White very bright picture on LVQ40HLA VisionQuest

[paulstef]

Hi,

I'm looking for a schematic of this mainboard 520-MSD119C-010C.

Google tells me that it;s also in the Sceptre X32BV.

The TV has a very bright picture which cannot be adjusted. The backlight CAN be adjusted though. Removing the signal output connector from the main board results in having only the dark backlight glow. So that seems to be ok.

The picture is so bright you can barely see the picture. It's as if brightness and contrast was adjusted to their maximum value.

The guy I have it from told me someone sat on the remote control and after that he had this problem. Sounds weird.

Anybody knows if a schematic is available or what could be the problem or if there needs to be firmware reflashed or something?

Thanks!

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

This might be another gamma IC failure. It can cause this effect, completely unrelated to main board. Can you supply a picture of the fault & a picture of the T-con board?

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Here is a picture of the fault. Brightness contrast and other picture related menu adjustments have minimum effect on the picture. The backlight adjustment works. The picture is colored.

I will post a picture of the TCON later.
I don't know much about the TCON, i.e. never had to replace one. Why does the picture go black when I remove the cable from main to TCON? I didn't remove the board yet...

Thanks

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

That definitely looks like a gamma fault.

The T-con converts the video signals that the main board produces into a format that the LCD panel can accept. It also typically generates the various voltages that a panel requires (VON/VGH, VOFF/VGL, AVDD, and VDD.)

When the power to the LCD panel is removed, because the T-con is disconnected from power, the pixels go back to their natural state; on most panels this is black. Earlier panels went white with no power.

Look closely at the panel. Can you see a dancing dots pattern? Every second dot flickering? This is one effect that can sometimes occur with a gamma fault due to mis-matched curve voltages.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Thanks for the explanation. No I couldn't see dancing dots, but I didn't look REALLY closely. Now I have it partly disassembled. Will have a look at it later.

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

No worries, we can still check the gamma curve voltages to see if the IC is bad or not.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Here is a picture of the TCON board. It says T400XW01.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Is the gamma IC the 48 pin TQFP package ? The datasheet says "voltage buffers that buffers reference voltage for gamma correction". If that's it it would be easy to replace. Otherwise, how can I actually check what's wrong with the TCON? I know they cost next to nothing but I still would like to find out what's wrong.

The other two big ICs look like a CPU and RAM but I didn't really look up their datasheets.

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

Yes, that's the chip.

Just above & to the right there's a series of points which are labelled VGAMA1 thru VGAMA22
https://www.badcaps.net/forum/attach...7&d=1398218428

Check the voltage on each point with the board connected.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Ok, I'll check them tomorrow. Thanks a lot .

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

So I have:

VGMA1: 15.29V
VGMA6: 15.16V
VGMA9: 15.16V
VGMA10: 15.04V
VGMA11: 15.04V
VGMA12: 6.64V
VGMA14: 4.9V
VGMA17: 4.13V
VGMA22: 0.35V

REF6: 14.45V
REF9: 14.3V
REF14: 4.8V
REF17: 4.08V

VGMAREF: 2.48V

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

Your gamma IC is bad. Replace the IC marked AS15 and it should be good. Alternatively replace the entire T-con board.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Thanks! I would like to understand better though.

How do I determine that it's bad? What voltages should there be? Are they similar for all panels? I assume the higher the voltage the higher the transmittance for the pixel?

Since this IC seems to be a simple amplifier how do I know it's not the input signal that's wrong?

Thanks

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

I will link you to a couple of pages from my ebook (still a big, big work in progress) to help you understand.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Great reading. Made certainly more sense to post this here than to explain it several times.

Anyway I guess I will have to read more on this subject. So the gamma IC actually creates fixed voltages in certain steps and the average of all of them should be half of AVDD, 15.77V in my case.

For the sequence I posted the obviously wrong voltages are 5x equal of 15V, is that correct?

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

Half of AVDD in your case is 7.5V and this is true for 99% of large LCD panels used in TVs. (Laptops & phones tend to use 6 to 9V AVDD...)

The voltages should either increase or decrease but they can only do one. They can't increase, decrease, increase. That's bad. That's test 1 and detects 90% of failures.

The second test is to virtually split the curve around the middle VGAMA (11 in this case.) For the lower values, try to mirror them about the axis so they line up with the upper values. As you can see this is not possible. This is test 2 and detects most other failures.

Failing that compare to the reference curve in the ebook...

Yes the real killer voltages are the first half stuck at 15V which is causing the LCD to drive all pixels full white for half the pixel cycle. (VGAMA22 & VGAMA1 represent the same light level... it's complicated.) This is why your picture is whited out.

I missed a figure in that text. It doesn't make much sense without it. Let's try this again...

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

It IS complicated. We will wait for your ebook. It will certainly be of great help for many people.

Will swap the IC and post the results. (I wonder why they burn so easily, must be out of specs or near max ratings somewhere)

Thanks a lot!!

[tom66]

Re: White very bright picture on LVQ40HLA VisionQuest

On EEVBlog forum, we haven't reached any consensus as to why these might be failing. So far, we don't believe it to be a thermal failure (they run about 40°C when working which isn't going to shorten life significantly) nor is it believed to be a board design issue (since it happens with tons of boards from different manufacturers.) Personally I think it's a design flaw, possibly leading to a thermal event (transient short) in the IC... but I'm really not sure.

[paulstef]

Re: White very bright picture on LVQ40HLA VisionQuest

Yes, it was the IC. TV works now. (just has one stuck green pixel)

Thanks Tom! Your help is much appreciated!