HP LaserJet 4 Service Manual - Page 259

50 SERVICE Error - Fuser Malfunction Warning The Fuser Assembly is HOT. Allow the assembly time to cool before servicing. The purpose of the fusing system is to maintain a constant temperature on the fuser roller. A high intensity fusing lamp, located inside the fuser roller, provides the high temperature required for the fusing process. While the printer is in standby mode, a temperature of approximately 172° C is maintained on the fuser roller. While the printer is in printing mode, a temperature of around 183° C is maintained. The main components of the Fuser Assembly are the fusing lamp and its associated fusing and pressure rollers, a thermistor, thermoprotector, and the AC controller/safety circuitry in the AC Power Supply. The fusing lamp is a specially designed 460 watt (220V), or 500 watt (115V) halogen lamp that operates at line voltage. The thermistor is a resistor whose value changes with temperature. At room temperature the resistance value of the thermistor should be between 180k ohms and 280k ohms.The thermoprotector is a switch designed for over-temperature protection, and opens when the temperature reaches 230° C. The thermoprotector will not reset when the fuser cools. It may be necessary to replace the thermoprotector or fuser assembly if the thermoprotector opens. The following conditions result in a fusing error: 1 Unable to maintain a temperature above approximately 180° C. 2 A temperature above 230° C. 3 A condition wherein a temperature of 165° C is NOT reached within 90 seconds after power-up. Note If a 50 SERVICE error message is displayed, turn the printer OFF for at least 10 minutes. Failure to wait the full 10 minutes before turning the printer ON results in a continuation of the error message and the shutting off of the fuser lamp current during this time, though the source of the error may have already been resolved. Troubleshooting 7-29