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Mysterious part in an old calculator

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Piotr Piatek
Guest

Sun Apr 17, 2016 9:17 pm   



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

Look165
Guest

Sun Apr 17, 2016 11:47 pm   



Seems to be a 4.7 Ohms resistor.

Piotr Piatek a écrit :
Quote:
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


Ian Field
Guest

Mon Apr 18, 2016 1:09 am   



"Piotr Piatek" <piotr433_at_pisi.com.pl> wrote in message
news:dnhnntFfidtU1_at_mid.individual.net...
Quote:
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.


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.

Look165
Guest

Mon Apr 18, 2016 12:49 pm   



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).


Piotr Piatek a écrit :
Quote:
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


Piotr Piatek
Guest

Mon Apr 18, 2016 6:48 pm   



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

legg
Guest

Mon Apr 18, 2016 8:22 pm   



On Mon, 18 Apr 2016 14:48:29 +0200, Piotr Piatek
<piotr433_at_pisi.com.pl> wrote:

Quote:
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


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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