Dell U2412Mb - Backlight works, blank screen

[will62]

Hello all,

I had a blank screen problem with this display that I repaired, and I thought that I would post it here in hopes that it would help someone else down the line.

Dell U2412Mb Ultrasharp
Power board: 4H.1GH02.A02
Main board:4H.1GH01.A10
Panel:LTM240CL01 by Samsung
Integrated T-CON: M240K04V0.1_HF

Problem: On powering on, LED was blue for a couple of minutes, but screen was blank/black. No OSD on screen. LED would then turn amber (sleep).

I verified that the power board was working and that the backlight was on. You have to confirm before LED goes to amber. I then made sure that the main board was sending the 5V to the integrated T-CON. The data sheet for the Samsung panel is available online if you need the pin out of the LVDS connector.

On the T-CON, I checked the fuse, which was good. I then checked all caps and diodes for shorts to ground. Found nothing there. It turned out to be the
power management chip SM4142. I could not find a datasheet for the chip to
check further before replacing, but I have listed below, voltages before and after chip change. The chip is available on Ebay for ~$9 shipped. It is a QFN-48 pin, so you're probably going to need hot air to replace it.

T-CON voltages before/after SM4142 change
VON: 4.5V / 25.7V
VSS1: 150mV / -5.6V
VSS2: 600mV / -10.1V
TVDD18: 200mV / 1.8V
TVDD12: 20mV / 1.2V
DVDD: 200mV / 1.8V
AVDD: 4.8V / 10.0V
VCC: 5.2V / 5.1V
PVDD: 5.2V / 5.1V
EVDD: 200mV / 1.8V

[rddube]

Re: Dell U2412Mb - Backlight works, blank screen

Wow, this is incredible diagnostic and work! Congratulations Will62, your post is very inspiring and informative.

Can you tell us how you replaced that chip? I suppose you used hot air, but at what temperature was your setup, because I guess those chips are heat sensitive, so you have to be right on the spot?

[will62]

Re: Dell U2412Mb - Backlight works, blank screen

Thanks. I used solder paste and air set at 350c. Slowly heat area from a distance, then come in to about an inch from the chip. Watch what the paste is doing, and you'll see as it separates between the pins. Reflux and clean-up each pin with a sharp point iron tip. I'm a novice at it, but there is no shortage of videos on youtube to point you in the right direction.

[rddube]

Re: Dell U2412Mb - Backlight works, blank screen

Will62, when you say solder paste, do you actually mean solder paste or flux or are they the same thing?

[will62]

Re: Dell U2412Mb - Backlight works, blank screen

They are different, and everyone has their favorites. Whatever you do, don't get the "dab-on" pen style of flux like I did. Get the tacky style in a syringe. Watch a few youtube videos on qfn soldering, and you'll probably see some using paste.

[rddube]

Re: Dell U2412Mb - Backlight works, blank screen

This is what I have, but both don't have any solder in them. I watched a few youtube videos, and there seems to be a kind with solder in the flux?

[will62]

Re: Dell U2412Mb - Backlight works, blank screen

There is flux core solder wire. For an example of solder paste soldering, watch the video below. Dave is pretty chatty, so you may want to skip to about 6:40
in.

https://www.youtube.com/watch?v=M_rO6oPVsws

Meant to ask you, do you have access to an oscilloscope? Your project thread is getting to the point where that would be most helpful.

[diif]

Re: Dell U2412Mb - Backlight works, blank screen

Dave is soldering to bare boards, Hence why he's using paste.
Paste isn't usually used if replacing a part as there's solder already on the pads.
Hot air to remove the old and the same to replace, depending on the solder after removing the IC. Maybe cleaning up the old solder by adding fresh leaded then using wick. Then I'd solder it like in this video.
https://www.youtube.com/watch?v=b7pvuOUN9oA
I find it easier to get better consistency with solder on pads and it doesn't blow where you don't want it.