I have got a scientific Sharp calculator, which I have been using since junior high. At school we currently need quadratic and cubic equations, but unfortunately my calculator didn't had the functions included. I didn't wanted to buy a new calculator, but a friend had a similar model. It had those function, so I decided to take a look inside to find differences, and finally got lucky.
Scientific Sharp Calculator
10KΩ 0603(1608) SMD Resistor
Phillips ph00 Screwdriver
Plectrum / cellphone lever tool
Hot air Gun / Soldering iron with fine tip
Optional: ESD protection
Step 1: Taking a Look at Your Calculator
Start by unscrewing and removing the Battery Cover. Under there you will find the main battery, who has to be removed, and two additional screws. After removing all the cases screws you will be able to take of the front cover. By carefully prying up the case using a plectrum ore one of those Cellphone lever tools, you will be able to completely remove the front keypad. In there you will find a PCB whit a single blobbed COB IC and some SMD components. On the PCB Silkscreen there is normally printed a table with all available model variants, and their component differences. You are looking for two empty solder pads near the blobbed IC, who are marked with 10KΩ on the table. In case of the EL-509X it is R21. All the other components on the table are just part of the solar charging circuit. Time to heat up the soldering iron.
Step 2: Time to Solder
Apply some solder flux to the pad and solder on the resistor. Normaly you shouldn't need to add solder, because there is already enouth solder left back on those coding resistor pads left from the reflow process. (they are propably using the same pastestencil for all variants) Short out the battery terminals to ensure that all capacitors on the pcb are completly empty. Reasemble the Calculator. Now you should have unlocked the equation functions and depending on the modell some additional features.
Step 3: Sharp EL-509W
Very similar the Sharp EL-509W. You can open it by removing the screws and and lever it open on the two guide rails for the cover on the sides. The Resistor on this model is R6.
Step 4: Sharp EL-W351XH
The Sharp EL-W351XH is a bit special. Because the full option model (EL-W506X) has a different button assignment than the base model, so adding the resistor will change the function of some of the function buttons. If you should decide to do the mod the Resistor is R31 and the new assignment is shown on the last picture.
Step 5: Sharp EL-W531G
The Sharp EL-W531G looks at first glance similar to the Sharp EL-509W, but it isn't. While the EL-509W can be opened by the two guide rails on the sides, the EL-W531G is opened by removing the front cover like all other models. The resistor on this model is R31 like on the EL-W351XH, but unfortunately the key alignment of some of the function keys will change like on the EL-W351XH.
Step 6: Sharp EL-W531TG
The Sharp EL-W531TG is easy again. The Resistor is again R31, like on the previous models, but the key assignment won’t change.
If you have got another similar Calculator who is not mentioned, and you would like i add, please feel free to send me a mesage wiht some pictures, and i will include it.