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Japanese calculator with 7-segment neon display

JapCal-front.jpg
This small calculator I found years ago on a flea market. I bought it for parts but after many years I found it back in my junk box and was able to fix it. It looks like I found the brand and model.

   Japanese calculator

Japanese calculator

This is one of those small mains powered desk calculators that were built in the early seventies. It only has the 4 basic arithmetic operators. Some key combinations cause erratic behaviour, which may indicate obscure special functions, but probably is caused by minor bugs in the calculator IC design. The neon display is very interesting: it's encased in a number of layers of glass and metal cemented together, not of blown glass. At first, I thought it was an incandescent display, but it turned out to be a neon display. Unfortunately, the window protecting the display has been lost. The brand name and type number may have been printed on it.

I found this calculator on a flea market long ago. Actually, I bought it to use the keyboard for some somupter experiment, but fortunately I never came to dismantling it. As I got more and more interested in old display technology, in early 2000 I decided to analyse the circuit to see what I could learn from it for my own nixie projects.


   Close-up of GI calculator IC

Close-up of GI calculator IC

There is only one (MOS?) IC in the calculator: type C-500 by GI in a ceramic case with a gold plated metal lid. The production date code is "7331". Recently, looking for more information on this calculator chip, I found out that this calculator must be a Royal Digital VIII-K calculator.

The supply voltage is -24 V for the IC and the display, and +140 V for the display. The display is driven by discrete driver stages (using discrete transistors, no IC's). There are 8 digit drivers and 8 segment drivers (including decimal point). The current limiting resistors are not in series with the anodes and anode drivers, as is often the case with multiplexed nixie tubes. Instead, current is limited by the segment (cathode) drivers.

When I turned on the calculator, the display remained dark. I measured the signals on the display. None. Looking for signals on all pins of the IC I found none either. The IC seemed to get power, though. The clock signal was missing, so I checked where it was supposed to come from. The clock oscillator, an astable multivibrator worked, but not very well. It turned out that the driver transistor that drives the IC was faulty. After replacing this transistor, I had a working calculator!

Copyright © 2001 by Onno's E-page         published 2001-08-05, last updated 2002-12-06 (typos)