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Calculators: Handheld: APF Mark 40

Size (approx):

65mm x 115mm x 33mm (max) (w x h x d)
Weight 92g not including batteries.

Power: 9.0v DC using 1 x PP3 size battery.  It accepts an adaptor (model 710 or 7515) through a socket on the left side in line with the on/off switch.   Quoted battery lifetime is 40 hours (alkaline), 20 hours (carbon zinc) and 10 hours (low quality carbon zinc – whatever they are!).
Case: The case is two-piece white texture plastic which is wedge shaped with a pronounced extra tilt for the display.  This also helps to accommodate the battery. A red display filter is slightly inset and tilted and gives a clear image but with limited angle of view.  The brand and model name are deeply moulded into the case. A smooth dark grey panel surrounds the keyboard.  The subtle coloured keys are medium travel, squishy type that still work well.
Display: 8 digit red LED with bubble lens and a ninth digit for negative and error indication. 
Features: Standard four functions and percentages
Age: 1975
Manufacturer: APF Electronics, Inc. (New York, USA), made in Japan, serial number 235473.
Comments: A nice miniature calculator that fits perfectly in the palm of your hand which, judging by the subtle colours was aimed at the lady’s market.   I quite like the idea of the (=/K).  Even though I would class this calculator as auto-constant, it reminds you that it’s there.  Good sound logic with excellent recovery makes this a little gem.

Components: 1 x cpu: (square spiral design) C-593 7529 7A (week 29 of 1975), 28 pin DIL 0.6" width
1 x display driver IC; Toshiba T1347 5E, 20 pin DIL 0.3" width
1 x 9 digit red LED with bubble lens display
2 x transistors
2 x diodes
2 x capacitors
4 x resistors
1 x transformer
Boards: The keyboard assembly is attached to the front by five screws and is joined to the cpu board via a 14-way ribbon cable. This main board has a small modular board power converter and is fixed to the back by plastic lugs.
Construction: Carefully pop the two lugs that you can see in the battery compartment.  Then hinge off the rear from the bottom to release the single lug.  Take great care not to force it or damage will result.

Logic comments: The (CE/C) key is used once 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 immediate left hand digit travelling in to the far left (ninth) digit thereby allowing full eight digit negative numbers.
There is automatic constant on all four functions, the K reminding you.
Divide by zero shows “C” in the far left (ninth) digit and is recoverable using (CE/C)
Overflow shows the result and “C” (or “E” if negative) in the far left (ninth) digit and is recoverable using (CE/C).


The manual measures 70mm x 112mm has twelve pages and is printed black on light blue paper.  It uses the marketing strap “Think and Touch” to describe the calculation logic.  It has plenty of examples and guides.  Printed in Japan, part No. 0404051.  There is a Warranty Certificate on the back page.

The calculator came with and additional single printed page “Full Six Month Warranty” which was needed for the new Federal Consumer Product Warranty Act.  Also printed in Japan.

Quote: "Modern electronic technology has provided a new tool for use in home, office or school."

Quote: "To simplify operation, your calculator is programmed for 'Think and Touch'...".