BUSICOM LE-80A Calculator from the 70's: Texas "Calculator on a Chip" implementation

[jebem]

I got this tiny little pocket calculator since some time ago.
The seller told me it was working but when I collected it in my mailbox it was not.
Basically it doesn't power on.

Today I took some time to have a look.

This is one of many implementations from different vendors that used the Texas Instruments TMS0105 "Calculator on a Chip" in the 70's.

My specimen uses a TMS0105BNC variant and has a date code of 7303 (1973, week 03).



As usual, someone forgot the LR1 batteries inside and it leaked corroding the copper traces in the battery holder section.
So I have just used an 4 x 1.5V AA external battery holder to test it.
It worked at first try.



It uses a Browmar Optostic 9-digit 7-segment LED display.
The display is amazing. Look to those tiny dots (LED's) on each segment!



The calculator do not have a dedicated "=" key.
Trying: 12345679 x 9 += I get this result with a "u" indicator (overflow positive?):
By the way, does anyone have information on the indicators used by these TMS0105 processors?



Trying: 12345679 x 9 -= I get now two indicators, "-" and "u" (overflow negative?):



Entering all eights and I get this "C" indicator:



And entering a negative number:




(to be continued)

[jebem]

Dismantling the machine.





Despite being so small, it packs no less than 4 printed circuit assemblies interconnected by the good old wire harness:
- Keyboard
- Processor
- Display
- Power Supply and Timing Clock





As I was in the mood to look a little deeper in the circuit, I hand designed a partial schematics by direct observation.

Despite being a standard design, it doesn't use the conventional driver ICs commonly seen in most implementations with this TMS0105 chip.
They drive the LED segments anodes directly and use 9 x discrete 3796-28 Si NPN bipolar darlington transistors to drive the LED cathodes.

Current consumption using fresh batteries (4x1.5V) is high for such a tiny machine:
- One single digit lit: 140mA
- All digits lit: 190mA



Battery holder showing the corroded contacts.