528 



BELL SYSTEM TECHNICAL JOURNAL 



internal assembly of this type of filter. The chassis consists of a pair 

 of perforated brass angles running the length of the assembly and 

 spot-welded to a cover at each end. Coils and condensers are riveted 

 to the angles, while the crystal holders are mounted with rubber shock 

 absorbers on studs extending upward from the ceramic bases of the 

 variable condensers. External leads are brought out through copper- 

 to-glass seals to terminals, as shown in Fig. 11, since in final assembly 



GLASS 

 COPPER SLEEVE ^^^ 



\%-\ LEAD WIRE 



^^^^^^^^^>-^^^^^^ 



EYELET 



0.5 



I 



ONE INCH 



Fig. 11 — Terminal used in hermetically sealed filters. The copper sleeve, bonded 

 to the lead wire by means of an insulating glass bead, is soldered into the container 

 of the filter in final assembly. 



the filter must be hermetically sealed to protect components from 

 moisture and dust. 



In wiring the filter special precautions are taken to prevent foreign 

 materials from being deposited on crystal plates, thereby introducing 

 mechanical damping, and from lodging in variable condensers, where 

 electrical leakage must be avoided. Internal connections are made 

 with bare tinned wire. Rosin flux remaining on soldered connections 

 is washed off with a solvent. Dust and other particles in variable 

 condensers are blown out with air. The filter then undergoes a careful 

 visual inspection and a 500 volt d.c. insulation test. 



At this stage it is generally necessary to adjust certain of the filter 

 elements, usually variable condensers, in order to compensate for 

 manufacturing variations in other elements and for parasitic effects 

 such as capacitance of the wiring to ground. A general view of the 

 testing equipment used for this purpose is shown in Fig. 12. One or 

 more of three methods of adjustment are employed, namely, (a) trans- 

 mission loss, (6) resonance and (c) capacitance. The first and second 

 of these are utilized on the channel filter, the schematic of which is 

 shown in Fig. 13. The two sections are adjusted independently before 

 the resistance pad, seen at the center of the figure, is inserted to 

 connect them. 



In transmission loss adjustment of the channel filter, the attenuation 



