youve probably put the belt on the wrong side of the wheel
sounds about right, cos u havent mentioned how much of a pig the belt was to put back on
trust me, you do not need to adjust the tension of the belt, its so tight its untrue
remember its a tensioning pulley, so the belt needs to be PULLED over the pulley, not the pulley touching the belt, so the belt is over the part of the pulley you can see from the side of the machine
ive seen that before, people do it because they dont think the other way is possible!
if im right, you need to get some implent behind the belt ( i use an old belt) and pull the belt over the pulley, and please, be carefull, as theres still lots of sharp edges, and please make sure its unplugged
hope this helps, it probably wont need a belt