I've a Sony MBX-266 (Z50CR MB S1202-2 ) which turns off exactly after 70 seconds (1 mins and 10 sec) and shows no display at all. I've already tried flashing both EC bios and MAIN bios , but still the problem remains same ! Anyone having idea of what to do next ?

if i am not wrong , this is model with radeon 7650m dedicated graphic card.

there could be a problem with GPU
try heating GPU up to 150C with hot air gun then see if its going to start.

Also, this model has a problem with memory controller. if you dont know what is going on, when you turn on the laptop, see if the cpu or gpu are getting power ( are they heating up)