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Calculators: Handheld: Sceptre 821

Size (approx):

68mm (max) x 138mm x 16-24mm  (w,h,d)
Weight 78g excluding batteries

Power: 9V DC using 1 x PP3 size battery.  It accepts an adapter (undefined) through a socket on the left side in line with the on/off switch.  There is a power save function after about 25 seconds.
Case: Two piece matt beige plastic case which is gently wedge shaped.  The top is printed with black text with the brand – but no model number. A brushed aluminium panel surrounds the keyboard and has black printed label for the switch and key labels.  The red plastic display filter is slightly inset and tilted in at about 30 degrees.  The keys are short travel hollow-clicking and work moderately well.  The rear of the case is taken over by a lot of black printed help text – but still no model number or serial number.
Display: 8 digit red LED with bubble lens.  There is no ninth digit for extra indicators.
Features: Standard four functions, percentages, constant memory and two function memory. Power save mode and display recovery.
Age: c1975
Manufacturer: Sceptre with no information about the manufacturer or serial number, but box states made in Hong Kong.
Comments: Cheap and cheerful calculator that looks like a branded National Semiconductor or Novus model.  There is a Novus 821 which I suspect is the same. Total lack of model or serial number on the calculator is unusual.  Quirky logic with no ninth digit, odd constant memory, “negative zero” bug and poor recovery abilities.

Components: 1 x cpu: National Semiconductor MM 5738N 544, 24 pin DIL 0.6" wide
1 X IC: (display driver) National Semiconductor DS8855N 548, 22 pin DIL 0.3" width
1 x 8 digit LED unit with bubble lens: National Semiconductor A1188 523
no other components
Boards: The main board (5738) sits loose above the keyboard assembly and is joined by a 18-way ribbon cable.  The latter is attached to the front with 8 plastic lugs.
Construction: Remove the four screws from the back and the rear section lifts off easily.

Logic comments: (CE/C) is used once to cancel the last number entered and a second press clears the whole calculator.
Input overflow is suppressed, inputting a ninth digit is ignored
After about 25 seconds a power saving mode just displays the first digit of the current number.  You recover this mode by pressing (D), for display.  If you just had a one-digit number displayed you do not know it is in power save mode.
Negative numbers are shown with a minus in the immediate left hand digit but as the ninth digit is not used you cannot see the symbol even though eight digit negative numbers are allowed.   Divide by 10 and you will see it again!
There is automatic constant on multiple and divide and the odd ability to add or subtract the constant factor (second number) to your calculation by using the (K=) key.   i.e. (2)(X)(3)(=) shows “6” and stores 3 in the constant memory.  Now (2)(+)(K=) will give you “5”.  This memory is not cleared by the (CE/C) key.
The percentage key is a divide by 100 key.
Divide by zero shows "00000000” and is not recoverable
Overflow shows the result the best way it can and hangs the calculator and is not recoverable
Memory store (MS) and memory recall (MR) does not have any indicator – you have to remember it.  To clear the memory store zero; (CE/C)(MS).
This calculator suffers from the negative zero bug.  Try (1)(-)(2)(=) to give “-1” then (+)(1)(=) gives “-0”

box

The box measures 71mm by 144mm and is 26mm deep.  It is therefore just big enough to hold the calculator.  Printed in purple, yellow, red and black it is a simple top and bottom end flap construction.  One side has a metallic sticker saying made in Hong Kong.