Casio FX-7500G graphics calculator repair

[Falowod_Jerzy]

Hi. I’m trying to repair my Casio fx-7500g graphics calculator, which suffered a drop while back and now part of display is not working properly. It can still do some calculations, so I think processor and memory is still good. I still tried putting on new solder to theirs pins, but it did not help to solve the issue.



I’ve checked ribbon connection between two boards, because it was the most suspicious for me due to broken hinge which I’ve already glued together. It is over EPROM chip labelled uPD27C256AG and under black sticker. It has pins labelled from 1 to 31. I’ve made a measurement and most of them has resistance of about 30 Ohms, but couple last have somewhere around 50 Ohms. So I think that connection is good.

 

While I was touching board to see if it changes anything, I discovered that if I put my finger somewhere around capacitors C104, C30Ps and blue oscillator, some more lines starts to appear (once I managed to get whole display working for a second), but calculator appears to crash. So I redone those connections as well. It also did not help. I checked oscillator with my oscilloscope (which is 20yr old and probably out of calibration) and if I measured it correctly it has 20MHz output signal, while datasheet states that it should have 2MHz. Also I desoldered one C30P capacitor for measurements and it showed 37pF, so it put it back because it looks good to me and i do not have a raplecement.

What I should try next? Or maybe this osclillator is the problem? I’m out of ideas.

[andy2000]

Based on the diagonal line running across the screen, it's clear that the display is cracked. 

[Falowod_Jerzy]

Based on the diagonal line running across the screen, it's clear that the display is cracked. 

I don't think it is crakced, I can get vertical lines to show up on the blank portion of the screen, while touching pcb somewhere around the oscillator and capacitors. And also I think that there wouldn't be any horizontal lines below that diagonal line if it was cracked. Maybe it's broken, but i don't think that sympoms of that are clear.