Casio Calculator RAM Upgrade

This will show you how to upgrade the RAM on a Casio fx-9750G plus graphics calculator from 32 KiB to 64 KiB. This may also work on similar 9750/9850 models which use a 32 KiB RAM module.

casio_9750_RAM_ROM_CPU

If you un-screw the back cover, you can see a RAM module, ROM module, and a CPU under an epoxy blob. In my calculator the RAM module was a Hynix HY62WT081ED70C. From looking up the datasheets online, I found that this was indeed a 32 KiB low-power CMOS SRAM module. More expensive models of this calculator such as the 9850G plus advertise 64 KiB RAM. The hardware between the different models in this series is very similar, and there is space on the PCB to mount a larger RAM chip. So in theory the fx-9750G plus may be expandable to 64 KiB of RAM by de-soldering the existing RAM module and soldering in a larger RAM module. However, the risk is that the firmware may limit the RAM to 32 KiB, in which case the RAM upgrade will fail. Luckily, it seems that Casio has designed their firmware to support both RAM sizes.

casio_9750_RAM_removed

The first step is to de-solder the existing RAM module. I used a heat gun and tweezers to carefully heat up the part until I could flip it off with the tweezers. Be careful not to burn the plastic connections to the LCD above the RAM module. Also be aware that the heat will transfer through the PCB into the LCD behind it, so make sure you apply the heat evenly and quickly. The picture shows the bare circuit board after removing the RAM module. There will be some residual solder left on the pads. This must be removed in order for the new larger RAM module to lay down flat on the PCB. To do this I used some copper braid as a solder wick. Once the area is flat you may want to clean the board with a little flux to ensure that the new RAM module will solder in place properly.

casio_9750_new_RAM

I found a suitable replacement for the RAM module online. The module I ordered was a 128 KiB low power CMOS SRAM, part number AS6C1008-55SIN, which cost a little over $3 NZD. Only up-to 16 bits of the address space is used by the calculator, so unfortunately you cannot use all the 128 KiB of RAM at once, but only 64 KiB at a time. If you really wanted you could solder a switch into your calculator to switch between the two banks of RAM in order to gain access to all 128 KiB of RAM. I decided against this. The picture shows the new larger RAM module soldered in place. Also note that you must solder a short on R20 just above the RAM module. Leave R21 as an open circuit. This connects the extra address pin to the CPU so that it can access 64 KiB of RAM.
The only thing left to do is to put the calculator back together and hope that it still works. In order to test the new RAM you can use the calculators hidden test menu. To gain access to the test menu turn you calculator off, then hold [a b/c], [F6] and press [AC]. Then enter the RAM section and the test will check the RAM is functioning properly over the entire address space. The picture below shows the old and new RAM test menus, confirming that the calculator recognises the new 64 KiB of RAM.

casio_9750_before_after_upgrade

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