432 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



struction of conventional panels. This also facilitates manufacture as does 

 a new method of mounting components of the "pigtail" type in parallel 

 thermoplastic strips and the division of the equipment into subassemblies 

 convenient for shop handling and so composed of circuit elements that the 

 same subassemblies can be used in more than one part of the system. Main- 

 tenance is facilitated and service interruptions reduced to minimum length 

 by arranging the units for interconnection by plugs and jacks so that a 

 defective unit can readily be replaced by a spare and sent to a maintenance 

 center equipped with adequate measuring equipment and manned by a 

 technically trained and experienced personnel. Engineering and installation 

 are faciUtated by packaging the equipment so that the maximum possible 

 portion of the assembly and wiring work is performed in the shop, and by 

 avoiding engineered options. 



The close packing of components in a relatively small space makes more 

 serious the problems of wiring, shielding, heat dissipation, accessibility for 

 inspection and maintenance, and major modifications. 



Unitized Construction 



This unit method of construction takes the form of conveniently sized 

 plug-in assemblies. It makes efficient use of the full 10-inch depth available 

 in the standard relay rack. The front of the unit carries the vacuum tubes 

 adjusting potentiometers and test terminals which need to be accessible 

 for routine system checking. Any space left over on the front panel is utilized 

 by voice and carrier frequency transformers. Other components are com- 

 pactly assembled inside the unit and are accessible only after the unit is 

 removed from its frame mounting. The external connections of each unit 

 terminate in a male connector which matches a female connector in the 

 frame mounting. Both connector assemblies consist of a molded phenoHc 

 rectangular block equipped with 20 gold plated contacts. These assemblies 

 are mounted by means of shoulder screws to give them a slight floating 

 action which relieves the strain on contacts and wiring when the units are 

 plugged in. After the units are plugged in they are secured to the frame 

 mounting by means of quick-acting fasteners. 



The plug-in method permits the testing of the units without expensive 

 jack fields, and allows the removal of any unit in trouble and its replace- 

 ment by a spare unit for immediate restoration of service. The defective 

 unit can then be taken to a maintenance point where adequate tools and 

 testing equipment are available for convenient repair work by experienced 

 personnel. This is especially valuable in the N-1 system where a majority 

 of the repeaters may be pole-mounted and many of the terminals located 

 in unattended or partially attended offices. It will be valuable in other loca- 

 tions by eliminating repair work from a ladder. For the handling of units 



