Problems with laptop brightness after changing screens

nox999

Commendable
Feb 18, 2016
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Hi, I've stepped on my laptop by mistake and I broke the screen. Luckily I had another semi-working laptop that I bought for spare parts and got the screen from it. It's working perfectly on that one. When I transplant it onto the "good" laptop it works, but is quite dim. I've tried everything software related (reinstalling drivers, Windows, fiddling with the brightness controls, testing the brightness while beeing in BIOS, so no OS is running), but to no avail, the max brightness is 70% at best.

I've transplanted the screen back to the spare-part laptop and the brightness is still fine there.

Any help would be appreciated.

My laptop is a HP Probook 4520s.
 
It sounds like the damaged laptop doesn't supply enough power to the screen/inverter, therefore the backlighting can't be turned up 100%. It's best to swap the screen out for an identical - or at least a similar/known compatible - replacement. You can find the make and model of the screen on the back of it; there should be a white sticker detailing what you need to search for a replacement.
 

nox999

Commendable
Feb 18, 2016
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The brightness is fine when the same screen is put in the spare-part computer.
 

nox999

Commendable
Feb 18, 2016
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I think we can rule out the screen. Both the intact screen and the broken screen have 100% brightness on the spare-parts laptop and both screens max out at 60-70% on the "good" laptop.
 

I must not be conveying this very well.

I'm not casting doubts about either screen. My prognosis is that the laptop you moved the screen over to - the laptop where you're seeing 70% brightness on - is not capable of fully driving that spare screen. It's not that the laptop is defective either, just that it doesn't supply enough voltage to fully power the backlight on the spare screen.

Now, what you haven't mentioned is whether the spare screen is an identical replacement to the screen you stepped on or not. Is the replacement/spare screen the same make and model as the damaged one?
 

nox999

Commendable
Feb 18, 2016
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The spare screen is identical to the broken one. In fact it's the original screen of that laptop in question - I moved it a while ago to the "spare-parts" laptop and now I'm moving it back.

I understood that the laptop is not fully capable of driving the screen, but the question is why? What might have broken when I stepped on it that this happens?
 
Ok, gotcha. Sorry that that took so long to clear up!

If it's an identical screen and the original one provided, it's hard to say what the problem is. The laptop actually should be able to drive it just fine. Did you also swap the LCD video cables when you swapped the screens?

If you can, post the make and part number of each screen here; if the part numbers really are identical, post just one of them. I'll look into detailed specs and try to find any discrepancies.
 

nox999

Commendable
Feb 18, 2016
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Thanks for ypur answers, and let me clarify the problem: I have 2 laptops, both are HP probook 4520s, HP1 is my good laptop and HP2 is the spare-parts laptop, both are working fine. One day by accident I break the display of HP1. Although the LCD shows signs of the accident, being partially black, the brightness of the readable part of the screen is 100%.

I decide to replace the screen so I take the display from HP2 (same model, both original displays) and I install it to HP1. The "transplant" is done by taking out the display with the connector from the MB, not just the lcd screen. While doing the switch I realise I still had the battery connected on HP1.

When I finished the job, and turned on HP1 to check the screen, I realised the brightness was poor (max 70% of the normal brightness) and also the "adjust brightness" scale was decalibrated (the scale is divided into 10 steps, the first 8 steps turning the brightness up, but the last 2 turn it back lower).

I take out the screen (straight out the MB, with the cable) and test it again on HP2, where it works perfectly. I put it back on HP1 and again it is dimm. I connect the laptop to an external monitor and the everything looks perfect on the monitor.

So, have I made everythig clear this time?
Thanks for your ideas, I've run out of mine.
PS: I don't have a multimeter.
 

nox999

Commendable
Feb 18, 2016
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From what I've read, LCD screens don't have backlight fuses like the LED screens, so theoretically the inverter should've taken that "hit". However, the screen, inverter, cables, and all that are fine since the display works on HP2. The problem must be on the motherboard, but where should I look?
 
LED screens are LCD screens. LED defines the type of lighting used, as opposed to CCFL backlighting, though both are a part of LCD technology (just FYI).

I wonder if this has something to do with the EDID(?). You can try a power cycle and removing the CMOS battery for awhile to do a full reset on the motherboard (on HP1), then try the screen again (if you haven't done so already).

What is the make and model/part number on each screen?
 

nox999

Commendable
Feb 18, 2016
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This is the serial number of the screen ltn156ar16-003. it seems to be LED :) So, it could be a backlight fuse, but how can that explain the fact that the display is partially lit, are there more fuses or can a fuse be half-blown :) ?
 
So, here we have two different view of troubleshooting. First is Prostar Computer, who believe it's the screen who is the problem. Second me, who believe it's on the motherboard the problem is. noxx999 is the only one who has the answer.:??:
 

Just to be clear, I don't think either screen is defective (besides the physical damage). We saw some recent EDID corruption, and I'm wondering if this issue has something to do with the EDID also. Hard to say, really.

@nox999, you may need to bring this to a good technician (not a big box retailer, like Best Buy's Geek Squad). I'm at a loss on this one.
 
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