Vanguard X-821

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Calculators: Handheld: Vanguard X-821 (aka 18PA)

		Size (approx):	93mm x 170mm x 25-35mm (w,h,d) Weight 220g excluding batteries.
		Power:	9V DC, 6 x AA size batteries double-decker stacked. Also accepts adapter (three pin 7.5V+9V DC 45ma+120mA) through a top side socket on the far right. The battery compartment has a ribbon-pull to aid removal
		Case:	Large blocky calculator with pronounced raised top area to accommodate the six batteries. Constructed of a two-piece matt black plastic case with substantial side brushed aluminium panels. The two vertical raised edges are painted white. A very stylish mini knob sit in a grooved section with an obvious punch-out for another one. It is accompanied by the yellow function key. The switch labels and branding along the top section are printed white whilst the extra key functions are printed in yellow. There is a tilted and very slightly proud red plastic display filter which is notable for being full-width when the display is actually a third of the width. The display is bright with a wide angle of view. The convex keys are short travel, loud hollow click type but work well to this day.
		Display:	8 digit red LED (flat display type) and a ninth digit for negative and error indicators.
		Features:	Four function with percentages, register exchange, square root, reciprocal, squares, change sign and five function memory
		Age:	1975
		Manufacturer:	Vanguard, Cahue Industrial S.A. Spain. Serial number 124506 on a label inside the battery compartment.
		Comments:	Very stylish but cumbersome calculator from this Spanish manufacturer. From the look of the extra switch panel (and other models on this site) this is a generic case used for many calculators. The logic is sound with good recovery only let down by the lack of cancel entry and the pseudo fixed decimal bug.

Components:	1 x cpu: Rockwell A4611PA 7527 A1211 (week 27 of 1975) 42 pin staggered DIL , 0.6"width 2 x ICs: ITT 492 7340 (week 40 of 1973) 14 pin DIL, 0.3" width 1 x IC: RCA CA3082 334 (suspect week 33 of 1974), 16 pin DIL, 0.3" width 1 x nine digit LED module: Optostic 2M R7H-122-9D 1575 3 x transistors 9 x diodes 27 x resistors 4 x capacitors 1 x transformer
Boards:	The main cpu board (18PA 200F 201F) is held to the front by two screws and sits on top of the keyboard assembly (18PA 220c 221c) which is held to the front by four spacer pillars. The two are joined by 9 wires.
Construction:	Remove the two screws on the back and the back section comes away easily. Gently pull out the front keyboard which hinges up from the top.

Logic comments:	The (C) key clears the whole calculator there being no cancel entry function
	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 far left (ninth) digit thereby allowing full eight digit negative numbers.
	A divide by zero shows zero and "o" in the far left (ninth) digit and is recoverable using (C)
	Overflow shows the result with "o" in the far left (ninth) digit and is recoverable using (C)
	Upper functions are selected by using a two key sequence: the yellow one first which is unlabelled
	The (X<>Y) key exchanges the current display for the contents of the last register
	Memory store is not indicated - you have to remember it
	The change sign function can be used in mid number entry
	Negative square roots are not allowed and result in a zero overflow status and is not recoverable
	This calculator suffers the pseudo fixed decimal bug: key in (1)(+)(0)(.)(0)(0)(0)(=) to give "1.000" which remain a fixed three decimal number until more digits are required.