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Calculators: Handheld: Plustron 108

Size (approx):

83mm x 165mm x 36mm (max) (w x h x d)
Weight 168g not including batteries.

Power: 6.0V DC using 4 x AA size batteries.  Can accept an adapter (6V DC 300mA) through a socket on the top side half way across.  
Case: The two-piece light and mid grey case made from smooth plastic.  Chunky, rounded edge design with a heavily tilted top section.  There is an oversized red display filter, which gives a reasonably bright display with limited angle of view.  Below the display, amongst some deep-set ridges is a recess with a black printed aluminium panel.  The long travel keys are squishy and slightly hollow sounding.  The keyboard surround is a matt black plastic section.
Display: 8 digit red LED with bubble lens with no ninth digit.  No leading zero suppression on number entry, display is left reading.
Features: Standard four functions and constant
Age: 1974
Manufacturer: No manufacturers details, made in Singapore, serial number 4181927. 
KP has also made me aware of serial number 4181753 - thanks.
Comments: Large chunky calculator that just about fits in the hand, but is equally at home on the desk.  Quirky functionality is typical of its time with the strange hidden exponent system and ?divide by zero? bug.  The original case is soft black plastic with card stiffeners in the front and back sections.  It has a bottom-side-top zip and carry strap.

Components: 1 x cpu: [Square spiral design logo] CZL-550 7404 (week 4 of 1974), 24 pin DIL  0.6" width
1 x IC: ITT 7350 7105 J, 16 pin DIL 0.3" width (suspect display driver)
1 x 9 digit bubble lens red LED with an additional full-length linear convex lens; Litronix 9098-002
2 x transistors
2 x diodes
21 x resistors
0 x resistor array
3 x capacitors
1 x transformer; Powertronics AA 1200 Hong Kong
Boards: The keyboard assembly (Mexico 3 1 74 4, CRC P/N N10028 Rev A) is bolted to the main cpu board (SPL 00 1-108and the whole thing easily lifts out.  The two boards are attached by 21 coated wires and held together with two screws.
Construction: Remove the two screws from inside the battery compartment.  The front section then hinges off from the bottom.

Logic comments: The (C) key is used to cancel an entry whilst a second press clears the whole calculator
Input overflow is suppressed inputting an ninth digit is ignored
Negative numbers are shown with a "-? in the far left (eighth) digit.  Full eight digit negative numbers are allowed, you just do not see the first digit anymore!
There is a selectable constant function; to use 5 as a constant key in (C)(5)(=)(K), now (2)(X)(=) gives ?10?.
Divide by zero shows ? where X is a cycling number.  I suspect the ?8? is a fast cycling number as you can change this display by dividing by 0.0 and see two cycling numbers.  It is not recoverable.
Overflow takes a little getting used to.  The calculator has an internal exponent (which you can never see) allowing numbers between 1 x 10-7 to 9.9999999 x 1079.  When you exceed this it just carries on at 100.


The manual measures 100mm x 148mm with a stiff cover printed in black and purple.  The other 11 pages are in black only.  Basic description and calculation examples are given.  My example has a number stamped on the back (4181367) which looks like the serial number.  I would have thought that over the years somebody has mixed manuals ? but the numbers are so close that maybe this happened in the shop or even the factory.  Printed in Singapore.