gearbox problem on a renault scenic manual ?

i have a renault scenic 1.6 petrol manual,old gearbox broke so i changed it butnot sure if its the right 1 as it is trying to go into gear/grinding (loud) when the engine is running and its in nutral but as soon as u put your foot on the clutch it stops, then you take it back off again and it goes back to doing it again. i've changed the clutch and back plate + the thrust bearing twice so its not that.......?, i was told that it could be the wrong gearbox......would that do that as i bought the new gearbox from a scrap yard......any ideas please help