Step 3: Removing the power supply
Remove two screws labeled "M4" (circled in the picture below) -- these screws are holding the tray guide in place.
Now you can remove the plastic tray guide (located on the inside of the printer cavity) ... press the small white locking tab pictured below, while sliding the plastic tray guide out and free.
Remove the metal power switch rod (lift and rotate to free the rod).
Unplug the power supply cables :
Unplug the two white ribbon cables as pictured below. Also unplug the large connector with multi-colored wires.
If you're using a LaserJet 4250/4350 printer, this next picture will apply to you. Unplug the four wires circled below. LaserJet 4200/4300 users can skip to the next picture.
Now feed all the unplugged wires through the slot pictured below, into the tray cavity. LaserJet 4200/4300 users -- you will be feeding a total of 3 cables through the slot. LaserJet 4250/4350 users -- you will be feeding 6 cables through the slot.
Removing the power supply.
Now remove the four screws circled in red below -- they are labeled "M4"..
On the other side of the printer, remove the 5 screws circled in red below -- this will free the metal access cover (yellow arrow), remove the metal access cover.
Gently, carefully, unplug the fan's connectors from the power supply. The wires on these connectors are easy to "tear" -- be cautious.
Now, at the back of the printer, gently twist the power plug to the right (shown below) to free its locking bumps -- then pull the power supply out to remove it.