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Calculators: Handheld: Bowmar MX55

(c) Emil Dudek 2010

(c) Emil Dudek 2010

Size (actual):

78.1mm(max) x 148.7mm x 34.6mm(max)  (w x h x d)
Weight 126g excluding battery
Power: Powered using internal sealed rechargeable batteries.  It  accepts an two-pinned adaptor (undefined though I found 4.5V, lower pin positive worked fine) through a socket on the right side in line with the display.  Rechargeable battery running time is quoted at 5 hours.
Case: Two piece plastic case construction, one matt, one high gloss black plastic with sculptured smooth edges.  The upper front has slightly raised plastic logo which sits in its own recess and is printed black and red with silver show through.  The raised edge that surrounds the keyboard and display area has a silver painted edge - which does not wear well.  The keyboard surround is spray-painted matt silver with a moulded "On" label for the switch. The red plastic display filter is heavily tiled and deeply recessed, giving a clear and wide angled display. The Klixon keypad keys are short travel with a prominent click.  The rear is dominated by a large sticker with numerous examples.
Display: 8 digit flat red LED with a ninth for negative and overflow and low battery indication
Features: Standard four functions with percentages
Age: 1974
Manufacturer: Bowmar/ALI 531 Main Street,  Acton, MA USA, assembled in Mexico.  Serial number 157336
Comments: Basic calculator that benefits from the flat (multi-dot) LED display which is better than the more common bubble lens type as the field of view is so much wider.   The logic is quite poor with good recovery but numerous fixed decimal and negative zero bugs.

Components: 1 x cpu: Circle integral design logo LC1552-B 7448 (date code week 48 of 1974) , 40 pin DIL, 0.6" width black plastic with sealed cap
2 x IC: Hitachi OM8895M 429 (date code week 29 of 1974), 16 pin DIL 0.3" width black plastic (display drivers)
2 x IC: Unknown make S7450 6347, 16 pin DIL 0.3" width black plastic
1 x 9 digit flat LED display unit: Bowmar Optostic with single linear lens
5 x transistors
9 x diode
7 x capacitors
9 x resistors
1 x transformer for DC voltage converter
Boards: The main cpu board (Bowmar PM905-411/B ) sits face down on top of the keyboard assembly (KB3119 10627 OP457 5111) and is held in place by four plastic posts.  The boards are joined by a 11 pin connectors.  The keyboard is fixed to the front with two plastic lugs and three blobs of plastic cement.
Construction: Remove the four screws from the rear of the case and the back will lift off easily.

Logic comments: The (C/CE) key is used to clear the last number entered whilst a second press clears the whole calculator.
Input overflow is suppressed, inputting a ninth digit is ignored
There is automatic constant on all four functions
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
Divide by zero shows zero and C in the far left (ninth) digit and is recoverable using (C/CE)
Overflow shows the result and C (or E if negative) in the far left (ninth) digit and is recoverable using (C/CE)
Low battery is indicated by a small "L" in the far left (ninth) digit
It suffers the negative zero bug: key in (0)(-)(1)(=) to give "-1" then (+)(1)(=) will give "-0."
It suffers the divide to negative zero bug: key in (0)(-)(1)(/)(1)(0)(=)(=)(=) etc.. to eventually get "-0.00000000"
It suffers the pseudo fixed decimal bug: key in (1)(+)(.)(0)(0)(0)(=) to give "1.000" which remains a fixed three digit number until more are required or you use multiply or divide

(c) Emil Dudek 2010

The image left shows the main cpu board removed from the calculator.

At the top you can see the single-lens Optostic LED display unit.

Just under this you can see the row of metal pins that locate in the sockets on the keyboard assembly.

There is an extra IC under the display - lots of components (and hence expense); typical in late 1974.

There are obviously at least two versions of this model as other internal shots on the web are quite different.