Casio CFX-9850GB Plus 32KB WE (White Edition): 64KByte "upgrade" reconfiguration

[jebem]

I believe this model was released in 2002 by Casio, but can't find any positive authoritative information about this.

I was looking to have this "White Edition" model in my collection since long time ago. It is a extremely popular machine, at least in my country, so there are plenty of used specimens for sale around here, although the asking prices are not that cheap.

Finally got this machine yesterday and it is in pristine working condition.
For a change, there is no need to clean battery electrolytes on this one. Still I wanted to see the internals.

General parts view.




Processor PCA and Keyboard/Display PCA.




Contrast preset potentiometer underneath the Processor PCA.




Processor PCA. Main components:
- Hitachi HCD62121 Processor as per Wikipedia, Martin Poupe, CasioCalc.org, Omnimaga, among others (the reference not visible as it is sealed & hidden away)
- BSI BS62LV1024-70 128KByte SRAM 70nS
- OKI MR27V802D 1MByte PROM 100nS
- MC74HC4066A CMOS Quad Analog Switch
- 4.3MHz xtal with embedded caps set

I can't find the Service Manual for this model, although it is easy to find it but for the previous CFX-9850G PLUS(ZX-935A) but this one uses different memory design. Apparently the Processor would be the same, but so far there is no positive proof.




SRAM reconfiguration to 64KByte.

The CFX-9850GB Plus WE has a SRAM factory set to 32KByte.
However the SRAM chip has 128KByte capacity.

Public information at several sources (for instance, here at CasioCalc.org) states that it is possible to "upgrade" the SRAM size by changing the RAM setup jumpers.

I can confirm that it worked fine on my machine as well:
- Factory set to 32KByte: R20 jumper out, R21 jumper in.
. Enable 64KByte: R20 jumper in, R21 jumper out.

It took me around two minutes to switch the jumper after spending one hour digging in the net.
Other machines may experience issues if the SRAM chip is defective.
Therefore it is essential to run the self test a few times to validate the new SRAM size for good operation.

Basically I have unsoldered the jumper from R21 and soldered at R22 position.
This jumper looks like a SMD resistor but it has a zero Ohm value.




Assembling the calculator.




The Reset push button membrane.



(continue)