Lexmark Optra 4049 Series, Service Manual

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2-19
4049-XXX
Fuser Theory
The fuser lamp operates off line voltage from the LVPS. The ther-
mistor 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 tempera-
ture. 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 con-
tinuously, the fuse melts and shuts off power to the lamp.
The thermistor output voltage is proportional to the fuser tempera-
ture and is on engine board J8-4. The voltage cycles from approxi-
mately 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 resis-
tance 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-