26 

 FORMS OF MECHANICAL CONSTRUCTION 



The 'traditional' form of mechanical construction for electronic equipment 

 is the panel and chassis arrangement of the type shown in Plate 26.1. The 

 controls are carried on the front panel with anything else which has to be 

 visible or accessible from the front, i.e. dial lights, fuses and certain connectors, 

 while all other items are carried on the chassis, with large components above 

 the 'deck' and small ones below it. If the assembly is to be used alone it 

 may be enclosed in a cabinet; if with a number of others a simple dust 

 cover suffices and the group are carried in a vertical stack on a 'rack' {Plate 

 26.2). In order for this to be possible it is necessary that the panel width be 

 standard, and until recently 19 in. was widely accepted as the norm. The 

 panel height is not, of course, similarly restricted. 



In recent years there have been considerable departures from this design. 

 The progressive reduction in the size of components has led many engineers 

 to feel that 19 in. is now too large, and a few firms have established a 

 narrower panel, only 14 in. wide (Plate 26.3). Furthermore, attention is 

 being increasingly focused on ready accessibility of components for servicing. 

 With traditional construction, to gain access to the interior of a unit one has 

 usually to: (a) break all connectors; (b) undo 4 bolts and withdraw the 

 unit from the rack ; and (c) undo further screws to remove the dust cover. 

 If it is known exactly what modification or repair has to be carried out, 

 one is now able to do it. More often there is an unidentified fault which 

 requires that power supplies, etc., be re-connected to the unit in order that 

 voltmeter and waveform checks may be carried out. Usually the various 

 flexible connections do not reach unless the unit is offered up towards its 

 original position on the rack and supported on a chair or steps, usually 

 augmented by piles of books. Much thought has been devoted to producing 

 alternative mechanical designs which simplify servicing procedures. Readers 

 who are interested are referred to an excellent review by R. H. Garner 

 (Mechanical Design for Electronic Engineers. London; Newnes). 



In apparatus for scientific work the need to be able to get at the interior 

 quickly seems to be even more vital than with the ordinary run of commercial 

 gear. It is not always possible before embarking on the design of a new piece 

 of equipment to specify exactly what it will be called upon to do. The time 

 will surely come when some new demand will be made upon it, often at a 

 few minutes notice during the course of an experiment. The writer therefore 

 ventures to describe his own scheme for mechanical layout. 



The apparatus is composed of small units measuring about 6 in. cube 

 (Plate 26.4) which are rack- or console-mounted (Plate 26.5). By removing 

 two thumbscrews the front cover may be drawn off a unit, revealing all 

 valveholder and potentiometer connections, and allowing waveform checks 

 and voltage measurements to be made to diagnose a fault. In this condition 



327 



