Mechanical calculators: Olivetti Summa Prima 20

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These are actually two machines: one with square keytops, and another with round ones. Both were manufactured in Argentina. There are still many Summa Prima 20s around, some made in Brazil.

I obtained the square-keytop one first, with many problems. The lower part of its case is in gray plastic. It was probably manufactured many years after the other one, whose case has a purple sheet metal tray – a charming Italian design detail, supposed to go well with the green plastic top. Nevertheless, the differences in the mechanism of these two machines are minor.

The round-keytop came to me working almost perfectly (one output digit was stuck), and smoothly. My only plans are a light cleaning of the mechanism and re-lubrication at key points. No disassembly is required. I'll also try to repaint the under purple tray, if I can mix paints to match the preserved color of its inside. I bought it as a way to help me understand its younger sister, so I'll do nothing to it until I can make the square-keytop work.

I found the repair of these machines much harder than any other I have. Whether I'll succeed or not in making the square-keytop work or not, remains to be seen. Their mechanism is much smaller, tighter, and apparently not very modular. Suffice to say that the Summa Prima 20 weighs half as much as the Summa 15, its predecessor.

Update: In May 2017, I finally got around to a revision in the round-keytop one, lubricating critical points, polishing the plastic cover, repainting the keyboard plate, the joystick dial, and the purple bottom.

Front view, restored keyboard plate and joystick dial


Left side  Left side view after restoration

Left side, notice the restored purple bottom and the overall aspect of the machine, as compared to the other picture, showing the right side as purchased.

 Left side view after restoration

 Right side view, as purchased

Right side

 Right side

Top view


Back view, made in Argentina


As purchased, in February 2016.

 As purchased

Olivetti ads for the Summa Prima 20 remind me of the TV series “Mad Men”

 Ad 1  Ad 2  Ad 3

More on this machine

An explanation of the design of the Summa Quanta 20, an electrical machine similar to the Summa Prima 20, a successor to the Summa 15. The calculating principle is the same

Olivetti Summa Prima 20 quick reference guide, in Spanish

Olivetti Summa Prima 20 User's Manual, in Italian

Have a mechanical calculator stored somewhere, and want to get rid of it? Send it to me!

c1966 Olivetti Summa Prima 20 S/N 99244041 (round keytops, purple bottom)

c1970 Olivetti Summa Prima 20 S/N A2-680109 (square keytops, gray bottom)

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summap20.txt · Last modified: 2017/05/14 18:27 by clodoveu
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