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Problem:
 
When warming up, your HP Laserjet 4 shows the following message on the LCD panel:
 
50 error
 

Solution:
 
This may have several reasons:

• A burnt out quartz fusing lamp:
  • A good fuser lamp should have about 6 to 8 Ohms (the 110 Volts version about 2 Ohms)
  • The lamp is connected to the right (larger) contacts of the fuser-unit
• A defect power supply:
  • While the printer is warming up, the power supply outlet for the fuser unit should
    provide about your mains voltage for some seconds.
  • If it does not, the triac or the optocoupler which switch this output, might be defect.