Fuser Theory; Fuser: 920, 921, Or 922 Error - IBM 3112 Service Manual

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 ther-
mistor 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 will cycle from approximately
2.0 to 2.3VDC as the lamp goes on and off during idle, and will be
around 2.7VDC 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
0VDC when the lamp is to be on and +5VDC 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 220V lamp installed in
a 110V 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 has blown, the fuser and the fuser wiper will be dam-
aged. 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 +5VDC; if it does not, replace the
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