Thinkpad T420 display backlight voltage issue

[nihar13]

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.

[RealBlackStuff]

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).

[nihar13]

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.

[nihar13]

Apologies for delayed reply.
I got a chance today to reopen the T420 and check the fuses.

Fuse F3 has 0.6 Volt when laptop is on. The display works fine via an external monitor.

Fuse F19 (just to the left of F3) has 3.3V.

Please help!

[axur-delmeria]

VBL may stand for "Voltage, BackLight". VCC3P is most likely 3.3v, since the LCD datasheet specifies 3.3v for its electronics.

If F3 is at 0.6v, then it needs replacement. I remember a post somewhere in this forum where the fuse blew but had a very high resistance instead of being completely open-circuit. It;s probably what happened in your T420.