ArjanH, You need to loosen all the socket-head set screws (Some people call them grub screws) that are holding the main gear, belt sprocket, one-way bearing and the collars that lock the main shaft @ the specified height, and the collar that grips the pitch control rod that runs up the middle of the main shaft. Some of the components might have more than one set screw. Also, make sure you disconnect the linkages from the head. It should slip out after this is done.
Al