Lexmark T520 Service Manual - Page 159

4520-xxx Fuser Operation Hot Roll Fuser The hot roll fuser is made up of two primary assemblies. The first assembly is the top fuser cover which contains the thermistor/cable, thermal fuse, cable assembly, left and right lamp brackets and cover. This is a complete assembly and the parts cannot be individually replaced as in previous Lexmark laser printers. The thermal fuse (two fuses in series) protects the fuser assembly and surrounding parts from ignition in the event of a failure in the fuser control circuitry by opening the AC circuit to the lamp. The thermistor assembly is designed to keep the temperature of the fuser hot roll within a predetermined temperature by being in contact with the fuser hot roll and changes resistance as the temperature of the hot roll increases. The thermistor resistance changes from approximately 150K ohms to approximately 260K ohms as the temperature of the fuser hot roll increases. The thermistor is connected to the fuser control board connector J2-1 and J2-2 and from J2 to J1-2 and J1-3 on the fuser control board to J16-2 and J16-3 on the engine board. J16-2 (THERMGND) and J16-3 (THERM) are connected to module U10. U10 monitors the change in thermistor resistance and sends a signal (HEATON) from J30-A25 through the interconnect board to CN3-9 on the LVPS to signal the triac to turn the fuser lamp on or off to maintain the correct fuser temperature. The voltage drop across the thermistor is proportional to the fuser temperature and varies from approximately 2.0 V dc (cold) to 2.3 V dc (warm during idle) to approximately 2.7 V dc during continuous printing. These voltages are assuming the fuser temperature is set to normal. The HEATON signal from CN3-9 varies from 0 V dc (lamp on) to +5 V dc (lamp off) and is connected to the cathode of an Opto triac to control the fuser lamp. The second part of the assembly is the fuser frame which contains the hot roll, backup roll, and associated parts as well as the paper exit guide assembly, narrow media sensor, fuser control board and the envelope conditioner solenoid. Diagnostic Aids 3-27