Remove All Ads from XDA
Find Your Device:
Or Continue to Thread: HD2 Pixel is dead?
9th April 2011, 09:36 AM |#6  
motoi_bogdan's Avatar
Senior Member
Thanks Meter: 345
Donate to Me
thanks for quoting T-Macgnolia.
yep, as i explained on those 2 posts in that topic one single pixel on hd2 screen's working with something like theese

- 3 subpixels
- each subpixels is made up of a pair of liquid crystal and driving transistor
- each transistor is linked to a processing chip (it organizes the screen into columns and rows, and drive them separately)
- each processing chip (the ones that trigger columns and the ones that trigger lines) are linked to a lvds bridge (that's some sort of protocol used to communicate to the gpu's analog output).
- gpu or other video processing subsystem

How to recognize various hardware failures occuring to the screen:

- white screen (nothing displayed) - this is pretty serious. As i've posted on my other topic, the crystals in the screen are normally transparent. If you apply various electric signals the matrix will actually produce colors by blocking various light wavelengths. If you get a white screen and you have already determined somehow it's not related to software issues, this most probably means faulty lvds bridge or connector (the display connector). Because the lvds is not communicating with column and row processing crystals - nothing is displayed on the screen. Entire matrix is powered off. Another cause can be a faulty GPU or video subsystem. If no video signal is being sent, the matrix will be also powered down. With crystal in their natural state, they will be transparent, thus - white light. Not user fixable, each part must be diagnosed, special equipment required.

- screen working but color lines appearing on the display or permanently there. You may have seen a laptop or tv doing this. The display works but there are some strange lines that are permanently there and either don't change color, or change it in a strange way. This happens when you have a faulty row or column driver. If that chip fails - all mosfet transistors that drive pixels in that region of the display will either stop working or work in an erratic way. Thus you get... a line (vertical or horizontal) on the display. Not user fixable, in 80% cases it requires another display as the faulty chip is in most cases embeded into the display module itself.

- screen works but with strange colors (inverted, red-ish, blue-ish or whatever). The lvds bridge - that feeds video signal to the processing chips on the display, contains a small subsystem called contrast & crominance setting. It's fixed on most displays (although i found some older pda's where this can be fine tuned). Simply put it's a small circuit made up of a negative voltage driving a PNP junction bipolar transistor working in a voltage divider circuit. The divider will actually... well.. divide the voltage across it and these 2 voltage resulted (together summed they give up the original voltage) are required for each corresponding column and row controller chip to know how to bias the pixel matrix. If you bias the matrix to hard (larger voltages makes the transistors to switch at smaller imput levels, so they are triggered more easier) the crystals will change transparency level faster, thus you will get a darker screen or strange colors. If in a normal situation you will need 0.3Volts to drive a subpixel to fully block light (on it's specific wavelength) a harder biased matrix will require only 0.2V - for example. However as the voltage varies from 0 to 0.3volts in a normal case of color reproduction by a subpixel, you reduce the maximum range by forcing the corresponding mosfet transistor to do at 0.2volts what is was supposed to do at 0.3volts. The GPU and LVDS bridge will still send signal from 0 to 0.3 but the display's transistors won't display anything different from 0.2 to 0.3 range. Thus you get color alterations. If you find this problem in a display, it means this specific subsystem (contrast/crominance) has either failed or it's drived improperly. Inverted colors means the transistor in the crominance block is actually either dead or has a faulty connection. Instead of driving the display with 0.3 volts it used -0.3 voltage. All hell breaks loose. By no means user fixable. This subsystem is also located on the display module level. Older pda's or smartphones have this thing mounted on the ribbon cable coming out from the LCM itself. Those have a chance to be repaired without changing the display.

- white spots on the display. Most visible on white background. I repeat, spots, not bunch of little points (pixels). Those areas will still display colors, but with some distorsions. I didn't covered these when explaining the lcd display hardware. Well simply put the problem is like this. LCD backlights are made up from small white led's. However, the problem is that a led is a point like light source. Light is more intensive near the led and decreases as you move further. So, if led's were simply placed on the bottom of the screen , the upper part will be darker or the backlight won't be evenly spread. In order to solve this, manufacturers use some sort of polarizing filters in order to deflect light and "move" it more effectively. 2 polarizing filters are used, they are 2 foils containing some veeery small mirror like material that reflects light only when it passes though them at a specific angle. By using 2 filters (horizontal/vertical) and other specific materials, the backlight assembly can spread the light produced by 3-4 small leds on the whole screen surface. The foils are placed together in contact with each other and their final role is to direct the light straight into the display's matrix in order to fall vertically over the liquid crystals and through them - to you. IF however the backlight assembly is damaged due to mechanic shocks for example, those thin foils can bend or change their original orientation. Because of that, light won't be diverted vertically, won't pass though the crystals at the correct angle and thus.. you'll see spots on the screen. Another way for this problem to occur is water or dirt getting into the backlight assembly. Both can affect either the orientation of the polarizing filters or simply leave dirt on them. Either by mechanic shock or dirt, this problem can appear. I have a samsung omnia with this problem, i heard about 1/4 of them have it due to poor design of the phone's case against mechanic shocks.
Not user serviceable. I disassembled laptop screens in order to repair them or to change polarizing filters but it's a very stressful job. The smaller the screen the harder to repair it. I don't think it's even possible to find replacement backlight modules for phone screens. You either have to live with the problem or change the display.

- bad pixels. If you see a color dot (they are either green, red, blue or combinations of these) you have either one or couple of "dead pixels/subpixels". If you see a white/black dot - you have a full dead pixel. Well, what's happening there?
- single color dot. - damaged subpixel. This is basically a mosfet transistor that's blocked into conductive state and can't switch off. The pixel is working (not dead) but can't be turned off or assume variations in displayed color.
- single black dot - if all 3 subpixels that form a pixel are having the problem above, you get a black dot.
- single white dot - 3 dead subpixels. No power is getting in. Either 3 transistors commanding 3 subpixels (one pixel) are incapable of switching on or their 3 crystals are incapable of changing their state (rare scenario). In most cases, we get this from transistor failure.
- bunch of color dots - if they are located in close distance to each other, suspect a mechanical shock to the matrix. If they form a geometrical shape, suspect logical damage (transistors improperly driven or simply not powered on at all). If the pixels form a line across the screen - well, you diagnosed the problem incorrectly, those aren't your usual "dead" pixels, see above - you may have a row/column controller chip problem.
Dead pixel - user fixable? In some rare situations (i mean .. pretty rare) varying the screen content very fast is a way of attempting to unblock a locked transistor. In some lucky cases this will solve the problem. I had some success some time ago, in some cases it worked after couple of hours of displaying those rapid moving/alternating colors, in other cases it worked but the problem pixel got stuck again after some couple of weeks, in most cases it never worked at all. Worth a shot, couldn't harm to try. There are programs that do this job for you, something like "dead pixel fixer" or do a search on either marketplace or google for an app for your OS.

These are the most common problems with LCD displays (all sizes). Of course there are the "exotics" but most people will never see one. As you can see, there is not much to do or repair with smaller phone screens. No replacement subcomponents, too small...
TV screens for example can be disassembled and various components replaced or repaired individually. Hope it helps or gives you something to read in the morning when drinking your coffee.
Last edited by motoi_bogdan; 10th April 2011 at 09:14 AM.
The Following 7 Users Say Thank You to motoi_bogdan For This Useful Post: [ View ] Gift motoi_bogdan Ad-Free