ZX5270 : blank screen ..oopps

Hi and hello everybody,
I 'm a desktop technician, not a laptop tech and i've some trouble.

Ok, so at the beginning, i would like to replace the 2x256 RAM, so i 've buy 2x512
when i puted my 1st 512mo (on the back) i've seen my left hinges (= broken !!!)
so i decide to desassembly this laptop.

I've remove :
- Battery
- HDD
- back plastic (over the cpu)
- copper and cpu (very carrefully, all pin ok)
- plastic (at the top of the keyboard - like that i've replaced the seconde RAM module)

After all part were assembled : when i press the start buton all hardware start but screen doesn't start.. Oopppss

- i've never touch electronic part with my finger (and touch metal before begin)
- so, i've try to remove et reseat new RAM and old ram module (many many time)
- remove battery and start with AC power only
- remove battery, unplugg AC power and press power buton 1min
- try keyboard function (TV out <-> LCD)
- unplug - plug CPU 2 times
- try to plug external LCD (blank screen to)

But i'ts always the same issue : blank screen.

Any suggestion will be the welcome !

Ferris
France