Texas Instruments TI-33 keyboard repair

Last night I tried using a Texas Instruments TI-33 calculator to do some inch-to-cm conversions and became angry at its unreliable keyboard. I got many double or triple presses and it took me ages to complete the task. So this morning I watched a guy who repaired some TI-30 (opens youtube in another window) and proceeded to check into mine.

TI-30 and TI-33 calculators are mechanically very similar if not identical. While heading for the keyboard (which is "sealed") I started feeling brittle stuff on my fingers, which revealed itself being the sponge totally worn out:

The sponge between keys and keyboard
totally worn out and disgregating.

I have no idea how to replace it, so considering how much the calculator might be used in the future (I have a small collection...) I tried a simple approach: what if I remove the sponge? Would the keypad work?

Off they came all keys, which are of two different sizes. I cleaned the sandy stuff with a brush, quick and easy. What I noticed using the calculator was that some keys were tilted with the longer side lifted. The reason was found to be a leftover from the manufacturing process, a break-off tab:

Many keys had leftovers of break-off tabs.

The history of pocket calculators tells us that in mid 1970's the development was so fast that prices were falling on a daily basis: I understand they were in a hurry building and selling them! I removed these leftovers and now keys sit as expected.

Nowadays in software terms I would call this an "Easter egg". a little surprise hidden in a product. I think I found one in the only PCB inside my TI-33: it is Texas Instruments' logo etched in plain sight on the right-hand side of the IC:

TI-33 only circuit board with bubble LED display and TMC0984L IC.

What does the square symbol on the other side represent?

I'm sure you are wondering if the keyboard works without the sponge. It does work, indeed! I don't know how it felt to the touch when new back in 1978, but I feel no difference before/after the repair except that now I don't get bounces and unreliable presses. Actually some keys suffer double hit, especially those in the centre of the keyboard, but I think it was a "feature" even in brand new specimen!

Now I need to find a manual for this model, or someone that used it and remembers how to use the three memories.