Use a piece of cardboard.
To see if the shutter curtain is hanging up, take a piece of cardboard like from a cereal box and block off a 1mm strip of the opening on one side of where the film goes. Then do the white test the others are talking about. You should see light when the shutter is released. Then try it a little further across but you will have to either use two pieces of cardboard or cut a slit (1mm) in the one you used. Then try that at the other end of the opening. In other words, try blocking off all of it except the left side, the middle and the right side. If you are seeing light at the 500 and 1000 speed then it is not the camera. If it is, that could mean it is time for a cleaning of the camera. Ric.