MBO 800

MBO 800
MBO 800

1973 MBO launched the model 800, a simple pocket calculator with four basic calculations. The unusual part of this calculator was, that it contained apart from the flourescent display tube two additional LEDs for indicating negative values and errors. At that time, LEDs were not as usual as today, which can you see on the style of the LEDs.


back side with open battery compartement
back side with open battery compartement

Case and Keyboard

The case of the MBO 800 is of a rather modest quality: The hard and thin plastic looks quite well, especially on the parts, where the white leather imitate is glued upon, but it ages and tends to break on used edges.

The case is not of a small size, but that was pretty usual for a calculator made in 1973. The battery compartement is styled a bit unusual, as you can see: The polarity markings are made as conductor lines on the PCB. This is a clever way to avoid losing stickers indicating the correct polarits.

Display and LEDs are beyond a large blackened plastic pane and allow a good readability even in bright surroundings. The pane in front of the display is angeled a bit and the position of the digits is marked upon.

In contrast to that, the keyboard is a very poorly made device: The keys have to feeling at all the bounce terribly so that it is almost impossible to enter the desired numbers. Worse to that, the mains switch has got massive contact problems.

While other keyboards can be opened and cleaned, this keyboard here is sealed and impossible to repair.

The problem with this keyboard not only appears on this very device, but also struggled me twenty years ago on an other specimen of the MBO 800.


Inside view
Inside view

Construction

The MBO 800 resembles from its construction with the large mainboard, which fills more than half of the case and the round VFD the state of the art of 1973, but there are still a few details, which make it different from other calculators:

The PCB has got detailed printings, even with the name of the ICs, looks manually populated and the calculator has got two large ICs looking almost identical.


details of the mainboard
details of the mainboard

A further noteworthy detail can be found on the connectors of the keyboard: Every single key has got an own line; normally you would have a matrix here, reducing the need for so many line. On the MBO 800 you have got 23 wires (including the power switch) which connect the keyboard with the mainboard, and this might also explain the sum of eighty(!) pins on the two large ICs.

Display

in 1973 it was still usual for a pocket calculator to have a round VFD. Those tubes had got a vacuum seal on the one side and lots of connector wires on the other side. The MBO 800 uses exactly that kind of display tube, of the type ISE DP 81A1, which is of course not very suitable for automatic assembly.


two really vintage LEDs
two really vintage LEDs

The display has got exactly eight digits and is because of this not capable to show additional information like errors or negative numbers. For this, two pretty vintage looking LEDs are used. Unlinke the display, which can best seen from the front, you have to look from above to see the light of the LEDs. A typical case of "form follows function", but not very economical.

CPU

On the PCB, two ICs with fourty pins are soldered, an ER 1432B (week 18, 1973) and an ER 1422B (week 17, 1973). Those two ICs are completely unknown to literature, so I have no idea about the functions (e.g. display driver, keyboard driver, CPU) of each of them. The manufacturer of these chips is unknown, too.


two ICs: ER1422B and ER1432B
two ICs: ER1422B and ER1432B

Calculating Capabilities

The calculating capabilities of the MBO 800 are rather normal for a calculator of 1973 and are quite limited. It is only able to do the four standard calculations, but already works with the standard entry logics using the "=" key. Quaintingly it has got a constants function which only works for multiplication and division.


displaying negative numbers
displaying negative numbers

A division by zero is properly indicated with an error and overflow in both directions also shows an error. On the multiplication, the overflow error is not indicated, unless sixteen (twice the range of the display) digits are exceeded. This can be fatal in your daily work!


displaying an overflow error
displaying an overflow error

The displaying of regular numbers on the division is a bit buggy, as the displays always shows the full eight digits, including the last zeros on the right side of the comma. So, the simple division 1 / 2 is shown on the display with the result of 0.5000000. A multiplication with zeros after the comma results in adding more zeros on the right: 2.0 x 2.0 = results in 4.00, and because of the constants function after the next "=" in 8.000, then 16.0000 and so on.


displaying error on division by zero
displaying error on division by zero

Summary

The MBO was, when it was launched in 1973, a pretty average calculator, which was neither famous for its calculating capabilities nor for its contstruction and of course not for its keyboard. But because of this, the price was probably rather low.

The specialities of this calculator are its display with VFD and LEDs and the calculating logics, which does not show (besides on overflow of the multiplication) wrong results, but as got some minor bugs, especially with zeros after the comma.

All this make the MBO 800 collecting worthy, and for me, this calculator was the first one, I came into contact when I was six years old. My grandfather, who bought it a few years ago, when he was already 78 years old, gave it to me, when he got himself (at an age of over 80 years) a Privileg MD 585.


calculator with original leather case
calculator with original leather case

Technical data

ManufacturerMBO
Model800
Year1973
CPUER 1432B
Display8 digit VFD
Number of keys 18
Functions+ - * /
Entry logic (classificationALG (BBD)
1 + 0.000 = 1.000 (the correct value would be 1.)
0 / 0 = Error
Power supply 4xAA
Size14,4 x 8,7 x 2,7 cm
Serial Number06216
ConditionB
Factsheet