I just purchased a TI-81 from a thrift shop, noticing it had some battery corrosion / leakage. I gently cleaned off all visible corrosion and installed new batteries. The calculator now powers up, but the display is very dark. Even at contrast level 0 (the lightest), I have to place it under a lamp to see the numbers on the display. Could anybody suggest what may be causing this?

I can get a photo and attach it if necessary.

Thanks!

What voltage are the batteries putting out? My TI-81 has fresh batteries (1.8V) and is comfortable around contrast level 5.

All four of the AAA batteries are fresh, putting out 1.6 volts each.
I also forgot to mention that if i perform a calculation, it disappears from the screen when I turn the calculator off and back on again.
Maybe the corrosion from the old batteries went a little deeper than the visible part of the battery compartment? I am somewhat nervous about opening the case unless absolutely necessary.

Thanks for your help.

On the TI-81, the screen is cleared when calculations are performed; That is normal.

Edit: a picture of the screen showing the contrast level would be nice, if possible.

Corrosion frequently goes past the battery compartment, which is basically why my TI-81 almost went to the trash instead of eBay. When it does, nothing external (that I've heard of) can fix it. I'd recommend taking a look at my repair guide and disassembling the 81. Don't pick away at any corrosion, and the clearest pictures possible would be greatly appreciated. I've never heard of a failure like this one, so as much information as possible here would help other people with the same problem.

Don't disassemble it completely; stop once you remove the metal RF shield under the back cover. And if you note any inconsistencies between the repair guide and your TI-81, PM me and I'll get it fixed.

I opened the TI-81, and peeled back the protective foil.
No signs of any corrosion inside the calculator.
Here is a picture of the screen.
[img]
http://imageshack.us/photo/my-images/39/ti81u.jpg/
[/img]

The contrast is at 0 - lightest setting and I have to illuminate it with a 40 watt desk lamp 6 inches above it for the display to be visible as in the photo. (That's why it's a bit lighter at the top). It is basically unreadable at normal room lighting.

Also, please note the simple calculations on the screen. They will go away when the calculator is turned off and back on. With other calculators I have (TI-82 and TI-84), they will stay on the screen until I press the "Clear" key, even if powered off.

This is completely normal. The TI-81 didn't preserve display contents when it was turned off.


That's really weird, how the screen is being so dark... That is probably outside of my experience unless the calc is sitting on my repair desk.
*points at KermM and asks for help*

Just for fun, can you take a photo of the back of the PCB with the protective foil removed? This almost sounds like a ribbon cable failure to me; I've had extremely minor ribbon cable failures that manifested as unpredictable contrast before.

How could I forget that the contrast data is carried through the ribbon cable?! I'd say this is almost certainly a ribbon cable failure. It doesn't necessarily result from corrosion; it happens sporadically.

Mansoul: try using imgur to host your pictures, as the quality of ImageShack makes analyzing the damage to the calculator rather difficult. There's also higher resolution available on imgur.

I can confirm this. The OS clears the screen when the 81 is powered off manually, though if it turns off automatically due to inactivity, the screen contents are preserved (and it stays in whatever app it was in last). Totally normal, just an unusual feature—the 81 was TI's first graphing calc, so it was quite different in a few ways from the later calc models.

Here's a picture of the back of the TI-81.

Thanks for clarifying the TI-81 poweroff behavior.

Also, I have another TI-81 (which doesn't appear to use a backup battery-confusing). Nothing ever appears on the screen when batteries are installed. Could this be a ribbon cable as well?

Ribbon cable failures can cause a wide array of issues. I fixed an 83+ that didn't turn on by repairing the larger ribbon connectors; those carry the power to the screen. However, it could be many things. If you have an ohmmeter, check the continuity of the cable. Each connection should be less than 15Ω.

Ok, sounds like a project.
Should I be checking the ribbon cable on the upper-right in the photo?
It looks like it's folded over the corner of the circuit boards.

I will be busy until after Christmas, so this thread will slow down a bit.
Thanks again for your suggestions.

Don't mess too much with the large ribbon cables, they're far more fragile and next to impossible to repair.
The two circuit boards are joined by a 17-pin ribbon cable about 3 cm across. This cable is the primary suspect for your calcs' problems. Try checking the resistance across the cable by measuring at the two exposed conductors just off the ends of the cable; perhaps I need pictures to go along with my instructions.

I'm going to update my website to fix a few inconsistencies in the disassembly instructions and make the information more relevant to ribbon cable repairs. And now that I have a working method to permanently repair a cable without soldering, I'll add those too.