714 BELL SYSTEM TECHNICAL JOURNAL 



single mechanical control. Remote selection of frequency may be by 

 a flexible shaft or by a pull-wire arrangement. 



The companion receiver, coded the 12-C, is a compact unit weighing 

 about 16 pounds. It has a superheterodyne circuit employing six 

 tubes and permits a quick shift between two fixed frequencies. High 

 sensitivity and unusual selectivity are obtained. The antenna circuit 

 is series tuned. The beating oscillator and modulator are combined 

 in one tube while a quartz crystal assures the correct beating frequency. 

 It is a plug-in unit similar to that used in the transmitter and ground 

 to the carrier frequency plus the intermediate frequency of 385 

 kilocycles. Two stages of intermediate radio frequency are used. 

 A duo diode triode tube is used as detector and first stage of audio- 

 frequency amplification. The second audio and output tube is a 

 pentode. Automatic volume control provides uniformity of output 

 signal. A ballast tube protects the filaments against voltage fluctua- 

 tions in the power supply. 



Space is provided for two crystals and remote selection of either, 

 frequency may be provided by flexible shaft or pull wire. The controls 

 are tied together through the receiver and transmitter mountings so 

 that the frequency of both may be shifted simultaneously from a 

 remote point. 



While in the aircraft system the air pilot has his receiver always in 

 use as he checks periodically on his route, it is obvious that the marine 

 pilot would not require this constant contact with the shore station. 

 To eliminate the necessity of loud speaker monitoring, selective 

 ringing is available. It works from the output of the radio receiver 

 with a conventional type selector which is stepped up in response to 

 the impulses received from a dial actuating the shore transmitter. 

 With this arrangement, any particular boat may be called, a bell 

 announcing the incoming call. 



For convenience of installation, the transmitter, receiver and 

 selective ringing unit are mounted in a small metal cabinet. The 

 cabinet provides protection to these units and incorporates the remote 

 frequency change equipment and a junction box. All the connections 

 between the unit mountings in the cabinet are permanent and termi- 

 nate in the junction box. Provision is made in the base of the cabinet 

 for two cables, one to the control equipment and the other to the power 

 supply. The cabinet is provided with rubber feet which permit its 

 installation in places subject to considerable vibration. One of the 

 features of the installation is the ready access to any of the units for 

 maintenance and test. The top and front of the cabinet may be 

 removed and each unit, being "plug-in" mounted, is easily removable 



