The front half of the calculator consists of a substantial sheet-metal frame supporting the keyboard and the two circuit boards. A full-length board immediately below the keyboard carries the eight display tubes and their drivers, while a half-length board at the bottom front carries the four processor chips.

The inverter and power supply module is located in the plastic housing under the display tubes, with the internal Ni-Cad battery in the black clip-on housing at the bottom rear.

The keyboard uses glass reed switches operated by moving magnets attached to the bottoms of the keystems.

The board carrying the reed switches is attached directly to a special two-slot edge connector for the two plug-in boards. Plastic card guides are attached to the inside of the frame.

The logic or processor board measures 75 x 85mm and carries four Rockwell MOS chips in 42-pin ceramic flat packs.

The part numbers are DC1152, NRD2256, ACM1156A, and AU2271C, with date codes from February to April of 1971.

The metal-can package is a CC1121 clock generator, producing a two-phase clock at about 45KHz (and a number of timing signals) from a 90KHz master oscillator.

The upper board carries eight individual vacuum fluorescent display (VFD) tubes, Iseden type DG10L. The tubes are 10mm diameter and 25mm high, with numerals 8mm high. They are attached to the board by flying leads from the base, and are supported by a metal bracket and rubber grommets at the top.

The numerals are formed from eight segments rather than the (now) usual seven, giving a script-like appearance with a distinctive half-height zero.

At the top right of the display there is a special two-element VFD tube which shows a horizontal bar for minus and a dot for error indication.

The lower section of the board carries nine display-driver ICs in 16-pin packages. There are six Hitachi HD3120 and three HD3121, all with date codes of March 1971.

This close-up view shows the geometry and the electrode structure of the DG10L tube. The filament (the vertical white line) and the control grid (or mesh) are clearly visible.

The "large-screen" display only has a single row of eight numerals, with the minus and error indicators at the top right and the power supply alarm indicator at the top left.

In common with many early calculators, the operation of the EL-8M has more than its share of "interesting" peculiarities.

Addition and subtraction work more or less normally in adding-machine fashion, using "+/=" or "-/=" after each entry.

Multiplication and division are both marked on the same key - which one you get depends on which "equals" key you press afterwards! For multiplication, use +/=; for division, just match the red division sign with the red -/= key. Successive calculations can not be "chained" together - it is necessary to use the "equals" key after every stage. For example, 2 x 3 x 4 +/= ignores the first operation and gives a result of 12 rather than 24.

The "C" key functions as both "Clear" and "Clear Entry", depending on whether any operations are pending. In the latter mode it clears both the number and the operation, and restores the machine to the state at the previous result.

The "Error" indicator lights for over-range or division by zero, and is cleared with the "C" key.

The decimal point floats within the available range, although the calculator attempts to preserve the number of decimal places entered if there is space available (eg 2.0 x 2.0 = 4.00). Results in division are always left-aligned with the maximum number of decimal places. There is no provision for roundoff.

The latching "M/D" key at the top right switches between memory functions and a "double-precision" multiplication mode:

• With the M/D key raised, the "M+" key adds the display to the memory register, "MR" recalls the memory, and "CM" clears it. MR can be used at any stage during multiplication and division, but only at the beginning of a series of additions.
• With the M/D key down, the calculator can multiply two 8-digit numbers and produce a 16-digit result. The second (lower) group of digits are returned via the memory register, and can be displayed by pressing the MR key. The C key returns to the high-order display, but the result can not be used in a continued calculation.

The power supply arrangements in the EL-8M are almost as interesting as the calculator itself.

The inverter and power supply module attaches to the card frame and the display board at the rear of the case, with the battery pack clipped on underneath. There are plug-and-socket connections for the battery input, and for the power supply output to the display board.

A specialised external AC power supply and battery charger connects to the socket at the rear. The power switch at the right has separate positions for AC and DC (ie battery) operation, with an OFF position in the centre.

The lamp at the left rear corner lights the "Alarm" indicator on the display.

The battery pack (P/N EL-84) contains six 1.2V 450mAh nickel-cadmium batteries, for a nominal 7.2V output. The assembly clips on to the bottom of the inverter module, and connects via a polarised 2-pin plug and socket at the top.

The calculator will operate with battery voltages down to around 5V, with reduced display brightness. Current drain varies from 60mA at 5V (300mW) to around 120mA at 7.5V (900mW).

This upside-down view shows two different versions of the external AC adaptor and NiCad charger.

Both units carry the same part number (EL-81) and have the same nameplate ratings. The larger unit has a multi-tapped transformer and a mains voltage selector, while the smaller (supplied with a later EL-816 calculator) has a fixed 240V input and carries a type approval number for the Australian market.

The charger produces two separate outputs which connect to the calculator via a short "curly cord" and a custom 3-pin plug. The (nominal) input rating is 5.5W, with outputs listed as 8.7V 150mA and 9.6V 230mA.

The charger has an L-C filter on the mains input at the right-hand side. On the output side there is a fuse, bridge rectifier, an indicator lamp and a couple of resistors, and a regulator module on a small daughter board. The 3-wire curly cord connects at the left rear corner. Note that there are no reservoir capacitors in the charger unit, nor in the inverter itself.

The regulator output connects directly to the battery in the AC and OFF positions of the calculator power switch, and through a dropping resistor to the inverter in the AC position. In the DC position the inverter is fed directly from the battery. If the AC supply is connected while the switch is set to DC, it lights an "Alarm" lamp on the calculator display to warn the operator that the battery is not being charged.

The daughter board carries an 8-pin "mystery" module (P/N RD165S) and a couple of resistors, and functions as a three-terminal phase-controlled regulator.

The module takes the unfiltered output from the bridge rectifier, and produces a regulated phase-controlled output whose conduction angle varies with load and battery condition.

This CRO trace shows the battery current waveform with the calculator operating normally with a fully-charged battery.

The vertical scale is 100mA/div, with zero in the centre. The mains frequency is 50Hz. The "shadow" is due to high-frequency current pulses into the inverter.

Current flows in to the battery as the mains voltage rises on each half-cycle. The regulator monitors the instantaneous battery voltage, and shuts down when it has reached a target level. Current then flows out of the battery to keep the inverter running until the next half-cycle.

The conduction angle varies from almost zero (calculator off, battery fully charged) to 180° with the battery flat.