Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

Is there any thing that I should check on the board for fault? I'm attaching the schematics of this board LA-6901P. Kindly help me fix it. Thanks

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

A similar fault happened on a Lenovo G450 on this thread:
www.badcaps.net/forum/showthread.php?t=38501
finally they concluded that the INVTPWM signal on pin 7 should be 3.3v.
it is generated by 3 signals according to page 31 in the schematic.
one of them is U1.
try to inject 3.3v on pin7 of LVDS.
If you get backlight, then try to fix the INVTPWM signal.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

Thanks for the input! Ok I tried providing 3.3V from nearby tiny capacitor to pin 7 of LVDS and backlight comes on. But blank display, ie. no image. As for U1 component I already replaced it as it was overheating (and shorted) last time.

Like I have been saying before that the whole screen goes deep dark and upon shining light I don't see any image, it is still the same case. I also tried keeping connected the pin7 with 3.3V while powering it on, the backlight comes on after few seconds when supposedly there is POST display with ACER logo.

EDIT: I suspect the lid close sensor is faulty. But I don't know where to find it (is it on power button board?). And how to bypass it?

Still looking forward for some more responses to help troubleshoot it. Thanks!

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

I traced voltages on KBC for LID SW:

Pin 16 LID_SW#/GPIO0A = 3.3V
Pin 99 LID_SW#/GPXOA02 = 0V
Pin 100 EC_LID_OUT# = 3.3V
Pin 101 EC_ON = 3.3V

I don't know exactly what that means though. Should I buy a KB930QF chip online? (I don't know how to program it) or this chip that I replaced is OK?

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

they all seem OK except for pin99. I think it should be 3v. this pin is generated by pin2 of JPWR1.
trace why pin2 of JPWR1 has 0 voltage.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

That pin is input pin. I traced it and it seems to connect with one of the resistors in parallel on the other side of the board near the northbridge chipset chip. Is it safe to provide 3.3V on that pin?

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

I meant trace the cable connected to pin2 of JPWR1 backward toward the power button board.
and see why 0v is fed into this pin.
unfortunately, the schematic does not say much about the power button board.

injecting voltage on pin2 seems risky.

find lid sensor on your laptop.
If you found it, please provide an image of the sensor and say where it is connected to.

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

please provide an image of the power button board.

by the way, remove the injection into pin7 of JLVDS1, because you don't need it anymore.

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

pressing FN+F6 may turn on/off the lid.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

Hmm... interesting... FN+F6 indeed works for backlight only when I keep providing voltage to pin 7 (doesn't show backlight without it). Backlight on and off using that keys BUT no display still.

EDIT: I think the lid switch sensor is on the power board because I could see a small block of magnet behind the LCD bezel below near the left hinge, the same place the power board is.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

lol by luck, today I received Acer 5750 for repairs. That one is coming on. It has same revision of board.
Tested the parts that seems faulty. That PQ41 on has same continuity beep and reading as the faulty board. The Pin 99 of KBC has also same 0V reading while screen on and displaying. And I also tried the power button board of the faulty laptop with this board and seems OK as well.

The only thing I now notice is different reading on U1 chip. The reading I getting on the faulty board is:
1 2 3
4 5

1 = 0V
2 = 3.3V
3 = 0V
4 = 0V
5 = 3.29V

But on the working board it is:

1 = 0V
2 = 3.3V
3 = 0V
4 = 3.26V
5 = 3.28V

So I took no time and replaced the U1 again, but still the same.

What is the use of PU9 chip? Because I getting completely different readings on that on both. The working board is getting:
1 2 3 4 5
6 7 8 9 10
(dot on bottom left corner of pin 6)

1 = 1.33V
2 = 1.06V
3 = 0.13V
4 = 5.06V
5 = 3.87V
6 = 0.30V
7 = 1.05V
8 = 5.01V
9 = 0.74V
10 = 3.29V

1 = 1.30V
2 = 1.05V
3 = 0.12V
4 = 5.08V
5 = 4.54V
6 = 0V
7 = 1.05V
8 = 5.06V
9 = 0.75V
10 = 3.32V

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

change graphic settings in bios setup between UMA and Discrete.
and see which one works.
but it is risky, because the laptop may lose the VGA video too.
then, you have to reset bios settings.

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

PU9 voltages seem similar.
U1 output can be fixed by injecting voltage on pin7 of JLVDS1.
So they are not important.

If the UMA/Discrete switching did not work, check whether clock and data are provided on JLVDS1 by PCH or Nvidia.
I mean: measure clock frequency of the signal between pin9 and pin10 of JLVDS1.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

There is no discreet graphics on this model.
Yes, the readings seems same for the PU9 chip so gonna ignore that.
Thanks for the help so far!

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

How to check the frequency signal between pin 9 and pin 10? I don't have an ocilloscope.

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

connect the two probes of multimeter to pin9 and pin10.
then measure frequency.
this is a differential signal.
so it is difficult to measure.
I only want to see whether there is any clock on this signal or not.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

0V reading on either DC or AC between the pin 9 and pin 10. Checked continuity and there is no beep but reading of 96. All checked while screen connected to LVDS and powered on.

EDIT: I have also traced both pins to the northbridge chipset (UMA) on the board which is just within the chip area. Doesn't seem to be shorted with either of the two or with ground.

[caspian]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

previously, you got 1.35v on both pin9 and pin10.
since they have the same voltage, you get a 0 difference on multimeter.
now, measure frequency of pin9 relative to ground without pin10.
then, measure frequency of pin18 (and other data pins) relative to ground.
if no frequency, it means PCH is not sending clock pulse and data to JLVDS1.

I suspect some signal from KB930 to PCH is not correct.
If you got time, check KB930 pins which are connected to PCH.
and compare them with the working Laptop.

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

It was 1.38~1.39V DC on Pin10 and 1.39V DC on Pin9 before I killed the board (PQ7 is shorted and needs replacing).

[FShahid]

Re: Acer 5750 P5WE0 LA-6901P, No Display, Doesn't Stay On

Didn't get the time to check it until today. I replaced PQ7 so it is back in working condition same as before.

I have decided to use the screen for parts and the laptop with my LCD TV via HDMI. So everything works except the touchpad (it gets detected in device manager though) but that doesn't matter as I will be using USB wireless mouse and keyboard instead. The laptop is running stable and quiet, most of the time fan is off when idle. At least it works!

Thanks a lot for the help caspian and dj_ricoh! Your input helped me understand a lot of stuff. Hoping to help you back too if I can.

EDIT: Touchpad works by pressing Fn keys combination and found Fn+F7 (not marked on keyboard) to enable touchpad. The 1.3M webcam works as well through the same LVDS cable.

Conclusion: I suspect when it initilly got dead it happened to break lots of components along with it all at once. Most likely the short happened near LVDS connector.
Of those components were the KBC chip, the i5 CPU, the LVDS backlight ic (U1), AND partially the intel northbridge chipset chip. Even though it works but currently all seems to point to the northbridge chip as by looking at the schematics it seems there are dedicated lines for VGA to UMA, LVDS to UMA with their respective CLK lines. Since the VGA and HDMI were not in use when it happened I think the LVDS lines are affected within the chip itself. The lines are not shorted anywhere apparently and the screen which is connected to LVDS is detected in system with correct native resolution, but no display even if backlight is on.
I was suspecting the LID switch but now I have confirmed that the LID switch is functional as when right now I have assembled the laptop and when I close the lid the laptop goes in sleep mode almost immediately.