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Mysterious part in an old calculator

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From	Piotr Piatek <piotr433@pisi.com.pl>
Newsgroups	sci.electronics.components
Subject	Mysterious part in an old calculator
Date	Sun, 17 Apr 2016 17:17:16 +0200
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Could someone identify the blue part (marked on the board SC81EIA4R7M), 
please?
http://www.pisi.com.pl/piotr433/misc/bluething.jpg

It's used in an old calculator Casio FX-602P, manufactured in year 1984. 
The part is connected across the power supply pins of the static RAM 
chip uPD444G. It could be a capacitor (but other capacitors on the board 
look ordinary and are marked differently), or perhaps a varistor or a 
Zener diode.

Here's the FX-602P circuit diagram drawn by me:
http://www.pisi.com.pl/piotr433/fx602sch.pdf
and a photo of the whole board:
http://mycalcdb.free.fr/galerie/Casio/casio_FX-602P.4.jpg

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Thread

Mysterious part in an old calculator Piotr Piatek <piotr433@pisi.com.pl> - 2016-04-17 17:17 +0200
  Re: Mysterious part in an old calculator Look165 <look165@numericable.fr> - 2016-04-17 19:47 +0200
  Re: Mysterious part in an old calculator "Ian Field" <gangprobing.alien@ntlworld.com> - 2016-04-17 20:09 +0100
  Re: Mysterious part in an old calculator Look165 <look165@numericable.fr> - 2016-04-18 08:49 +0200
  Re: Mysterious part in an old calculator Piotr Piatek <piotr433@pisi.com.pl> - 2016-04-18 14:48 +0200
    Re: Mysterious part in an old calculator legg <legg@nospam.magma.ca> - 2016-04-18 09:22 -0500

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