The Museum of HP Calculators

HP Forum Archive 06

[ Return to Index | Top of Index ]

9815A Printer Question
Message #1 Posted by Mike on 19 June 2001, 11:43 a.m.

I have an almost New condition 9815a. The thing is spotless inside. It is as tough this thing was purchased and put aside and never used. Maybe because of the printer problem. The print head is working fine but the paper is not advancing.

What is happening is that during the printing of a line, the printer solonoid energizes at the beginning of the line and releases at the end. It is not actuating on each of the line feed pulses. I can feel the solonoid energize for each pulse but it is as though the energy stored in the coil is not being released between pulses.

I think there may be some defect in the path that is used to release the energy. I don't have a schematic and will try to buz this out later this week but was wondering if the components are all on the main circuit board or if some might be embedded inside the colonoid coil.

OR, might there be another problem that I have not thought of. I know the ratchet mechanism works ok because I have tested it manually. The return spring seems fine.


Re: 9815A Printer Question
Message #2 Posted by Tony Duell on 19 June 2001, 2:06 p.m.,
in response to message #1 by Mike

As far as I know, the solenoid on these printers is just a simple solenoid -- there's nothing hidden inside it. And the solenoid drive circuit on the 9815 is also fairly simple -- just a transistor switch (albeit with a current sensing resistor) But I think your problem is mechanical. IIRC there's a little bit of foam on the end of the armature lever (or maybe the solenoid core). This turns to 'glue' with time and (a) causes the mecanism to stick and (b) allows it to stick from residual magnetism holding the armature on the solenoid core. I would start by taking the printer apart. Remove it from the machine -- unplug the connectors for the printhead, solenoid, and paper out sensor, then undo the 4 screws on the bottom of the machine. Take off the paper sensor board (2 screws on top) to get it out of the way. Take out the 2 screws holding the bar across the front of the printer (caution -- it's spring loaded), then remove the bar, the springs and the printhead hybrid circuit (take great care when handling this). Take out the screws from the right hand end of the printer, and take the frame apart. The solenoid comes free, as does the roller/armature assembly (don't let the return spring fly out). Clean off the remains of the foam, and if there's any problem from residual magnetism, stick some _thin_ foam onto the end of the solenoid core. If it's not a mechanical problem, I'll have to look over the driver again, I guess...

Re: 9815A Printer Question
Message #3 Posted by Mike on 19 June 2001, 2:53 p.m.,
in response to message #2 by Tony Duell

There was some black goo all over the place. The first thing I thought of was that somehow this goo was tacky enough to hold the armature. I cleaned it the best I could without disassembly and it made no difference.

The other thing I noticed was that in order for this to be an effective mechanism (which it is), I would have expected a crisp snappy action. But it seemed a bit sluggish.

Maybe there is some more goo somewhere that is gumming up the works.

I will do as you suggest and take it apart tonight and give it a thorough cleaning. I was a bit nervous about taking it apart because there are some adjustment screw (it looks like) on the aluminum part behind the print head and I was worried about getting something out of calibration.


Re: 9815A Printer Question
Message #4 Posted by Tony Duell on 19 June 2001, 4:23 p.m.,
in response to message #3 by Mike

A couple more comments. Firstly, these printers do feel a bit sluggish (or at least the ones I have do) -- there's a fair amount of rubber damping stuff, etc in them. Secondly, I've taken apart this type of printer in my 9815 and 9825, and I've never had problems with losing the adjustment settings. Don't remove the printhead from the aluminium heatsink block -- take out the bar I mentioned and pull the printhead and heatsink out together. The only other adjustment in the printer is the back stop for the armature (the locknut and slotted shaft at the front right). Don't touch this unless you know it to be out of adjustment. Other than that there's no problem in taking one of these units apart and getting it back together again.

That was it - goopy
Message #5 Posted by Mike on 19 June 2001, 11:32 p.m.,
in response to message #2 by Tony Duell

That was the problem. I disassembled the printer and sure enough, it was much goopier than I thought.

I cleaned the sticky black stuff off and sealed the surface with some tape (temporarily) and tested it.

It worked great. The font height was too high because I think there was a pad at one time between the armature and the adjustment screw (is that correct)? I put some material (again temporarily). If there is what should the material be and how thick?

The printer works great now. I just don't know if the surface that comes into contact with the solonoid core needs some padding to replace what was dissolved. How thick?

I have another question but I'm going to put it into another thread so the subject will make sense.

Thanks a bunch.

Re: That was it - goopy
Message #6 Posted by Tony Duell on 20 June 2001, 2:08 p.m.,
in response to message #5 by Mike

As I remember it : There are 4 little rubber pegs in the holes on the end of the armature. They are fairly stiff rubber, and project about 1mm from the face of the armature (I would have to take one of my machines apart to get the exact dimensions). There's also the soft foam on the solenoid core, but that's less important. On the adjusting screw there's a pad fixed to the end of the screw, I think. It wasn't metal-on-metal contact here. If that's just worn, try screwing that screw in a bit -- so that the ratchet can only pick up one tooth on the advance knob (this will make sense with the unit in front of you). That might be all it needs.

Don't take anything apart
Message #7 Posted by Mike on 20 June 2001, 2:36 p.m.,
in response to message #6 by Tony Duell

It is working ok now. I can figure out what needs to be done to quiet it a bit. Right now it's metal on metal so I put some temporary padding to quiet is a bit. Works well. Thanks,

[ Return to Index | Top of Index ]

Go back to the main exhibit hall