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Calculators: Handheld: Colex 812-R

Size (approx):

93mm x 150mm x 32mm (max) (w x h x d)
Weight 196g excluding batteries.

Power: 3.0V DC using 2 x AA size batteries.  It accepts an adaptor/charger (2.4V, 150mA DC) through a socket on the top side to the far left.  The back panel quotes a full recharge time is eight hours.  Oddly enough the battery compartment could hold 4 x AA size batteries but two of the terminals are block off by a piece of foam and missing two out of four terminals.
Case: Two piece glossy white plastic case with a third (slight proud) section acting as front and keyboard surround in black smooth plastic.  At the top a black printed metallic sticker sits in its own recess and holds the brand name and on/off switch label.  A green plastic display filter is flat and inset, with white printed text for the number positions.  They keys are very reminiscent of a Commodore Pet (computer) with inset black printed metallic labels.  They are medium-long travel with a soft click and strong bounce-back.
Display: 8 digit green VFD with a ninth digit for negative indication
Features: Standard four functions with percentage, square root, reciprocal and five-function memory.
Age: 1975
Manufacturer: Colex, made in Hong Kong, serial number R13433
Comments: Large-sized calculator with attractive keys that do not tend to survive well. It appears that this was a generic case as they have gone to some effort to stop you putting four batteries in! The logic is quite good with excellent recovery and really only let down by the strange divide by zero bug.  Negative root recovery is very unusual.  Similar functions and insides to the Busicom/Colex 812-SL.

Components: 1 x cpu: Mostek MK50311N 7518 (date code week 18 of 1975), 28 pin DIL, 0.6" width black plastic
1 x 8 digit (plus "E" symbol) VFD unit; single glass tube with round front face
2 x transistors
17 x diodes
5 x capacitors
3 x resistors
2 x resistor arrays
1 x transformer
Boards: The main cpu board (Colex Ltd -I3V 812-1) sits on top of the keyboard assembly upon four long metal pillars, screwed in place.  The keyboard is attached to the front with seven screws and joins the cpu board by 15 flying long wires.
Construction: Held together by internal lugs only so be careful you do not damage your example.  Start with the right hand edge.  From the rear press in the front of the case edge whilst levering out the seal with a blunt, soft  instrument.    The back will lift off but it attaches to the adapter socket quite strongly. 

Logic comments: The (C) key is used to cancel an entry whilst a second press  clears the whole calculator
Input overflow is not suppressed inputting a ninth digit causes the number to flash which is recoverable using (C)
Negative numbers are shown with a "-? in the immediate left digit travelling into the far left (ninth) digit thereby allowing full eight digit negative numbers.  However, the ninth digit "-" sign is bigger than the other central horizontal segments.
There is automatic constant on all four functions
Overflow shows the result flashing and is recoverable using (C)
Divide by zero shows zero flashing and is recoverable using (C) but it then suffers a bug: key in (9)(/)(0)(=) to get a flashing "0." then key (C)(2) to show a display of "0.2".  This does not happen if you use (0)(1/X) where recovery using (C) is perfect.
Negative square roots are not allowed and result in the original number flashing which is recoverable using (C) - really neat!
Memory store is not indicated - you have to remember it
Overflow in memory stores the result, shifted.
No other bugs found