HP48GX everything works perfectly except the 'ON' key?


Okay I have a HP48GX and everything works perfectly except the 'ON' key.

No amount of pressing at the base of the bezel (or anywhere else) does anything.

Whats more I have the schematic and I checked the 'ON' key with a meter and it is totally functional!

According to the schematic the 'ON' key connects VBAT (the battery +) directly to pin 86 on the CPU. I checked this with a meter and it seems that is exactly what it does.

The circuit and key seem to be functioning exactly as shown in the schematic.

Yet the 'ON' key (nor any of it's various combinations, 'C', 'A & F', 'E', '-', '+', etc.) Does not turn the calculator on nor when it is running turn it off. The reset doesn't seem to help either.

Does anyone have any thoughts, theories or suggestions?


Sounds like the chip has gone bad. Possibly a result of ESD.


Hello Bruce,

Can you fix the LCD of an HP-48GX? I have one where the lCD permanently shows a vertical line. I also think the LCD has a browse when you increase the contrast.


Edited: 21 Oct 2012, 3:16 p.m.


I probably can, although it depends on how extensive the damage is of course.


I'm curious how you fix the LCD -- or does fixing here mean removing the bad LCD and replacing it with another?


Well it depends on what is wrong with the display.


1: Cleaning the elastomeric connectors and their accompanying pads on the mother board (as well as the contact points on the display's liquid crystal film).

2: Replacing the elastomeric connectors if needed

3: Shimming the elastomeric connectors to provide better contact with the motherboard. This is needed because over time these connectors distort or soften.

4: Repairing the connections and / or traces leading to the display.

5: Sometimes it is possible to repair the actual display. I am still experimenting with this though.

6: Replacing the display. (As you suggest).

This may not be an exhaustive list.

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