Having got back into making weird noises over the last few years, I decided to try and recreate this formative experience. I racked my brain trying to remember what that calculator looked like - I thought it was a Texas Instruments one, and after a look at the Datamath Calculator Museum (http://www.datamath.org/) I got a little ping of recognition when I saw the TI-2550 (actually it was the number I remember - the calculator itself I remember looking very snazzy, compact and futuristic not like the clunky brick I saw online :-) And there was a reasonably priced one going on eBay!!
So here is the result - I think my expectations of synthesizer capabilites are rather higher these days - in my memory it wasn't restricted to about 3 notes or drowning in static, but here we go - it might still be interesting. I used a Moog MF Drive pedal for the distortion/lowpass (to try to make the static less harsh)
The pitch seems to be generated by RF interference from the keyboard matrix scanning and changes depending on the number of keys (and which keys) pressed together. It shows up in medium wave AM band. Tuning around changes the timbre of the sound but it still only ever manages about 3 notes plus an open drone - but just look at all that crazy stuff happening on the display!!!
I will probably open this up and see if I can get more sounds going on with it - I am wondering about bringing the display multiplexing into it - the refresh seems slow so maybe it can be played out at audio frequency - I wonder if the display signal can be fed back into the keyboard matrix scanning to get some crazy feedback loop going...?