Fuser
If you are getting a "50 Service Error" there are 2 causes for this. The fuser is one and the AC power block is the other. Here's a quick test to tell which it is. Open the cover, turn on the printer and then try the following test - in that order. Open the top cover and insert a small screwdriver into the small hole which has a lever to signify the case is closed for about 10 seconds. This is the hole right above the case release lever, and is long and narrow. Put a screwdriver in side this hole when the cover is open. If you see a bright light on both ends of the fuser for a few seconds, then the problem is not the fuser and you should replace the ac power block. The printer needs to be plugged in and turned on. You should see a really bright glow on both ends of the fuser - you can't miss it, it's real obvious. If you need to replace the fuser it can easily be removed by taking out 4 screws without having to disassemble any other components.

AC Power Supply
If this is bad you will also see "50 Service Error". This is the power supply which also contains the power switch and that which you plug the printer into the wall. To remove you will need to remove the fuser first and then by removing a few screws it will easily unplug.

Motor & Gear Assembly
If you are getting no movement of paper when the printer is supposed to print you may very well have damaged gears on this assembly. If the printer makes a loud buzz as opposed to it's usual humm when powered up the motor itself may be shot. You can replace this as a unit or by removing 4 screws you can separate the motor from the gear assembly. To remove this from the printer you will need to first remove the fuser, the AC Power Supply, and the black plastic cover, via a single screw. It would be a bit easier to also remove the DC Power Supply to get at one of the screws though not essential.

Pick Up Roller
These do get warn, and you will see "13 Paper Jam". If your paper is not picking up very well, thus leaving black smudges on the paper or only going in part way the Pick Up Roller may need to be replaced. One way to test for sure if it is a bad pick up roller is to remove the paper tray, and with a piece of cotton saturate the rubber pickup roller with rubbing alcohol. Before the alcohol evaporates try to run the printer again. If it now works prior to the alcohol evaporating your problem is infact the rubber pickup roller. Remove the DC Power Supply to get at this Roller. Sometimes you can clean these and get a some more life out of it. While you are fixing the pickup roller assembly take a look at the seperation pad on the assembly, they do wear out as well.

DC Power Supply
If the paper is not getting fed properly it is possible the DC Power Supply is no good, but very unlikely. If you do need to replace remove the front steel panel and then take out the 3 screws which secure it to the base. A common problem with the DC Power Supply are the pads on two solenoids attached to a PCB board which is attached to the DC Power Supply. If these pads get warn they somtimes cause the solenoids to stick and the printer will not operate properly. Common symptoms are mis-alligned printouts. Simply increase the size of the pads by using electrical tape or purchasing some small inserts to replenish their thickness. These same pads at can also become sticky. What happens is the DC solenoids of will accumulate core magnetism, which will prevent reliable release of the armature. HP apparently put rubber pads on their solenoids to break the intimate metal to metal contact and prevent the residual magnetism from being a factor. Unfortunately, the material they chose - the rubber pads - gets gummy over time. So the cure is to cover the two sticky pads with a none stick cover.