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HP9815 - repairing the paper advance mechanism
Posted by Mark Edmonds on 19 Oct 2008, 8:29 a.m.
If your HP9815's printer is not advancing paper correctly, this may be a solution.
Two ways to identify the fault that is fixed here.
1. Does the paper advance slowly, intermittently and by very small amounts (one pixel line)?
2. When you print something, does the printer make a single "click" noise instead of multiple "click" noises?
If yes to either of those questions, read on.
Basically, what has probably happened is a foam pad has decayed inside the printer mechanism and left a horrible tar-like goo over part of the mechanism.
Step 1 is to open the machine.
1.Unplug and remove the fuse holder inside the paper roll bay.
2. Turn the machine over so the underside is facing up with the keyboard at the front.
3. Remove the top two screws. These are the top cover screws.
4. Remove the four screws in a large central rectangle underneath the keyboard area. These are the keyboard screws. The four screws in the smaller rectangle on the left hand side are the printer screws. Leave these for now.
5. Hold everything together and turn the machine over. Lift up the keyboard section a little and remove the rear top cover.
6. Now remove all connectors attached to the main keyboard assembly. There is one underneath that should be done first otherwise it might get pulled up unevenly and damage the pins. Put the keyboard assembly to one side.
7. Disconnect all leads to the printer assembly - 4 connectors then unscrew the printer assembly from the under side. Lift out the printer.
8. Hold the printer assmebly so you are looking down the platen axis with the manual advance wheel nearest. There are three screws on the end panel that must be removed to disassemble the printer. Once undone, lift the end panel off. Some parts of the printer are under slight tension so it will expand out. Hold your hand round it to maintain its shape.
9. Now you can disassemble the printer which is easy. Just lift bits out and make sure you note the positions for re-assembly later.
10. There are two areas that need cleaning. The reside left by the decayed foam is really nasty. It is a sticky tar that is difficult to clean and wont wash off your hands easily. Try to avoid touching it.
11. The first area to clean is on the magnet or metal bar attached to the stator. The area where the platen advance ratchet lever contacts the metal bar should be black and sticky.
12. To clean, I used a Stanley knife blade held at a narrow angle (like cleaning glass) and scraped the goo off. Once you have got most of it off, use a spirit rag over the bar to clean the last bits off.
13. The second area to clean is the platen ratchet advance lever. This is a metal pad which gets pulled to the magnet then released where upon a spring pushes it out and it engages with a notch in the advance wheel that pushes the platen round a small amount. The metal pad on the advance lever will be covered in the same black tar and because this is sticking to the magnet bar, the mecahnism never releases properly and the paper doesn't advance. Clean using the same technique as before. You will also find four mounting holes that need cleaning out. I used a toothpick for these.
14. Once you have got everything free of the tar residue, reassembly the entire machine, switch on and test.
15. What about replacing the foam pad? I didn't do this for two reasons. Firstly, I have no idea how big it should be and how dense the foam is. Replace it with something too hard and the lever will not be able to run its full travel. Also, the only purpose of the foam that I can see is to reduce noise slightly. As the printer is noisy anyway, I don't think there will be much difference. If I ever find out exactly what sort of material and what size is needed, I'll update this post.
15. If all is OK, the printer will now advance paper correctly and instead of a single click when you print, you will get a rapid-fire series of clicks, once for each line of pixels.
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