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Repair and troubleshooting help for the HP Elitebook 840 G2, a 14" laptop computer from HP.

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LCD Backlight Not Working

Problem: Computer turns on but there's no backlight. The screen is on and working when I point a flashlight on it.

Diagnosis: short-fuse/capacitor needs replacement.

Note: All related software / OS / Bios based approaches have been exhausted.

Any help with the LCD screen circuit board and fuse/capacitor replacement steps will be deeply appreciated.

Details:

  1. HP EliteBook 840 G2
  2. Core i5
  3. Intel Graphics - 1366 x 768
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Hi @thisisfine ,

Here’s a link to the schematics for the motherboard (I think, check the motherboard “board number” to verify).

On p.44/77 you will find the motherboard’s EDP cable connector pinout. Pins 38-40 are the backlight power wires.

If there is no power on these pins, you will have to check the PVBAT source to find out where it comes from.

Also be aware that the “backlight enable” lead (BLKT_EN) needs to have voltage on it to turn the power on and off in the panel. This voltage is controlled by the BIOS chip (L_BLKT_EN)and by the lid switch sensor (LID_SW#) which is mostly found on the power button board so you may want to check that it is working OK as well. Not sure what this voltage is in your model. It depends on the design of the LCD panel circuit by the panel maker. Some I’ve found in the past require a signal voltage of +3.3V to enable the backlights but others have been +5V and it requires 0V to disable.

There should always be power on the video cable connector backlight power pins to the panel. It is turned on and off in the panel, not on the motherboard by the backlight enable lead.

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