Printronix L1024, Maintenance Manual

Page: 131 / 372

7–35 Chapter 7 – Fuser Subsystem
Temperature Control Section (Thermistor Troubleshooting)
To detect the temperature of the fuser roller, the engine controller sends an analog signal, TMRPWR, to
the Stepper Motors Controller Board. This signal exits the board at connector CN8–1 and is passed
through cabling to the thermistor. This, in turn, is fed through the thermistor and back to the Stepper
Motors Controller Board at CN8–2 as TMRSG.
The heat control thermistor is mounted so that it makes contact with the fuser heat roller at about the
center of the fuser roller.
The thermistor has a negative thermal coefficient; when the fuser temperature rises, the resistance of the
thermistor goes down, and vice-versa. Normal operating boundaries of the fuser are in the range of
182
°
C to 188
°
C. During normal operation, when the temperature reaches the lower trip point of about
185
°
C, the engine controller turns on the HEAT signal. Then the lamp heats the fuser roller until the
temperature reaches about 186
°
C—however, this temperature might be different, depending on the
setting of the temperature on the Control Panel. At this point, the engine controller shuts off the HEAT
signal. There will be some thermal lag, so the temperature might exceed this set point by 5
°
C or more. To
measure the temperature, use a temperature probe. Access to the fuser is limited, so obtain the smallest
possible diameter surface-reading probe.
Thermistor Malfunction Symptoms
There are several conditions where you might suspect that the heat sensing control circuit is
malfunctioning:
S
If a “THRMISTR FAIL” or “HEATER FAIL” error message is displayed on the front panel.
S
If the thermal breaker needs replacement (see previous section).
S
If the printed characters are not fusing to the paper correctly.
1. A “THRMISTR FAIL” or “HEATER FAIL” error message signifies one of the following
conditions:
a. The thermistor circuit has opened up or shorted, or the fuser never reaches the correct
temperature.
b. Units other than the thermistor circuit could be at fault.
The thermistor condition can be checked by removing the fuser heat shield cover. Unplug the
thermistor lead. Measure the resistance of the thermistor. At normal ambient temperature
(
≈
20
°
C), the reading should be from 150 k
Ω
to 200 k
Ω
. If you find an open circuit, replace the
thermistor (see fuser removal and replacement).
CAUTION: Be sure to check the heater lamp and thermal breaker according to
the first portion of this section before you continue. An open
condition of either the lamp or the thermal breaker will cause the
thermistor to appear defective (by not heating up the fuser roll).
2. If you found the thermal breaker to be open during testing, thoroughly check the thermistor
section using Step 1 above and Step 3 below.
3. If the print on the fused paper feels excessively raised, then there might be an
undertemperature condition at the fuser. First, confirm that the fuser temperature set via the
control panel matches the type of paper being used.