Radio broadcast .. (1922-30)

FEBRUARY, 1927 A FOUR-TUBE "LAB" RECEIVER 369 R.F. STAGE DETECTOR FIG. Originally the grid leak and condenser were used to obtain rectification but in the rebuilt "Lab" receiver the C battery method of detection is employed. Its use insures against overloading of the detector tube. Both system are shown here THIS REAR VIEW OF THE RECEIVER Clearly denotes the use of bakelite strips to in- sulate the volume control and regeneration con- denser from the metal panel. The Sangamo condenser which is employed as a protective condenser in series with the regular Precise neu- tralizing condenser may also be seen. The cabled leads are run along the rear edge of the base- board and are terminated at the Yaxley plug receptacle care of, will inter-couple with adjacent tuning circuits so as to cause uncontrollable oscilla- tions. One way to overcome this difficulty is in the use of a type of coil which will not set up such a great field. Coils of the confined field type, such as binoculars and toroids, are satisfactory on this score but are not generally as efficient as solenoids. To employ the solenoid but not experience inter-coupling effects, a sheet of aluminum, if placed between the adjacent in- terfering coils, will decrease the inter-coupling effects to a negligible minimum. Such a shield has been employed for just this reason in the revamped four-tube set described here. The interstage shields, manufactured by the Alumi- num Company of America, fit in to this plan extremely well. See Fig. 4. The circuit itself is shown in revised form in Fig. 5. Changes, purely of a mechanical nature, have also been considered and, with the circuit changes, were incorporated in the present receiver. MECHANICAL ALTERATIONS THE mechanical alterations consisted of sub- stituting one of the new Aluminum Company panels for the insulated panel. These metal pan- els are to be obtained in several finishes, such as walnut, mahogany, etc., and make the outward appearance of the receiver pleasing indeed. FIG. "Before "shows the original volume control system. "After" shows the system now employed. With the volume control in the latter position, the output of the radio-frequency amplifier stage may be decreased without affecting tone characteristics At the time of going to press on the January RADIO BROADCAST, in which was described the two-tube "Lab" circuit, it was not possible to show the use of Marco illuminated controls because of the insulation problems involved, which at that time had not been solved. Since then these controls have been provided with insulated bushings which allow their use in re- ceivers employing metal panels. They are employed in the receiver described here. It will be noticed by referring to the circuit diagram that neither of the two tuning conden- sers have either of their sections at ground po- tential as in other circuits. Since the condensers are mounted upon the dial frames and, in turn, the dial frames attach to the panel, it is easy to see that, unless some insulation precaution is taken, the condensers will have a common con- nection between them which is not at all desir- able. It is absolutely necessary then, that some means be employed to insulate the dial frames from the metal panel. The Marco Company ^-Secondary Terminal -__ Lugs RADIO BROADCAST Photograph Mounting Brackets' Gnd. G / Ant. Gnd. Insulated Mounting Strip Ant. CenterTapon La L i = 8 Turns 85 Turns Gnd. 1= 26thTurn 2- 59th " 3-89th " 4-118th « 4 Solenoid- 2g" diameter Binocular -If" (each half) diameter 26 Turns 77 Turns Back Plate of CardweM Condenser--' No. 25 Cotton Silk covered Wire used to wind all Coils FIG. 3 Complete coil specifications necessary for winding the tuning coils used in the receiver are given here. Three coils, instead of two, are now employed in the "Lab" receiver. The extra coil takes the form of an antenna loading coil