Printer and Photocopier Troubleshooting and Repair Collection : HP LaserJet III cold fuser              
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HP LaserJet III cold fuser

(From: Flemming (flemming.gottsche@get2net.dk).)

Where the fuser is cold and the relay trips, check Q153 for leakage (C-B) which results in current leaving the base and charging C158. When C158 is charged to about 3.3 V, Q153 is turned on and the relay trips.

The relay that is tripping is RL101, which actally is located on another PWB than the transistor. This relay has a very distinct sound, which has been mentioned over and over again in postings to various news-groups. Both PCBs are placed in the rear right corner of the HP LJIII.