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Two main problems had to be overcome before the case depth could be reduced to 9.5mm: a special keyboard had to be designed and a way had to be found to power the calculator from miniature mercury button cells. The first problem was overcome by using the printed circuit board itself as the lower contacts for the push- buttons. The case is therefore constructed in two halves with the push buttons in the upper half and the printed circuit in the lower. Supplies to the push buttons are connected by little pieces of spring material soldered to the printed circuit board which make with contacts in the upper half of the case; this looks rather crude.
The problem of using batteries with very limited power capacity was overcome by switching off supplies to the calculator chip (made by Texas Instruments) in between clock pulses and relying on the gate capacity of the m.o.s.f.e.ts on the chip to store the information. The ratio between on and off periods varies between 1:3 and 1:40, depending on what the calculator is doing. Further power saving was achieved by using a low clock pulse rate, by causing the display to flash after a few seconds of steady reading and by powering the display directly from the battery eliminating series dropper resistors which dissipate power. We understand that Texas Instruments do not guarantee correct operation of their calculator chip under the pulsed power supply conditions present in this calculator. However. Sinclair give a five-year guarantee which covers everything except misuse.
We thought the on /off switch (a piece of plastic pushing a piece of bent flat spring material on to the printed circuit board) and the fixed / floating point selector switch were difficult to operate and were not examples of particularly good engineering practice.
The production tests carried out on the calculator do not include a temperature cycling test which is unfortunate as if the calculator were left in direct sunlight, say on the back seat of a car, for a few hours one could not be sure that it would still function correctly.
Sinclair Radionics are to be congratulated for a brave and clever design but it is a pity that more attention was not paid to detail and sound engineering principles.