THE L3 SYSTEM — QUALITY CONTROL IN MANUFACTURE 993 



possible by conventional manufacturing techniques. Initial production 

 experience indicated that the "natural tolerance" of the 200A resistor is 

 better than the ±4 per cent "A" limit originally specified, and this limit 

 was reduced to ±3 per cent. 



3.5 2504-TYPE TRANSFORMER 



The 2504-type transormer, due to its critical function in the operation 

 of the L3 amplifier circuit, has necessitated the use of completely new 

 materials and methods not usually associated with transformer pro- 

 duction. For instance the windings of this transformer are formed by 

 diamond grinding the threads in fused quartz forms upon which a silver 

 coating is bonded by a firing process. The grooves are then filled with 

 electroplated copper, thus producing a winding intimately bonded to 

 the quartz forms. Since the whole manufacturing process is unique and 

 must be controlled in each small detail, application of statistical quality 

 control procedures is essential. 



Control of the leakage flux of this transformer, capacitance across the 

 high impedance winding, and the capacitance to ground from the high 

 potential terminal of its high impedance winding is of particular im- 

 portance. To achieve the desired control of these characteristics, care 

 must be taken to maintain a number of mechanical dimensions to toler- 

 ances far more precise than those on any transformer previously made 

 by the Western Electric Company. 



For example, the inner diameter of the quartz form, bearing the outer 

 winding, is held to 0.7280 =b 0.0005 inches, while the thickness of the 

 form between the inner diameter and the root diameter of the threads is 

 held to 0.0310 ± 0.0005 inches. Dimensions of other component parts 

 of the transformer must be held to comparable tolerances. To do this, 

 it has been found necessary to keep all facilities — chemical, electrical 

 and mechanical — under careful control at all times. 



The facilities provided for the machining of the component parts are 

 such that all critical dimensions which affect the final overall electrical 

 characteristics of the transformer are held to approximately one-third 

 of the design tolerances. By this means, it is expected the resulting 

 transformers produced will meet the desired distribution. In the initial 

 stages of production, records are being kept on 14 mechanical dimensions 

 on each unit, checking them at each critical state of manufacture. The 

 use of extensive mechanical tolerance controls on the components of this 

 transformer is necessitated by the non-adjustable nature of its design. 

 If controls were not applied, it would be impossible to obtain the close 



