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Calculators: Handheld: Serd Bonn

Size (actual): 73.7mm x 119.4mm x 28.3mm (max) (w,h,d)
Weight 100g excluding batteries
Power: 9.0V DC, 1 x PP3 size battery.  It accepts an adapter (undefined) through a socket on the right in line with the on/off switch.
Case: Two piece case in beige and and dark brown matt plastic which is gently wedged-shaped.  Above the display area is a metallic plate with black text for the brand which sits in its own recess.  A flush purple plastic display filter give a good easily viewable display.  The small keys are soft and springy and work quite well.
Display: 8 digit red LED with no ninth digit
Features: Four standard functions with percentages
Age: 1975/6
Manufacturer: Serd Deutschland (Germany), made in Japan, serial number 1120744S
Comments: This German brand issued a number of calculators with the names of German cities.  Sturdy enough calculator with simple functions.  The logic is let down by seven digit negative numbers, a strange post division error and input overflow.   

Components: 1 x cpu: Mostek 50281N MT 7541 (week 41 of 1975), 28 pin DIL, 0.6" width black plastic
1 x IC: Toshiba T1347 5-D (April 1975), 20 pin DIL, 0.3" width black plastic
1 x nine LED module with integrated bubble lens (SL1892 B)
0 x transistors
0 x diodes
0 x capacitors
2 x resistors
1 x variable resistor
Boards: On first opening you will see a metallic coated piece of cardboard protecting the main board, sitting loose and grounded.  This is a bit odd as the case is plastic, so would not cause any electrical problems and you don't often see R.F. shielding in a calculator!  The main board (Ref: 704 R-41X181 2 - (the two being a smaller number, so it looks like a version number)) sits face down on the keyboard assembly loosely supported by two plastic pillars.  They are joined by two 11-way ribbon cables.  The keyboard assembly if fixed to the front with four screws.
Construction: Remove the single screw from the rear.  This does not help a great deal as there are internal plastic lugs holding it together on the top and a tight fit on the other edges.. Start by pushing in the rear section on one side and travel around - careful, you may damage it.  Eventually the back will lift off. When putting it back together start with the top edge.

Logic comments: The (C) key on the first press can be used to clear the last entry whilst a second press clears the calculator completely, i.e. it is a C/CE key
Input overflow is not suppressed, keying in a ninth digit causes and error state which is recoverable using (C) - for a second register number
Negative numbers are represented with a ?-? sign in the immediate left digit - but as there is no ninth digit you are limited to seven digit negative numbers
There is automatic constant on all four functions - using the second register number
Overflow shows the result flashing and is not recoverable using (C)
Divide by zero shows a flashing zero which is not recoverable using (C) - if fact it gets very buggy: key in (5)(/)(0)(=)(C)(5)(=) to get "0" then (5)(=) to get "1" - so its has managed to transpose registers!
The percentage key just divides the current register by 100 for one press only
No other bugs found