My son got caught in a downpour with his calculator in his backpack. It had water under the screen but seemed to dry out OK. However, whenever he turns off the calculator, the screen stays on (all white pixels).

I ran diagnostics and everything looks OK except maybe the RAM test (I'll explain shortly).

I reflashed the OS to make sure it wan't a software problem. What's interesting when the calculator is connected to the PC is that when I turn it off, I can hear the USB connection being dropped by Windows (an audible beep is made when establishing and breaking a USB connection). So the calculator thinks it is being shut off, but the LCD isn't complying.

So I never ran the diagnostics before, but the RAM test may be showing an issue. When I run it, the words "Testing RAM..." are shown. If I run the test not connected to the PC, the words slowly fade out but the screen stays on. I think the RAM is supposed to turn off the calculator, so this behavior may be as expected. The calc is off when the words are gone (but the screen is still white).

When I run the test connected to a PC (charging), the words "Testing RAM..." stay black and displayed on the screen. I don't think the calc is off since pressing ON does not turn it on. If I disconnect the USB then the words dim again slowly until gone, and pressing ON turns the cal back on again. In both cases, the test shows as green as if it passed.

So something is preventing the screen from turning off. Any suggestions? I ordered some tools so I can take it apart and look around next.

Ouch, that water damage could've really messed up the calculator.
I'm surprised, but thrilled, you knew to run the diagnostics! So with the LCD test, the black pixels displayed properly? They didn't turn white? If they didn't turn white, then it may be an issue of the backlight refusing to turn off.

As for that RAM test, you're getting expected results (besides the slowly fading out to white). What the test is suppose to do is display "Testing RAM..." for a few seconds, then turn off the calculator. When it's plugged in to a computer, it waits until you unplug it before it proceeds.

My best guess is that the LCD controller got damaged/ corrupted by the water. I'm honestly not sure how you would fix it since I think it's soldered to the motherboard. Perhaps someone here with more experience than me can help you, but you may want to be prepared to use your warranty (if it still applies). Next time make sure your son is keeping his calculator in a rainproof case or in a better spot in his bag.

Yes, the black pixels all displayed properly.

Your description of the backlight refusing to turn off sounds like the perfect description for the problem. Even though the rest of the calculator has "shut down" when power is turned off (CPU, USB, etc.), the backlight still stays on.

Is it possible there could still be some water in there completing a circuit to the backlight even though everything else has shut down? Odd that it isn't shut down through the motherboard with everything else.

Maybe try just letting it dry out for a while. And see what happens when it runs out of battery. (Drying first )
I thought just reading it at first that it was the OS, but I didn't realize there was water involved until later.
Good luck fixing!

My guess is that there is a transistor or something that allows the calc's processor to conditionally power the LCD even though they operate at different voltages. That circuit has been bypassed, either due to rust from the water or from water that remains in the calculator. Opening it up might allow it to dry and make it easier to see what is causing the problem.

Well I took it apart, cleaned the board and brushed it. Then I dried it out with a hair dryer. The backlight still stays on.

Hmm. Seems as though there is nothing more that you can do but give it time. You should contact TI about a warranty as well, perhaps you could get a new CE.

You've voided your warrenty by opening the calc, I think. It looks like you are going to just have to pay for one :/

I would still check. It is worth it to do at least that.

Just a quick follow up.

In a last ditch effort I left the unit plugged in (charging) with the all white screen on. Sometime in the 5 days or so I left it unattended, the screen turned off! It now works perfectly fine!

I'm guessing the power flowing non-stop dried out whatever last circuit still had some moisture in it.

$100 saved!