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Calculators: Desktop: Citizen 121F

(c) Emil Dudek 2011

Size (approx):

185mm x 255mm x 40-60mm (w,h,d)
Weight 1,250g excluding cable


Mains (220-240V 50/60Hz, 4W) power only through a large three pin standard connector.
Display: 12 digit green VFD with additional symbol cluster on the left for memory (M), error (<) and negative (-) indicators. The display has upper commas for thousand notation (and unused lower ones).
Features: Standard four functions with percentages, square root, four function memory and register exchange.  Switches for constant, rounding and fixed/floating decimal.
Age: 1977
Manufacturer: Citizen Business Machines Inc., made in Japan.  Serial number 81090116


This late mains-only VFD desktop is larger and heavier than most and is very well built.  The viewing angle is a bit too low for my liking so you have to sit some distance away from it.  Logic is OK except for the strange handling of input overflow and archaic Reverse Polish Notation.  There were at least four other versions that used the uPD286c, M58617-22B and M58623-23P ICs, and one that used split ICs

Case and Design: The base and top section is made of matt light grey plastic whilst the third display section is matt dark grey plastic.  The vertical green display filter is deeply inset to give a clear bright image.  However, the angle of view is quite low down because of that. It has white numbers painted on the inside - which you cannot see!  Above the display is a black printed metallic label that sits in its own recess.  

Keys and Switches: They keys are well laid out, large hollow sounding click with long travel.  A row of switches to the far left switch for decimal point selection (A0234F), rounding (down, 5/4) and constant (K). The raised labels for these switches are unprinted. The on/off switch is on the right hand side, nearly at the top.
Construction: Remove the four screws from the bottom and pull off the knob on the decimal point switch. The whole front section will come off easily.  The base is extremely sturdy and needs to be for the whopping transformer.  A fuse holder sits loose in the top area.

Boards: The main keyboard assembly (ref TK-145) is fixed to the base by screws into four metal pillars - both pairs of which have their own separate earth cables.  Twenty connecting wires join at the bottom to the main cpu board (ref 1D-001a-1) which sits loosely in some guide lines.  There are a surprising amount of diodes underneath the keyboard and a multi-pin connector brings power from the transformer. A third board is mounted at a very steep angle to support the display. 

A lot of the components have German text so it is likely that it was constructed in Germany.

Components: 1 x cpu: Texas TMS1273NL KBSP 7745 (week 45 of 1977), 40 pin DIL, 0.6" width
1 x IC: Sharp IR1403 611, 16 pin DIL, 0.3" width
1 x 12 digit green VFD display: flat tube with left symbol clusters
0 x transistors
22 x diodes
6 x capacitors
21 x resistors
4 x resistor arrays
1 x mains transformer: INL-2A 7122
2 x fuses

Logic comments: (CE) is used to cancel the last number entered and (C) to clear the whole calculator.
Input overflow is not suppressed, inputting a thirteenth digit results in an overflow that is not recoverable.  However you can still carry on calculating with the overflow in place!
Constant function is switched in with the "K" switch and operates on multiply and divide only
The rounding switch can select round down ( ), round up/down (5/4)
The decimal point switch can be selected between floating "F", floating "0,2,3,4" and  financial mode "A" for $x.xx entry
Negative numbers are shown with a minus in the far left symbol cluster allowing full twelve digit negative numbers
Negative square roots are allowed and display a negative result
Divide by zero shows "0.00000000000" and "<" in the far left symbol cluster and is recoverable using (CE)
Overflow shows the result and "<" in the far left symbol cluster and is recoverable using (CE)
Memory storage is indicated by "M" in the far left symbol cluster
The register exchange key (EX) exchanges the answer for the first number entered
The add and subtract keys are rather archaic RPN.  To do 3-4 you input (3)(+=)(4)(-=)


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