Ajax 2500

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Calculators: Handheld: Ajax 2500

		Size (approx):	78mm x 138mm x 35mm (max) (w,h,d) Weight 126g excluding batteries.
		Power:	6V DC with 4 x AA size batteries. It accepts an adapter (undefined) through a socket on the left side in line with the first row of keys.
		Case:	Stylish designed body made from a two-piece matt plastic case in silver and black. The boldly contrasting white keys are short travel with a loud click and work well to this day. The upper area is prominently raised and contains a black printed metallic sticker with the brand name which sits in its own recess. The concave, heavily tilted red plastic display filter gives a bright but limited viewing angle image.
		Display:	8 digit red LED with bubble lens and a ninth digit for negative, error and memory indicators.
		Features:	Four function with percentages and three function memory
		Age:	1974/5
		Manufacturer:	Acme Electric Company (Finsbury) Ltd., UK, made in Hong Kong. Serial number 13649.
		Comments:	Nice solid design that for once, doesn't need the expanded top section to accommodate a 9V battery. The logic is sound but let down by the lack of recovery and awkward memory system. The original case is soft brown plastic with a flip over top flap that tucks into a front cut-out band. It has Ajax printed in silver on the back. From the main board this appears to be a early branded Radofin calculator and I'm told looks exactly the same as the Radofin 2500.

Components:	1 x cpu: Mostek MK5022A 7420 Y Korea (week 20 of 1974) 28 pin DIL , 0.6" width ceramic package 1 x IC (Display driver?) NEC 517 DS8855N, 22 pin DIL, 0.3" width 1 x nine digit LED module with individual bubble lens: 6082-7449 Singapore (date code week 49 of 1974) 2 x transistors 1 x diodes 4 x capacitors 6 x resistors 1 x transformer
Boards:	The main cpu board (Radofin 2500-01) is fixed to the rear section by melted plastic lugs and joins to the keyboard assembly (Radofin 2500 CT005) via 12 wires.
Construction:	Remove the single screw from within the battery compartment and, surprisingly, the front black section lifts away easily, hinging from the bottom.

Logic comments:	The (C/CE) key is used to cancel entry on the first press and clears the whole calculator on the subsequent
	Overflow on the input of a number is suppressed, typing in a ninth digit ignores the last number
	There is automatic constant on all four functions
	Negative numbers are flagged by a "-" sign in the immediate left hand digit travelling into the far left (ninth) digit thereby allowing full eight digit negative numbers.
	A divide by zero shows zero and an upper "n" in the far left (ninth) digit and is not recoverable
	Overflow shows the result with an upper "n" ( "o" if negative) in the far left (ninth) digit and is not recoverable
	Memory store is indicated by a lower "L" in the far left (ninth) digit
	Memory operation is a two keystroke system: use (M)(+) to add the contents of the register to memory, (M)(-) to subtract and (M)(C/CE) to clear.
	Memory overflow loses the original contents
	The percentage key just divides the current number by 100 (once)

	This scan shows the front section removed with the Radofin board held in place by the two melted plastic lugs. Notice also that the LED display module is held by four melted plastic lugs as well - intensely manual in its assembly. The blue wires connect to the keyboard assembly, the yellow to the display and the red (positive and negative) are the main power lines.