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has a more even quality and is more consistently reproducible; this is of importancehaving regard to the stringent requirements which have been placedon the individual apparatus units. The work of assembly, especially soldering,is greatly simplified, as all soldering points lie in one plane easily accessiblefor the soldering tool and clearly visible when inspection is carried out. Inthis way, an appreciable improvement in the quality of soldered joints can beexpected.The etched wiring board places completely new mechanical demands onthe electrical components. For example all the inductor and transformer typeshave been redesigned so that they are suitable for this new wiring technique.The plug and socket connexion between the wiring of the apparatus unit, viz.the etched wiring board and the bay cabling has also had to be redesignedowing to the introduction of the etched wiring board. In short, it can be saidthat starting from the etched wiring board, the design of the apparatus unitshas been completely changed.The miniaturization which is provided by the transistor, and to which ofcourse contributions have also been obtained from other steps forward inthe field of components-for example, better ferrite material-has meant a reductionof space requirement to a third of that for the earlier method of construction.This means that the optimum bay is no longer a tall double-sidedbay but is a single-sided bay of approximately half the depth. A logical treatmentof all the new points of view coupled with the experience obtained, forexample as regards maintenance, from the earlier method of construction hasled to a completely new design of all the mechanical details, even as regardsbay design, which are used in carrier systems.We are convinced that by dealing with problems consistently and by avoidingunsatisfactory compromises between earlier and new methods, a designof transmission material has been successfully obtained which does justice toall the demands placed by the customer and by a rational production on amodern method of construction.II. Mechanical DesignGeneralFor a layman, the mechanical design problems associated with the developmentof a new construction technique for transmission equipment may appearrelatively simple compared with, say, those in selector switch design, wherethe mechanical parts, mostly moving parts at that, are directly included in thesignal path, whereas in transmission equipment their main function is only tosupport and contain the electrical components and their wiring.That is correct as far as it goes, but the design problems are of quite adifferent kind. Transmission technique uses considerably higher frequenciesthan exchange technique, so that in the mechanical design much greater accounthas to be taken of electrical requirements, such as for instance screeningbetween different pieces of equipment or units. A further example is thatby its nature transmission equipment is composed of a great number of differenttypes of products, so that for a design to be economically competitiveit must fulfil stringent requirements for flexibility and universal applicabilityof its component parts.Previous Mechanical Design PrinciplesThe earlier construction practice for transmission equipment was to use adouble-sided 19-inch rack with a maximum height of 2 590 mm (8' 6"). The
units were either connected by plug and socket or built up on panels of thefull width of the rack, with appropriate dust covers.Because of the large number of sub-units, this construction was relativelycostly to produce, while at the same time its exploitation of available spacewas not very high by modern standards.Aims for the New Mechanical DesignThe aims in designing for the new construction technique can by and largebe summarized as follows:a) Transistors should be used where feasible instead of electron tubes.b) The units should be simple to manufacture and involve as few productionsteps as possible. This can be achieved by using printed wiring techniques,which moreover results amongst other things in a more uniform andcompact product; at the same time sub-units are avoided.c) The mechanical construction of the equipment should be so arrangedas to permit a sub-division into functional units from the electrical point ofview. For the sake of greater flexibility these units should be of plug-in typeand therefore replaceable.d) Space utilization should be high, i.e. the design should be compact, butnot at the cost of reduced accessibility (e.g. for repairs) or excessive weight.The weight could, however, be kept low by using light alloys in large measureas constructional material.e) The bay should be single-sided for increased flexibility in station planning,and should keep within certain maximum dimensions. Nevertheless, itshould be possible to accommodate channel modems, signalling and powersupply equipment for 60 telephone circuit terminals on a single bay side.iiimiimini!f) The number of mechanical components in the design should be as lowas possible, and as a consequence the field of application of each mechanicalcomponent should be as wide as possible.g) Routine maintenance measurements on the equipment should be possiblewithout having to remove any dust covers.h) The equipment when mounted in the bay should as far as possible beprotected from dust and electrically screened.Principal Dimensions and Modular SystemA design which fully meets both the essential requirements and the desiderataset forth in the above aims has been realized in the new constructiontechnique.The design work has resulted in a bay construction in which the principaldimensions of the tallest type of bay are 2 743 X 670 X 236 mm (9' X 26.4" x9.3"). A bay with this height is sub-divided vertically into 73 height modulesof 36.5 mm (1.43"): the height module previously used was 44.4 mm (1.75").Sub-division horizontally into width modules depends entirely on the unitconstruction principles and is described in conjunction therewith.Fig. 2Channel translating bay, type ZDG 801with some of the cover plates removedx 2526The rack and dust-covers are finished in pale blue-green stove enamel.Compared with the earlier construction practice, the new technique hasinvolved a complete re-design. As an example of the result, the new channel117
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