How to test LCD display?

[starLED]

I have on older netbook MSI WIND U100 and LCD screen is not working.
Before screen stopped working there was a one issue: when screen was in a certain position there were red lines on the screen, and if I move it slightly screen was fine.
I thought it was probably wires pressed by the hinges.
Then one day there was a smoke near the hinges and screen stopped working.
I have disassembled screen and I have tested LVDS cable with a multimeter and every pin (wire) has a continuity, I don't see any shorted wires.
Now I am not sure if the screen is faulty, LVDS cable or something else?

 

[ramondo]

Have you removed the flex cables going thru the hinges and inspected them? Also the flex are usually bonded to the screen and a bad connection there will cause lines on screen. If something smoked then it should be visible.

 

[starLED]

Yes, there are no visible burn marks.
While the screen is connected, I have tested continuity from the pins on the motherboard to the pins on the lcd screen, and they all show continuity.
I don't know if all pin wires are connected properly though, or maybe are shorted, but it doesn't seem bad.

Can you suggest me some tests?

 

[rjenkinsgb]

If the cable shorted in the hinge, it may have damaged the connections at the laptop motherboard, or blown the feed components to that connector. It is unlikely to be a fault in the screen itself.

It must have been a power connection, as signal wires would not have sufficient current available to cause burning.

 

[starLED]

I checked LVDS cable again and there is a broken wire somewhere, I will try to find the m*****f*****.

 

[danadak]

Smoke makes me think backlight DC/DC. Its LED backlight.

Regards, Dana.

 

[starLED]

After closer inspection I found two wires burned and melted together in LVDS cable on a place where hinge bent them.
They are very small, but I think I can peel off insulation and solder them back together.
I have one question though.
Wires are not made of copper, will there be an issue if I use copper wire?

 

[starLED]

I joined wires together and LVDS cable is fully connected, but screen doesn't turn on.

What next?

 

[rjenkinsgb]

Get the schematic from here, if you do not already have it:

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.

elektrotanya.com

See the LVDS connector pinout on page 21. Check each of the three power feeds to that; two different 3V ones and the 5V one.

Most likely one of those is missing - if so, look at the power control circuits for those, also in the lower half of that page; Q39 for the 3V LCD feed and Q3 for the 5V LED supply.

 

[rjenkinsgb]

PS. The screen cable is almost certainly copper (or silver), but likely many separate "Litz wire" style conductors to achieve the required flexibility. Each "wire" may be made from a lot of much thinner insulated strands, which makes it rather difficult to solder properly.

Litz wire - Wikipedia

en.wikipedia.org

Another type "tinsel wire". That uses tiny copper (or possibly again silver) tape or ribbon, spiralled around an insulating fibre or yarn core, with thin insulation over the whole thing.
Again, the idea is for long term flexibility. I doubt that would be used for high frequency signals, though - Litz wire is perfect for that application.

Tinsel wire - Wikipedia

en.wikipedia.org

 

[rjenkinsgb]

For info, there are a few screen cables on ebay:

Msi U100 Laptop Screen Cable for sale | eBay

Find great deals on eBay for Msi U100 Laptop Screen Cable. Shop with confidence.

www.ebay.co.uk

I would not advise that you continue using a repaired one after getting the machine working.
It's virtually guaranteed to soon fail again at the same place, as it will be less flexible there and be under more strain.

 

[starLED]

Thanks for the schematics, I've been looking for that.
I've checked voltages.
3V are on the 4, 6 and 8 pin, like on the schematics.
5V doesn't show on either pin, there is 0,01 V on the 12 pin.
3V are also on 28 and 30 pin.
1,3 is on the 26 pin.

 

[rjenkinsgb]

OK, so check around Q3, the 5V power switch transistor.

If that has 5V to it but not out to the connector, it's a good guess that it has blown.

 

[starLED]

I found the problem.
Transistor Q3 is fried.

I need to replace Q3, its P-IRLML6402PBF-SOT23
Is this available?

 

[starLED]

I can't find IRLML6402 MOSFET SOT 32 locally.

https://www.infineon.com/dgdl/Infineon-IRLML6402-DataSheet-v01_01-EN.pdf?fileId=5546d462533600a401535668d5c2263c

Can someone tell me what can I buy something similar that will do, from this link:

Kelco - Proizvodi - FET/MOSFET tranzistori u SMD tehnici

 

[rjenkinsgb]

Can someone tell me what can I buy something similar that will do, from this link:

This one is near identical; slightly better ON resistance & higher max current:

Kelco - Detalji proizvoda - P-MOSFET, 20V, 4.7A, 1.56W, 0.05Ω, SOT-23

 

[starLED]

Thanks man, you helped me a lot.
I will fix it soon as I get them, I will post results.
I owe you a

P.S. Beer, not a girl...

 

[starLED]

I replaced the Q3 transistor and I soldered broken LVDS cable wires, and the screen works!
That's the good news.
Bad news is that the keyboard doesn't work.
I have no idea what is wrong, I didn't removed keyboard or do anything with it.
BIOS doesn't show "keyboard not present".
Touchpad is working though.
Left is new transistor Q3, right the screen working.

 

[starLED]

Keyboard connector is out, I will fix that.

 

[starLED]

OK, so everything was working fine, until I had short again in LVDS cable.
I soldered LVDS wires for testing purpose, but unfortunately wires touched and caused short with a bit of smoke, nothing bad.
I checked transistor and it's fine, but now screen doesn't show clear picture, but instead it shows blue lines.
I will buy new flat cable, I hope it's just a cable issue, that it's not the screen dead.