April/May 1980, Issue 7

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Impact Matrix Printer


by D. R. Hunt


printed using an Imp printer.

The Nascom Imp printer is a comparatively cheap dot matrix printer, printing 80 characters per line on plain paper using an imported American mechanism and electronics designed by Nascom. As Nascom do not have an electronic assembly facility, the Imps are constructed by a sub-contractor in Scotland. The mechanism is designed for bi-directional printing (more about that later), and will accept friction fed unperforated paper up to 8.5″ wide, or tractor driven perforated paper up to 9.5″ wide.

The Mechanism

The mechanism chassis is a through shaped 16g steel pressing, cadmium plated to avoid corrosion. Although not very rigid without the remainder of the mechanism, the ingenious use of the 12g aluminium platten and the head guide bar as stretchers produces a remarkably stable box like assembly. Paper and ribbon guides do not form a structural part of the chassis, and are simply laid in slots in the main chassis stamping. Two screws through the chassis side-plates secure the head guide bar, and two screws at the rear secure the back of the platten, the front of the platten being supported in slots in the side-plates towards the front. These four screws poll the whole chassis together, trapping the paper and ribbon guides at the same time. By constructing the mechanism in this fashion adjustment of the critical head to platten distance is particular easy to achieve, and the whole idea leads to a very compact structure.

The 7 needle print head is transported across the paper by a peg which engages with a three turn continuous spiral cut in what appears to be a nylon (or delrin) shaft, approx. 1″ in diameter, driven by a stepper motor via a miniature toothed belt. The head transport motor is secured to the left hand side-plate. Head to paper registration is maintained by the precision ground steel guide which is part od the chassis pressing. A nylon gear driven take-off at the right hand end of the head spiral shaft drives the ribbon feed.

Paper feed is derived from a second stepper motor mounted on the right hand side-plate, via nylon reduction gearing which drives the aluminium forward friction feed roller directly and the tractor feed via another miniature belt drive. The tractor assembly is a separate part of the mechanism and is secured to the chassis by two large screws. The tractors have levers which friction lock on to the tractor bars, and it is possible to position the tractors at any point across the paper width. Friction paper feed is made possible by a rubber roller which may be engaged with the front friction feed roller by a small lever mounted in front of the ribbon cartridge. The rubber roller is held in place by strong springs and are adequate for most purposes, but have been found to slip when ‘interleaved carbon’ telex rolls have been used (an unfair test perhaps?).

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