Printer and Photocopier Troubleshooting and Repair Collection : HP LaserJet III powers on but appears de..  
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HP LaserJet III powers on but appears dead

"When I flip the power switch on, I can hear it power up, but the none of the lights comes on and the LED panel is also dead."

(From: David J. Pittella (ddc_pitt@ix.netcom.com).)

Since you say the unit 'powers up', I assume you hear the engine power up (fan, drive motor, etc). Assuming this is a Lajerjet III (not IIIp or IIIsi, etc.), you can do an engine test by removing a small cover on the bottom of the right side, removing this cover will expose a metal plate with a hole in it. The hole exposes a switch on the DC controller which will print simple line test pattern. This will at least confirm that the engine is working.

The front panel LCD display and status LED's are driven by the formatter board. The formatter gets its +5 power from the DC controller via (2) screws that secure the formatter to the CD controller. If you remove the bottom plastics and bottom pan, the formatter is the large main board. The DC controller is the smaller board with several connectors attached to it.

I am also making the assumption that the cable leading from the formatter to the front panel is attached at both ends?

You may also want to remove any options, (memory, optional I/O) as one of these options could be preventing the formatter from initializing and possibly lead to a blank display. BUT, the best guess here is that the formatter is not getting power - check those screws between the DC controller and formatter!