HP LaserJet 4 Service Manual - Page 260
LaserJet 4 Plus/4M Plus/5/5M/5N on the DC
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Table 7-10 50 SERVICE Fuser Malfunction Checks Checks Is the Fuser Assembly correctly seated? Is the Fuser Bulb/ Thermoswitch open? Action Reseat the Fuser Assembly. Allow adequate time for the error message to clear (10 minutes) and turn the printer ON. The fuser bulb and thermoswitch are wired in series. To check if one or the other is defective, perform the following: 1. Remove the Fuser Assembly from the printer. Is the Thermistor defective? 2. Use an ohmmeter and measure the continuity between pins (see Figure 7-6, callout 1). It should read about 2 ohms (about 4 ohms for the 220 volt fuser). If an open condition is detected, either the fusing bulb or the thermoswitch is defective. Replace the defective component (refer to Chapter 6) or replace the complete Fuser Assembly. 1. Remove the Fuser Assembly and let it cool to room temperature. Does the error persist? 2. Use an ohmmeter and measure the resistance between pins (see Figure 7-6, callout 2) It should read about 220 K ohms +/- 40 K ohms at ambient room temperature. If the thermistor is either open or shorted, replace the thermistor (refer to Chapter 6) or the complete Fuser Assembly. 1. Verify all cabling is correct. 2. Replace the following components in the order given: Power Supply Assembly DC Controller PCA See Figure 7-11 for Fuser Assembly location and Chapter 6 for removal and replacement, as necessary. Note The 50 SERVICE error message can be cleared immediately by shorting the C202 leads on the HP LaserJet 4/4M (or the C205 leads on the HP LaserJet 4 Plus/4M Plus/5/5M/5N) on the DC Controller. Remove the Formatter PCA to access the DC Controller as shown in Chapter 6. 7-30 Troubleshooting