Hello, Need to fix this new lcd tv (50 lbs) on this appartment wall. Can't find any stud with a stud finder, seems to be no nails, sound doesn't change when I tap different places with a hammer. I unscrewed this electrical box (see pic in attachment)... from what I know, the box should be fixed to a wood stud, which I don't see. Can I conclude it's a concrete wall ? or what ?