Backlight off

Hello,

Normally this PC has LED backlighting... but it may depend on the type of panel used by the manufacturer... These LEDs are powered by a small transformer (switching power supply) which should be located in the lid, just below the panel... It can be faulty... There may also be one or two LEDs that have shorted in the panel and this short circuit cancels the power to the other LEDs...

If it lights up briefly, you can see it as a shaded area at the bottom of the screen... This lighting is a strip of LEDs, every 5 mm the shadow comes from the LEDs that are off... but there is a nuance between off in cut and off in short circuit... it happens, and on TVs too.

The remaining issue is the connection between the motherboard and this backlight transformer; it can be cut in the hinge and, depending, it can be separated from the display cable...

Everything still depends on the type of panel used... some have a 40-pin connection, with the backlight towards the LEDs, all integrated in the panel... And others only have 30 pins, there is only the LCD matrix display and the backlight is separate with its own cable independent from the LCD... See the panel reference written on the back.

Thank you for your response, Jeannets.

In my case, it’s this panel: https://www.amazon.fr/gp/product/B01068597E/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

There is no separate transformer. The screen is powered directly by a Y-shaped cable connecting the motherboard to the screen connector and to the webcam. I tried replacing this cable and the panel, but the screen still doesn’t turn on.

So, could the problem originate from the motherboard?

OK, so this panel would be a LP156WHB according to the site... I would have preferred a readout or a photo of the back of the panel to avoid errors...

Already it is listed for 30 pins on Amazon and 40 pins on the LaptopScreen site

Here is the Datasheet for this panel https://www.panelook.com/LP156WHB-TLC1_LG%20Display_15.6_LCM_parameter_25929.html; it is indeed a 40-pin, so there is only one Cable that contains the LCD panel and the LED backlighting is around 20 volts... Indeed, there is no transformer... everything is included in the panel signal interface at the back of the panel "included in panel signal interface"

--- In the PC on the motherboard level, there should also be two "Y" outputs; one is the LCD display outputs from the graphics card (or chip) and the other is the LED backlight supply...

-- The brightness could also be at minimum, via keyboard commands Fn + F8 for example..??
.. It is also necessary to be sure it is not Windows power saving at minimum... To tell the difference, display the BIOS... or boot the PC from a Live CD.. Linux for example to be really different

After seeing all that, one must look at the motherboard more closely, particularly the micro fuses, bad solder joints or grime, signs of liquid (coffee)
and in the end, it is screwed, one must think about the motherboard, indeed, but not admit defeat at the first obstacle..!

Here is the label of the original screen


The screen connector received matches well (same location and same number of pins). Moreover, aside from the backlight being off, I do have the image.

- I only have a single ribbon cable going towards the screen (except for the wifi antenna) and the graphics card is integrated into the motherboard

- I tested a boot under live USB Ubuntu and I do not have backlight any more and thanks to an external monitor I was able to verify the brightness settings.

I took the motherboard out and at first glance no damage (apart from a few crumbs, apparently it eats often on its PC ;)). I confess I do not know where to find the fuses.

Connectors and screen are good, but it doesn’t say whether they are 30 or 40 pins... Generally, apart from counting them, it’s written on the back interface of the screen. But the data sheet is formal, it’s 40 contacts.. So LCD + LED

Your photos... even zoomed in aren’t bigger than the original... You should rather place it on the site "cjoint.com" very clear and with good definition.... and you paste the link here

Here is an image of a CMS fuse

And here is another image of a CMS fuse burned on a motherboard in the bottom middle

And there are also those that are dead without being visible, you have to test continuity with an ohmmeter..
Usually the indication "F" or "F2" for fuse No. 2 is indicated on the PCB print... C = capacitor R = resistor L = Inductance... there you go with good eyes and a magnifying glass to find them Or a good photo and I’ll also look

They are often grouped in the area where the battery supply arrives, the four tabs.

Coffee on motherboards is also very bad (the coffee spill) when it dries it becomes conductive with a resistance value..

That’s it for tonight..

Thank you for the time you’re dedicating to me.

I’ve inspected the board and found (I think) several fuses that all beeped on the multimeter and none that seem burned.

here are a few more precise photos of the board, if you would be so kind as to take a look, that would be great.

https://www.cjoint.com/c/JFbjHzTVONe

https://www.cjoint.com/c/JFbjI6QMcFe

I watched your photos, it’s better, I spotted two fuses PF2 and PF1. The beep indicates continuity, so not burnt out... I didn’t see anything else in that style.

But there is a battery (in blue) you should measure; it should be 3 volts or more—let’s say at 2.8 volts it’s bad... This battery powers the startup config memory.

You can also short-circuit it for 3 seconds to perform a CMOS reset. It isn’t necessarily related... But I’ve seen stranger.

If I understand correctly, you replaced the screen, for nothing—the original fault is still there...

In my opinion, the complex and smart circuits are in the interface board on the back of the screen. and the motherboard only provides the DC voltage which should be around 20 to 24 volts... but this voltage doesn’t go directly; it must go through a regulator that shouldn’t be far from the connection to the screen... this distribution uses several conductors in parallel because the currents are quite high there...

These indications may help you think while you search.

Moreover today is Pentecost... The festival for the spirits.

The battery voltage reads 2.83V.

I reset the board by shorting it for 3 seconds, reassembled the setup temporarily to test, and there is still no change.

This time I think the problem is beyond my skills and I’m wondering whether it would be worth consulting a professional (would they bother trying to repair the board or simply replace it?) or if I should order a new motherboard myself (risky that it won’t work any better) knowing that I can still reconnect the screen.

Yes, although the stack is a bit weak, it doesn’t seem to come from that side... Moreover, the reset didn’t change anything.

-- Entrust it to a professional, in my opinion, he won’t even try to repair the motherboard, it’s not profitable and few know how to intervene at the component level, even with an established diagnosis... We also have to understand their financial imperatives...

There remains Le Bon Coin and eBay to find an identical or very close motherboard

here is one for $59 with free shipping https://www.ebay.fr/itm/NM-A281-8S5B20G380-Motherboard-for-Lenovo-G50-45-LAPTOP-AMD-EM6010U-A/292591650126?hash=item441fd2454e%3Ag%3AzAQAAOSwngxeelOs

I’m going to check with her to order the motherboard and I’ll keep you informed if that fixes the problem.

Thank you very much for your help and your time, it was greatly appreciated.

Yes, I’m interested... Thanks in advance and good luck.

If you order that one, it comes from China, it takes three weeks.. (free shipping)

Here is the schematic of your motherboard https://drive.google.com/file/d/10l41ofoRqbj5lOTYikzZN0z0kyS6CL05/view

Page 33 this must be what powers the LEDs of the backlight...

 B+ to + LERVDD POWER

Hello, so to turn on the backlight you need to press FN - or FN + I hope it will work for you.