Well I had help from a different forum to check components from VBSTx pins of TPS51620 , I remember caps C552,C553,C534 were chipped off from the motherboard when it came in...
![Не видал? :ne_vi_del:](./images/smilies/bn.gif)
so I decided to check today and found out that there was a dry joint between C534 and R522. Problem has been solved..
![Восторг :ya_hoo_oo:](./images/smilies/bp.gif)