Lexmark Optra N model 245 Service Manual - Page 46
Fuser Theory, Fuser: 920
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4049-XXX Fuser Theory The fuser lamp operates off line voltage from the LVPS. The thermistor is in contact with the fuser hot roll. The system board reads the thermistor resistance and toggles a TRIAC in the LVPS which cycles the lamp on and off to maintain the proper hot roll temperature. Inside the fuser shroud there is a thermal fuse which is in series with the lamp; if the TRIAC fails and leaves the lamp on continuously, the fuse melts and shuts off power to the lamp. The thermistor output voltage is proportional to the fuser temperature and is on engine board J8-4. The voltage cycles from approximately 2.0 to 2.3 VDC as the lamp goes on and off during idle, and will be around 2.7 VDC during continuous printing when the Fuser Temperature is set to Normal. In Power Saver mode the lamp is left off until a job is printed. The engine board signal to the LVPS TRIAC is on J4-16. The voltage is 0 VDC when the lamp is to be on and +5 VDC when the lamp is to be off. The lamp can be tested by briefly connecting this pin to ground. Fuser: 920, 921, or 922 Error A 922 error during a POR caused by low line voltage can usually be cleared by doing another POR. If the fuser lamp is lighting very dimly, check for a 220 V lamp installed in a 110 V machine. Check all the connectors to the exit sensor card and the lamp power connections at the left end of the fuser. Make sure the thermistor is making contact with the hot roll. Check the continuity of the lamp, the thermal fuse, and the fuser power cable. Check the thermistor resistance at engine board J8-3 to J8-4; it should be from 100K Ohms to 260K Ohms when cold; it may be as low as 2.3K Ohms when hot but increases quickly as the fuser cools. There should be no connection from either pin to ground. If the thermal fuse is open, the fuser and the fuser wiper may be damaged. Before replacing the fuser and the fuser wiper, check the voltage at engine board J4-16 as you POR again. It should stay at 0 until the 922 Error is displayed and then go to +5 VDC; if it does not, replace the engine board, the fuser, and the fuser wiper. Next, mea- 2-19