Radio Broadcast (May 1928-Apr 1929)

22 RADIO BROADCAST MAY, 1928 terference, power leaks, and other sources of noise are not bothersome. When the receiver is first put intooperation, the inductive trimmer should be set in midposition, the set screws on the tuning condensers loosened, and then some local station should be carefully tuned-in by moving the two tuning condensers separately. This process is necessary in order to get the two circuits in "step." The set screws are then tightened, and any slight variations on other stations may be compensated by means of the trimmer. If the two circuits are not properly lined up, broad tuning and lack of sensitivity are certain to result. The reason is self-evident from Fig. 3, which shows the resonance curves of both tuned circuits as well as the combined curve for the entire amplifier when the two circuits are properly lined up, as at A, and when they are not, as at B. Whenever a station seems to come in at two slightly different dial settings, we have the condition illustrated at B in Fig. 3, and a slight simultaneous readjustment of the trimmer and the tuning dial will correct matters. In tuning for distant stations, the use of regeneration in the detector circuit will be found of great assistance. As a result of the use of the screen-grid tube in the r.f. stage, the detector may, if desired, be permitted to oscillate, in which case stations may be picked up by their carrier waves, without annoying the neighbors. In order to obtain smooth regeneration it is advisable to try several different values of grid leaks and also different values of detector plate voltage. While almost any type of tube may be employed as a detector, the 112-A will generally be found to be more preferable than either the 200-A or the 201-A. Although a good 200-A is more sensitive than the 112-A the latter will probably A B FIG. 3 Showing the effect of having the two gauged condensers "out of step." Broad tuning and lack of sensitivity result. At A the circuits are properly lined up, while at B they are not be found less microphonic and certainly in every way a better tube for general use than the less-expensive 201 -a. Some of the 222's are inclined to be rather microphonic and may cause trouble when the loud speaker is placed quite close to the set. This difficulty is generally only encountered when the volume control rheostat is in approxi THE LAYOUT OF THE FRONT PANEL IS BEAUTIFULLY SYMMETRICAL SHOWING THE DIS he antenna inductance consists of POSITION OF PARTS two variable-doubled coils in series mately the mid position. A slight readjustment of the rheostat in most instances will correct the trouble, unless the tube is a poor one. The screen-grid voltage should be approximately 45 when using 135 volts on the plate of the r.f. tube. This voltage is not critical, however, and a variation of a few volts in either direction does not make an appreciable difference in performance. In the first audio stage a ux-201-A should be used while in the second stage a high-mu tube should be employed. A ux1 7 1 a is recommended for the last stage unless there are no "local" stations, in which event the use of the 1 12-A with proper C bias will result in increased volume. The following parts were used in the fivetube receiver as described and illustrated in this article: List of Parts National Single-Dial Tuning Unit, No. BD 222, with No. 28 Illuminator National ist-Stage Impedaformer National 2nd-Stage Impedaformer National 3rd-Stage Impedaformer National Tone Filter Aerovox 0.00025-Mfd. Moulded Mica Condenser Aerovox 0.001-Mfd. Moulded Mica Condenser Two Tobe 1.0 Mfd. Bypass Condensers Precise Midget Condenser, 50-150 Mfd. Five General Radio Sockets General Radio R.F. Choke Yaxley Filament Switch Carter 20-Ohm Rheostat Lynch 2-Meg. Grid Leak with Mounting Lynch 0.8-Ohm No. 4/5 Filament Equalizor, Mounting Lynch 15-Ohm Filament Equalizor, Mounting Westinghouse Micarta Panel, 7" x 18" Westinghouse Micarta Sub-panel, 10" x 17" Fuse Clip for 222 Tube Cap Wire, Solder, Etc. Two Eby Binding Posts The three Impedaformers and the tone filter are shown in dotted lines in Fig. 1. The various leads to the batteries are not taken to binding posts, but the leads themselves are made sufficiently long enough so that they may be cabled by the constructor and lead directly to the batteries.