The Gamma sensor is located just to the left of the transfer belt unit (in the main body cavity. If you slide out the process unit (Drum, developing, transfer belt are on the one assembly that slides out on rails) you should see the unit i am talking about. There are three sensors on the assembly two are the colour registration sensors and the one in the middle is the Gamma sensor. They are shielded by spring loaded small plates that operate by solenoids. These can be manually liftes to reveal the sensors.