Asus pu551l not turn on (mainboard pu551la rev 2.0)

Hi everybody
My ASUS PU551LA (REV 2.0), after a week powered off with power adapter disconnected, is not booting anymore.
If I plug the adapter, the battery is charging, the power led lights but after pressing the power button there is no sign of life.

I've found the Boardview file of the motherboard and the schematic of PU551JA (that looks 99% compatible).

I've disconnected the battery and all devices except screen (I removed also the RTC battery, hope it's not a problem at this stage...) and checked all signals and power rails involved in the power on sequence (attached).



All signals up to PM_SYSPWROK and BUF_PLT_RST# seems good (they rise to 3.3V when I plug the adapter in or after I press the power button), but the display doesn't light and there are no beeps. The cooling fan sometimes starts (for a while) and sometimes not…

Since the boot process arrives at this step, I suppose EC and PCH are good, am I right?
Do you have any suggestion on what should I check next? Don't tell me the CPU has gone…
I've ordered a logic analyzer to check if the flash chip is read properly. Is it possible that the problem is in the flash chip?

Thank you for you help

Machine napisał(a):Measure the voltages on pins 12, 14, 17, 18, 22 of J4511.
Does it work on an external monitor?

Thank you a lot for pointing me in the right direction!!!
It took a little for me to get and external monitor to test, but today I did: the external monitor shows the BIOS screen ! I was wrong thinking about bios flash issues. Thanks again.

The voltages on J4511 connector are:
22: about 19V
12: 3.38V

14: 0V
18: 0V
(I hope I measured them properly , I was a little afraid to short adiacent pins on the connector...)

17: 0V

Going more in deep, pin 17 is 0V because L_BKLT_EN is 0V and drives D4504 low, while LCD_BACKOFF# is 3.3V.



Would you be so kind to help me with further steps ?
Thank you a lot again.

Thanks a lot for the detailed and valuable information!

My bad, I wrongly assumed that all the signals were directed from motherboard to LCD panel so I made the measurements with the LCD connector detached (to have easier access to the pins and to avoid that possible shorts in the cable or any issues with the LCD panel could affect the measurements, since I was searching for possible problems within the motherboard).

I re-measured the signals with the connector on and now both EDP_HPD and BL_EN are 3.3V.

So I assume the motherboard is ok and I'm going to replace the LCD panel.

Again, thank you all for the valuable help.