CASIO fx-4000P scientific calculator from 1986

[Csaba Tizedes]

(12-05-2015 03:25 PM)jebem Wrote:  This specimen had poor LCD contrast.
The issue was fixed after cleaning the battery holder connection pads on the PCA.

I have two workable units, one of them in perfect condition, the other is my daily used machine for calculating dawn-sunrise-noon-sunset times for my location.

Both of them has the problem with contrast: typically the contrast set is OK, but sometimes goes too poor and sometimes too dark, but the battery is new. Have you got any idea how this is fixable? If I turn off/on the unit the contrast set is fixed for a long period then starts again the changing.

Another question: the second unit is arrived this Monday and really like a new but as I can feel the bracket [(] [)] keys response is poorer than others. I will clean the unit inside, my question is between the keyboard foil and PCB is it required to clean or that will kill the contacts? Is it possible to clean the foil itself? What is the right cleaning liquid?

Thanks for the answers!
Csaba