Potentometer (the power button turny setting thing) or the Power supply.
It's unlikely to be much else to be honest. I'd suggest it's the potentometer as Chinese made ones are generally crap. (sorry China but they are )
I'm no electronics wizz (I can just about wire a plug) so I'm not the best person to advise on how to replace but I do know it's a fairly simple job (and cheap,as in change from a $5 bill)