Mysterious part in an old calculator

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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

Re: Mysterious part in an old calculator
Seems to be a 4.7 Ohms resistor.


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Re: Mysterious part in an old calculator


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As a totally wild guess, I would've said a polyfuse - but not if its ACROSS  
the supply pins.

Its probably semiconductor based, maybe a clamp protection diode - but they  
usually don't look that different to ordinary ones.

If it is semiconductor - its probably designed to sacrifice itself to save  
the chip.  


Re: Mysterious part in an old calculator
Everybody should learn how to read a schematic !

This component is in series with the power supply, so it probably is a  
protection resistor (low value).



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Re: Mysterious part in an old calculator
Thanks to all who responded. The device is indeed connected across (i.e.  
in parallel to) the power supply pins, so it cannot likely be a low ohm  
resistor or a fuse. It appears to be a tantalum capacitor. The measured  
capacitance is about 4.75uF. Photo taken after removing excess solder  
with wick:
http://www.pisi.com.pl/piotr433/misc/bluething2.jpg


Re: Mysterious part in an old calculator
On Mon, 18 Apr 2016 14:48:29 +0200, Piotr Piatek

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It occupies the right footprint for an early smd tantalum and has a
suitable part number (odd to see that on silkscreen) but I've never
seen this particular construction.  

The presence of tack-soldered radial tantalums elsewhere in the
assembly made this an unlikely guess...

If guessing was requested, I'd have suggested an early TVS type for
smd applications. Possibly part of an external wallwart protection
scheme. This would have a similar part numbering system and footprint.

RL


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