Lenovo laptops power on but no display

[Bturlea]

Hello guys!

I have 3 different laptop with same problem: Lenovo ideapad 100( MB: NM-A681 ), Z50-70 (MB: NM-A272) and yoga 700. all of them power on, fan spinning, power light on, battery charging but no display.

Made some measurement on edp port and i found that +LEDVDD 20v, +3VS 3V, but nothing on +LCDVDD_CON, DISPOFF# and INVT_PWM.
I've tried replacing the display, programming bios and ec and no difference.

Can anyone explain to me how video start sequence works on lenovo laptops?
Any suggestion what to check?

 

[ThienBui]

Is the laptop original or has any repaired yet?

 

[Bturlea]

Lenovo ideapad 100( MB: NM-A681 ) i've seen it working.
Left if on a side for 2 weeks as needed a new palmrest and after the part arrived and replaced, on first boot display didn't show picture but windows sound was there, and on secound boot no picture, no sound.