First return the Cleaning unit (color black @ left side) you will see 2 shaft at the drum carriage lower the cleaning unit but dont lock( 2 lever w/ spring) it, then put the Selinium Drum then insert the shaft. Next the developing unit right side and screw it. Engage now the cleaning unit make sure the lock insert the hole both side. Then put on top of the drum the charging corona & CEL. In sert the whole drum carriage inside the copier . good to go