: 2
Joined: Sun Aug 16, 2020 5:43 pm
Location: Ahmedabad, India

Re: Thinkpad T420 display backlight voltage issue

#3 Post by nihar13 » Mon Aug 17, 2020 7:20 am

RealBlackStuff wrote:
Mon Aug 17, 2020 1:41 am
Get schematics here: https://www.laboneinside.com/lenovo-thi ... c-diagram/
Also check the backlight fuse F3 near the LCD-connector (page 32 in the above).
This is very helpful. Thank you. I have 1 more question: I know for sure that the Pins38,39,40 on the LED screen connector should be VLED voltage (7-21 Volts) as per the datasheet attached (page8), whereas I see from the schematics on page32 that there are 2 input voltages (VCC3P and VBL20) on the Mobo connector. Which voltage would be ideally connected to the VLED voltage? Would it be VCC3P (logical inference from the "check fuse F3" mentioned by you!!!) or VBL20? Depending on that I can check the continuity of the cables in the display cable.

Link to LED screen datasheet:
https://datasheet4u.com/datasheet-pdf/L ... ?id=908656

Thanks.

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