How to Open Your HP48

Since this is a constant topic on comp.sys.hp48, I thought I'd take some pictures of my calculator to show how to open it from the back. In fact, one of the things that worried me about opening my calculator was the imprecise nature of the text directions. But, with these pictures, I hope it will make it easier and less stressful to open your calculator! I'm not giving instructions as detailed as the ones available in pure text form, since I think that with the pictures, it should be a lot easier to figure out how and what to cut.

I should note that I took these pictures after I was all done opening my calculator, so some pictures may show holes you haven't made yet. Don't worry, just pay attention to the directions!

Credits: Thanks to David Fenyes for his excellent instructions about how to open the calculator from the back. In many cases, our instructions parallel each other, since I'm adding my pictures and ideas to his description. Also, thanks to Sean Cier and Mike Sherman for their patience and the use of an SGI, IndyCam and SuperVHS camcorder to capture and manipulate these images.

Here's an overview of what you will do to open the calculator.

1. Back up your data first. Although the calculator should theoretically be able to retain memory without batteries for at least two days, back up anyway!
2. Take out the batteries and remove the IR cover from the calculator.
3. Make access holes behind the IR cover and cut through the four rivets near the screen.
4. Make two access holes in the battery compartment, one by the plus battery terminal and one on the other side of the compartment. Cut through those two rivets
5. Make holes to cut four rivets underneath the bottom "ridge" in the battery compartment.
6. Take apart the two halves of your calculator!

Here are some tools and supplies you will need.

1. An X-acto knife with a good sharp blade. Sharper is better. David Fenyes suggests a number six blade, though others will work well.
2. A wide, flat, thin blade of some sort. This will be used to pry on the case, so the wider and thinner and blunter the better. Otherwise, the plastic the case is made of may be "dented".
3. A drill with a small bit, anywhere from one quarter inch (5mm) to one sixteenth inch (2mm). I used a drill press, which made the work much easier because of the extra control I had over the motion of the drill bit. Obviously, not everybody will be able to use a drill press, but if you can it may make your job easier.
4. The slick backing paper from a sticker.
5. You might want long, thin pieces of metal to spring the metal clips that hold the two halves of the calculator together. I used some stiff pieces of spring steel. Alternatively, you can use a thin bladed screw driver.
6. Matchsticks or similar size pieces of material to hold the pieces of the case apart as you separate it in the last step.
7. If you want to "restore" the structural integrity of the plastic rivets, you will need about four and one quarter inches (10cm) of fourteen gauge copper wire. It should not have any plastic insulation on it.
8. Good light. This is a must, especially for looking into the mysterious inner recesses of your calculator. A bright desk lamp is fine.

Here are the main instructions.

1. Back up your data. To do this, you may either back it up via IR onto another calculator or use a serial cable connected to your computer. For those of you who don't know how to do data transfers, it is explained in detail in Chapter 27 of the HP User's Guide included with your calculator.

2. Snap off the IR cover at the top end of the calculator. Turn the calculator so its back is facing you. Use the wide blade described in number two on the tools list to pry gently in the crack where the IR cover and the calculator back meet. Pry about half an inch from either end of the IR cover. I find a twisting motion works well. That will disengage one (or maybe both) of the molded hooks on the back of the IR cover. If you need to, repeat the prying process in the other end of the crack. With a little care, you probably won't mar the plastic. To avoid any risk of marring the plastic, you can try prying off the IR cover with your fingernails, but I've found that difficult.

3. Next, remove the battery compartment cover and three batteries from the battery compartment, and do not replace the cover, as you will be working in there later. Then peel off the two foam-backed metal contacts in the battery compartment and stick them on the piece of slick paper (number four on the tools list). Set the battery compartment cover aside with the IR cover and the foam-backed battery contacts.

4. Now that you've done that, you're ready to begin actually cutting into your calculator. You need to cut through the four rivets at the screen end of the calculator. There are two on each side of the screen. In order to cut them, make two holes as shown in the picture below.

   

   The location of the holes is obvious, and you can see the four vertical rivets in the holes (They're the circular things). The piece of plastic in between the rivets is another structural piece which gets sliced through just like each rivet. More will be said about this piece later. To make my access holes, I drilled two rows of holes and smoothed the sides with an X-acto knife. I decided to make mine big enough so I could see everything I was cutting in there. As long as you cut where I did, or slightly lower in the picture, you won't hit anything. The important thing is to angle your cutting blade down, away from the circuit board, which is attached at the top of the rivets.

5. Now proceed to the battery compartment. Here's a straight-on shot of what mine looks like after I drilled holes to open the case.

   

   The first thing to do is to cut through the two rivets behind each upper corner of the battery compartment. Here's a picture of the one on the right.

   

   The rivet is the little knobby thing sticking up in the middle of the picture. This is one of the trickiest rivets to cut because of the close quarters. Here, you need to make sure you can see what you're doing, because if you drill or cut wrong, you can hit the circuit board. So take this one nice and easy. Try to cut it relatively high up, maybe even higher than I did in the picture. I could have made a bigger access hole, but I wanted this to be as small and neat as possible, so I made my life a little more difficult than necessary. Now, do the same thing for the rivet in the corresponding position behind the left hand corner of the battery compartment. Sorry this picture isn't as clear as the previous one, because the battery clip was bent down in front of the hole.

   

   To make this hole, bend the positive battery clip until it is pointing straight up. Your life will be considerably easier this way, and I doubt if the hole could be made if the clip wasn't bent up. To make the access holes for the two rivets behind the corners of the battery compartment, I drilled two columns of holes vertically along the wall of the compartment, smoothing the sides with a X-acto knife.

6. Now, tackle the four rivets underneath the batteries. Their locations are shown clearly in this picture. I'm sorry about how dark it is, but you can see exactly where the four rivets are.

   

   To cut these rivets, I'd recommend cutting around them, as I have in the picture, and also drilling right down on top of them. Although it may not look this way, all four rivets are not connected to any surrounding plastic. That's the important part.

7. Finally! All ten rivets are sliced, if you did it right. The only change in structural integrity you should notice is that you should be able to squeeze the two halves of the calculator together slightly if you press the space above the screen, where the HP logo is. Also, the two halves can now be separated slightly in the same place. If you can't move the two halves at the top, here's why. The piece of plastic between the two rivets attaches the two halves of the calculator slightly above where you (and I) made the cuts in those rivets. So, what you need to do is gently pry at the top end of the calculator (the part above the screen) until it pops off the rivets. The place that joins the rivets to the calculator appears relatively small, so it won't take too much pressure. Or, you can try angling an X-acto knife to cut any attachments. The spot I'm describing is shown in a picture near the end of this page.

8. Now for the big step. To physically separate the two halves of the calculator, you need to pry the two halves apart by inserting a small, flat screwdriver blade or spring steel (numbers two or five in the list of tools) in the slots down each side of the keyboard. The slots are located by the [MTH ]key, the [NXT] key, the [ENTER] key, the [backspace] key, the [rightshift] key and the [minus] key. Yes, you do need to fit something down those slots! The trick to opening the case is that there are seven metal clips that hold the case together. They are located near each of the slots at the edge of the keypad, with the exception of one located at the bottom end of the keypad, near the [decimal point] key. The clips are on the top half of the calculator, and they face out and up. They lock under ears on the bottom half of the case, which stick toward the center of the calculator. To separate the two halves, you need to insert a blade into each slot so it gets down past the top half, then pry the bottom half out, pushing the top part of the blade you're using towards the center of the calculator. You can use the matchstick pieces to hold the calculator halves apart, but I found they weren't necessary. Before you start prying, you need to make sure that the positive battery contact is not hooked on the edge of the battery compartment. It has a hole in it that hooks on a "ear" on the edge of the compartment, and if that's hooked, the two halves won't come apart. It has a tendancy to spring back into place all by itself, so check it periodically as you open the case. To open the case, I first pried by the [NXT]key, as shown here. I also used spring steel to do the prying, not a screw driver, so I don't know how well a screw driver would work.

   

   Then, I pried by the [MTH] key.

   

   Then, I pried by the [backspace] and [ENTER] keys.

   

   Those four clips were the easy part. I know of no easy way to describe how to get the remaining three, so take a look at the following two pictures. This one shows one of the metal clips up close. The dark green part underneath the metal clip is the top edge of the calculator (the circuit board was upside down for this picture).

   

   This one shows the locations of the three metal clips down the side of the calculator. They're the light rectangles just above the dark green edge of the calculator. One is all the way down at the left end of the picture, one is just to the right of center, and the third one is one quarter of the way in from the right hand of the picture. I should also mention that the keyboard is at the left-hand end of the picture, facing down, and the screen is at the right-hand end of the picture, facing down.

   

   Basically the only thing you can do is pry in the slots by the [rightshift] and [minus] keys. But two of the last three clips aren't exactly under those slots, as they were with the previous four. Those two take some more careful work. These also require you to pry a little harder than you did for the previous four. As you try to get these three clips separated, keep gently twisting the two halves of the calculator. When you get one clip popped off, you will be able to carefully swivel the two halves of the calculator apart, with some more careful twisting and coaxing! Then this is what you will see - the front and back halves of the calculator!

   

9. Now, here are some other shots of the circuit board of the calculator, for those of you who are interested. This one is a close up of the area where the memory expansion is done. The shiny metal area below is under the battery compartment when the calculator is held upside down. In this picture, you can also see the locations of the six rivets at this end of the calculator (The hollow tubes). As you can see, you need to be careful of the circuit board when you're cutting through the rivets behind the corners of the battery compartment!

   

   The RAM chip is the smaller chip on the left. The gold pads in a row above it are the connectors for the expansion slots on the GX. Pin 1 is at the right in this picture. When I have some more time, I'll do a page about how to expand the RAM. For now, this is all you get :-) I believe the 74HC00 goes on the 14 pads on the right side of the PCB. The other 14 pads, to the left of the RAM, are for the 74HC174 which does address line decoding to enable the 32 banks of RAM the GX can address.

10. And, for those of you interested in the IR end of things, here's a view of the other end of the calculator.

    

    Although I haven't verified this, I believe this picture covers enough area to be helpful to people who want to upgrade the IR. If I do an IR upgrade, I'll do a page about it, but don't ask me about IR upgrades, because I don't know exactly what to do.

11. This is a closeup view of the four rivets you chopped through near the screen of the calculator. By the way, the processor is the large chip in the middle.

    

    If you look carefully, you can see that the rectangular piece of plastic between the two rivets was attached to something, and that the plastic has been slightly "sheared" off. That's what I was referring to previously, so if the top end of your calculator doesn't pry apart, look where my rectangular piece was attached and use a little imagination to figure out where it goes inside the calculator :-)

12. Now, you can put it back together. First, clean up the surfaces of the rivets you cut through. If you want to, you can cut lengths of the 14 gauge wire and insert them in the hollow rivet centers. David Fenyes recommended 1cm. long pieces, but I didn't find them necessary. When you snap the case back together, line up the ten pieces of wire sticking out of the rivets with the holes in the rivets in the other half of the calculator. Snap the two halves together in the reverse order you took them apart, starting by snapping together the bottom clips, then working up the sides of the case. You should hear clicks as each clip snaps into place. Then, bend the positive battery connector back down the way it was. Take the foam-backed battery contacts off the sticker paper and stick them back where they go. Snap the IR cover back on, and you're done! I hope it works!