The problem with ASUS Transformer Book T100TAF

[alfredleo]

1) After one month of usage with windows 8.1, monitor turn black and won't turn on. I fixed this. Opened the back of the case and removed battery. After the battery was placed back Transformer worked one week.
2) Then suddenly almost the same happened (monitor turned black and won't turn on). But this time I removed micro SD card and heard windows sound. And if I put it back there was a sound again. So I thought that there were some trouble with monitor. I opened back case of tablet and saw that something with the name of "6R8 434 RTR" was out of order. You can see the attached pictures. I am wondering what is that square thing and if it can be fixed. I do not trust engineers here as they generally do fry the board with their irons and surely won't fix it. I am from Uzbekistan and sending that back to ASUS will cost like 100-150$. I was fan of ASUS devices but now wondering of the quality ASUS provides as that thing worked only 2 months.

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[arvika]

Wait for fzabkar reply Element on your photos is coil. Can you write model of motherboard? - there should be description directly on logic board.

This is data recovery forum mostly. You can write on www.elvikom.pl too - forum treats mostly on hardware fixes etc. You can write there in English of course.

[labtech]

Seems like your tablet may have been sold as a refurbished item. Does this ring a bell?

[fzabkar]

It's a 6.8 uH coil, but there appear to be additional problems.

Despite its name, the following forum has some excellent techs who may be able to help you:

http://www.badcaps.net/forum/

This forum is for data recovery.

[alfredleo]

Thank you for your help I posted on badcaps.net forum. It was sold as a new one not refurbished. The model of motherboard is T100TAF WIFI T3 MAIN BOARD REV. 2.0 PCB.

[fzabkar]

It appears that badcaps is not as helpful as I thought.

ISTM that the symptoms point to a failure in the supply for the backlight. If you shine a torch at the display screen, do you see an image? If so, then it's a backlight fault.

The annotated photo clip and circuit diagram are my best guess as to the layout of the damaged area of the circuit. The IC appears to be a boost converter with either a 5-bit or 6-bit DAC for selecting 32 or 64 brightness levels.

I suspect that the backlight is shorted. This would have caused the failure of the 6.8uH coil, Schottky diode, and possibly the R01 resistor. With the display connected, measure the resistance between ground and the "display voltage" test point. A resistance of zero ohms would confirm that the display is shorted. Verify that the short goes away when the display is disconnected.

Verify that the V+ voltage is present on either side of the zero-ohm link and at inductor L.

Choose a high speed diode of at least 1A or better as a replacement for D.

Panasonic's ELL5PS6R8N coil, or equivalent, may be a suitable replacement for the 6.8uH coil. You will need to confirm the dimensions.

http://industrial.panasonic.com/lecs/ww ... 000CE7.pdf

Code:

           ___    ___
    V+ o--|___|---UUU--+----------------+
                       |                |
          0 ohm   L   --- C1            |
                      ---               C|
                       |        6.8 uH  C|
                       |                C|    D            R01
                      ===               |                  ___
                      GND               +----->|---+---+--|___|--o
                                        |          |   |       display V
           .----------------------------|---.      |   |
           |  .-----.     boost         |   |      |   |
     ___   |  |     |   converter       |   |     --- ---
   -|___|--|--|     |                   |   |     --- --- C2, C3
     ___   |  |     |                   |   |      |   |
   -|___|--|--|     |    .-----.        |   |      |___|
     ___   |  |     |    |     |        |   |        |
   -|___|--|--|     |    |     |        |   |       ===
     ___   |  | DAC |----| PWM |        |   |       GND
   -|___|--|--|     |    |     |        |   |
     ___   |  |     |    '--.--'        |   |
   -|___|--|--|     |       |        ||-+   |
     ___   |  |     |       |        ||<-   |
   -|___|--|--|     |       '--------||-+   |
           |  |     |                   |   |
           |  '-----'                   |   |
           '--------------------------------'
                                        |
                                        |
                                       ===
                                       GND

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[fzabkar]

Sorry, I think I erred with the DAC. Instead I believe that those six inputs (?) may be the current returns for 6 WLED strings. There are 6 additional resistors below the chip. I'm wondering these are the current sensors for each of the strings.

This chip looks similar:

https://people.xiph.org/~xiphmont/thinkpad/MP3388.pdf

[arvika]

Signature xx= usually means Richtek chip. On this photo it is 89= , but I can't find datasheet for it.

[fzabkar]

This chip seems to have the correct pinout and package style, but wrong marking code:

RT8543, Richtek, 6-String 43V White LED Driver with Boost Regulator, marking 0N=, WQFN-20L:
http://www.szfujiawei.com/aspweb_editor ... 257154.pdf

[fzabkar]

ISTM that the LED5 string may be the culprit and that the corresponding MOSFET within the boot regulator IC may also have been damaged as a result. The current sensing resistors for each LED string are inside the IC, not external as I had guessed.

I wonder if the LED5 string could simply be disconnected to avoid replacing the IC. The 6.8uH coil and Schottky diode would need to be replaced, and the R01 resistor if necessary.

Attachments

RT8543_Pinout.jpg (42.97 KiB) Viewed 10290 times

T100TAF_LEDs.jpg (128.35 KiB) Viewed 10290 times