I have GBC Savantaa sigma AAS instrument with automatic burner rotation facility. Yesterday, my staff changed the burner head from Air-acetylene to Nitrous oxide-acetylene. Since then the burner angle is fixed at one point; 10 to 20 degree rotated from angle zero. No matter which angle I set, it will rotate and go to that particular position. I have checked different burner heads, cleaning the whole assembly of burner mounting but result is the same. What could be the issue??