The Museum of HP Calculators
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Remove two screws from under the lower feet. On card reader models, remove two screws from under small caps near the AC adaptor socket. On non-card reader models, remove two screws from inside the battery compartment. Now the painful part: Remove two screws from under the top corners of the large label on the back. You can carefully peel this back and replace it but it probably won't look like new. A hair dryer might help to soften the adhesive.
Additional screws hold the spine to the front case. When the spine is removed, all of the keys and switches will be freed so be careful. The silicon grease under the slide switches is meant to be there - don't clean it off. Be sure to line up the pins and holes in the keyboard, spill guard sheet etc. when reassembling. If the keys don't feel right, your alignment is incorrect. Be especially careful when disassembling card readers. A large number of very small parts will come loose.
Remove the battery. Remove two screws from under the top feet. The keyboard will then pivot way from the case on a hook at the bottom. The clearance of the battery contacts is tight and you may need to push them through. On reassembly, make sure the dust guard attached to the contacts ends up in the correct position before screwing the case closed.
Remove the battery. The upper rubber feet are attached on one end and free on the other. (The right one is attached at the bottom and the left is attached at the top.) Pry up these two rubber feet and remove the screws underneath. Remove two more screws from the battery compartment, and one from the charger socket.
Remove two screws from the battery compartment. The case now splits at the top and is hooked at the bottom. Pivot the case parts apart. It takes a fair degree of force (enough to scare many people away) to pivot the cases apart. Reassembly is much easier.
Early models have the chips held in place (without solder) by a backbone. If you disassemble this part, make sure you note where the chips go first. The chips will be loose!
Remove the batteries. There are four screws underneath the feet that are held on with adhesive. Peel up the feet and remove the screws. The back will come away easily. Peel open the protective film which is held closed by adhesive at the back. The IC chips are behind the display and another piece of protective film will need to be removed to get to them.
Remove the battery pack. There are four screws underneath the feet that are held on with adhesive. Peel up the feet and remove the screws. The back will come away easily. This calculator will not work properly if the case halves are not held securely together. Overtightening the screws can strip the threads and lead to a malfunction.
Remove the battery pack. Remove the 6 visible screws on the bottom and the bottom comes off easily.
Remove the batteries. Remove the card reader or back filler plate where card reader would go if unit has no reader. The HP-IL module and any RAM or ROM modules can stay in the unit, or you can remove them. (If you have a memory module in the card reader slot, it should be removed.) Carefully remove the 3 pads on the bottom of the unit. Remove the screws from the battery compartment and one from each corner of the case. (One of the screws is in the card reader expansion area.) The case halves may now come apart easily. If they do not come apart easily, check for two additional small screws on the back near the HP-IL module. These are present on some versions. (Probably only metal-backed units have the extra screws.) You'll need to remove the two closest to the module door (near the edge of the case).
Remove 7 obvious screws from the back and 4 not-so-obvious ones that are under the feet.
Use a coin to unscrew the battery pack and remove it. Use a coin to unscrew the large expansion door and remove it. Remove four screws from the back (two are in the memory compartment.) The back will feel tight especially on the battery side. Careful prying and shaking will separate the two case halfs. On reassembly, make sure the back is evenly closed with all ports properly seated before screwing it together.
Remove two screws in the front and two under the front sides. The top pivots up and locks into place. Press the latch inward to return the case to the closed position. The the CRT is a high voltage device. Take great care around it and the power supply.
Remove the easily visible screws on the top of the case allowing the top to slide. On the HP 9810 and 20, remove the four screws from under the keyboard area and remove the keyboard to slide the top panel off the front. Beware of high voltages from power supply capacitors.
Remove the paper and the fuse in the paper compartment. There are ten screws on the bottom of the case. Four near the serial number are set deep and hold the printer in place - leave these alone. Remove the six other screws. (The six screws are not deeply set in the plastic case.) Carefully turn the calculator right-side-up and lift the darker keyboard/display piece up slightly so that the rear top case and printer cover may be removed. You can then disconnect cables in order to rotate the keyboard/display piece all the way forward. Beware of high voltages from power supply capacitors.
Most of the newer models including the Pioneers, the Champion/Clamshells and the 48 series are not designed to be disassembled. It's very likely that disassembling one will leave you with junk!
If you feel compelled to disassemble one anyway, you generally start by carefully removing the metal keypad plate. You can use a hair dryer to warm and soften the adhesive and you must proceed slowly and carefully to avoid bending. (It's unlikely that you will be able to restore this plate to its exact original condition.) Too much heat can damage the calculator. Then carefully drill off the tops of the heat-stakes.
On reassembly, you may be able to remelt some of the heat-stake remains, or use glue or tiny screws. It's essential on most models for the two halves to be held together firmly and this can be difficult to achieve.
You may find more detailed instructions on disassembling the HP-48G elsewhere on the net.