Model: Spell master Radio shack
Battery: 4 x 1,5 V type C
Info: The Radio Shack Spellmaster) was manufactured in 1988 and represents the last design iteration of the Speak & Spell family. The printed circuit board (PCB) itself is much smaller than the original Speak & Spell design.
The calculator is housed in a rectangular plastic box finished in imitation woodgrain.
The main circuit board and the display tube board are fastened to the base with a pair of light metal brackets. A 2-pin mains connector and the power transformer are mounted at the rear of the box, with the power switch attached to the moulded top cover. The transformer secondaries are hard-wired to the circuit board.
The display uses a set of miniature eight-segment vacuum fluorescent tubes, Futaba type DG10R1. (The small eighth segment at the centre right gives a "proper" numeral 4, but the processor only drives the usual seven segments). Eight tubes are used for the numerical display, and one (at the right) for polarity and error indication.
The keyboard is built as a single module and is attached to the main board via a cable and edge connector.
The TMS-0105 series single-chip processors contain all of the calculator circuitry with the exception of the power supplies and display drivers.
The rather untidy circuit board has a simple power supply at the rear and the TMS-0105 chip in the centre. The anode drivers for the nine tubes are at the left, with the eight segment drivers (seven plus decimal point) at the right. The keyboard attaches to the edge connector at the bottom.