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Calculators: Desktop: Prinztronic MC66

Size (approx):

180mm x 230mm x 30-60mm (w,h,d)
Weight 1290g excluding cable

Power:

Mains (240V 50Hz, 4W) power only through a permanently fixed mains cable of considerable length.
Display: 12 digit green VFD with additional symbol cluster on the right for memory (I), negative (-) and constant (K) indication. 
Features: Standard four functions with change sign, constant toggle and three function memory.
Age: 1973
Manufacturer: Prinztronic: Dixons International, made in Japan, serial No. D 6609497.
Comments:

 

A sturdy, heavy desktop from the early 70s which has very basic functions but a quite advanced case design.  The light blue plastic has nearly gone green with UV ageing on my example.  The logic is sound (if you can get used to the RPN logic) but the lack of error recovery (typical of Rockwell ICs) is a pain.

Case and Design: A three-piece calculator made from smooth dark blue base, smooth light blue middle and matt dark blue top, which is practically wedge shaped.  The green display filter is angled at over 80 degrees and so sits very deeply in its recess. It has digit numbers one to twelve printed in white below the numbers.  Two ridged white plastic sections sit in recesses that surround two unbalanced key clusters.  A prominent black plastic, raised and silver painted piece is stick on for the brand and model number.

Keys and Switches: The keys in light blue, white and red, are short travel loose-click type and arranged in a mess.  The (+=) stands prouder than the rest.  The on/off switch is situated on the left hand side, nearly at the top.
Construction: Remove the four screws from the bottom  and the whole front section will come off easily.  The key board units are attached to the front and are gathered into two large keyboard assemblies (attached with 11 screws), one individual key on the left (K) and one on the right (M+) which doesn't make much sense to me!  It is attached to the main cpu board by an 11-way edge connector that could probably take 25 connections as most are unused.  The individual display tubes are held in a metal framework attached to the main board.

Boards: The main board holds all the components and is attached to the rear section by four large screws with rubber washers.  The display holding brackets sits above the glass tubes.  This assembly hides the power supply components.  As a full size board there is space to spare and the various bits are well separated. 
Components: 1 x cpu: Rockwell 15352PA 7331 (date code week 31 of 1973), 42 pin staggered DIL, 0.6" width
3 x display drivers(?): Toshiba TM4358P 3-F (June 1973), 16 pin DIL, 0.3" width
12 x 1 digit green VFD display individual tubes: ISE DG10F1 GGA (or GHA) Japan (I suspect that the G numbers are date codes or batch numbers)
1 x green VFD symbol cluster: Futaba SP10D7 3H
0 x transistors
5 x diodes (2 on the PSU area)
5 x capacitors (3 on the PSU area)
53 x resistors (4 on the PSU area)
1 x mains transformer
2 x fuses 

Logic comments: (C) is used to cancel the last number entered and a second time to clear the whole calculator.
Input overflow is not suppressed, inputting a thirteenth digit results in an overflow that is not recoverable
Constant function is switched in with the (K) switch and operates on multiple and divide only (it is not needed with (+=) or the (-=) key as these operate just as a constant does.  When in use a "K" is displayed in the upper far right symbol cluster
Negative numbers are shown with a minus in the far right symbol cluster allowing full twelve digit negative numbers
The change sign key (+<>-) can be used in mid number entry
Divide by zero shows  "0.0.0.0.0.0.0.0.0.0.0.0." and is not recoverable
Overflow shows "0.0.0.0.0.0.0.0.0.0.0.0." and is not recoverable
Memory storage is indicated by "I" in the far right symbol cluster although there is no memory "II" to indicate a second memory
The logic is semi-Reverse Polish Notation RPN so to do 3-4 you input (3)(+=)(4)(-=) or (3)(+=)(4)(+<>-)(+=)