how can i fix my d600?

take it to a computer shop... i think to what you have said. the dc socket internal soldered connections are loose and are shorting out when you put the end of the adapter in.
dont use it anymore, because the damage can cause a circuit burn out on the board where the dc socket connects.
the cost of a simple either replace or resolder is average £85.00 to £125.00 (depending on damage report)

the workshop
August 2009