Printer and Photocopier Troubleshooting and Repair Collection : Testing the erase lamps on an HP LaserJe..  
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Testing the erase lamps on an HP LaserJets

(From: Tony Duell (ard@p850ug1.demon.co.uk).)

There's a very quick test for the erase lamps in these printers (SX engine). Open the cover, and swing it right open. There's a plastic post that comes down from the cover at the back left with 2 contacts in it. It goes into a slot on the left side of the fuser when the printer is closed.

Check for continuity between the 2 contacts in the cover. I have no idea what the right resistance would be, but it's less than 1k. If it's open-circuit then most likely one of the lamps is burnt out. There are 5 of them, in series.

If the lamps are good and there are still erase lamp problems, then remove the fuser, take the cover off the left side of it (1 screw on top) and unscrew the PCB under it. Check the 2SD1414 power transistor Q331 on this board (it's the erase lamp driver). If that doesn't do it, check the components around it. R335 (1.8 Ohm) is the current sense resistor in Q331's emitter circuit, L331 is a filter choke in series with the lamp supply, and Q331 is a current sense transistor that removes the base drive from Q331 if the thing overcurrents.

I suppose it could be a problem with the cable back to the DC controller or even the DC controller itself, but that's unlikely.

If you've still not found the problem, though, post again and I'll think again.