I bought my Texas Instruments TI-83 calculator back in 1999 for when I took my HTX degree (Higher Technical Exam).
Since 2012 it began showing various symptoms of ageing that just got worse as time went on.
It could freeze during calculations and get stuck with dots moving in the upper right corner, as if it got stuck in a infinite loop, this could only be fixed by removing the backup battery.
It would not turn on at all, removing the batteries no longer helped and the calculator seemed dead, when I looked at prices for a new TI-89 I decided to repair my old one instead, despite it have some 20 dead pixels from a hard blow to display.
The freeze, unresponsive and suddenly dead calculator is caused by a flat ribbon cable connecting the two circuit boards in the calculator. There are some data lines with communication between them and when this link is disturbed it can freeze doing calculations or it just will not turn on at all.
If your calculator shows up with a black screen after a battery change, try to turn down the contrast setting by pushing the yellow 2nd button and arrow down button alternating.
The black screen appears because the calculator remembers your contrast setting that you gradually turn up as the batteries are drained, so when new batteries are in place they will light up the whole screen as they can supply full current.
Disassembling the TI-83 calculator
It is a relative simple repair that will require nothing more than a set of small screw drivers, flat and star will do even for the Torx5 screws used for the enclosure.
You will also need a knife, soldering iron, some thin wire and a steady hand.
First remove the battery cover, batteries, backup battery cover and the backup battery.
Remove all six enclosure screws.
Remove the two screws holding the foil in place.
Locate the flat cable that connects the calculator unit to the display unit.
Remove the flat cable from the contact surfaces by peeling it off with your fingers, cut the black support ribbon with a knife and clean the contact surfaces for glue.
Solder a wire straight from each point on the calculator unit to the display unit. I used a single strand of copper wire from a multi core machine tool wire, nothing special, just a thin copper wire.
Almost as good as new!