Radio Broadcast (May 1928-Apr 1929)

FIG. 3 The Rice short-wave receiver circuit employing the shielded coupling coil waves between 15 and 200 meters. More than three turns would, possibly, give greater signal strength above about 75 meters. In some cases better results may be had by reversing the leads to the plate coil. Fig. 3 shows the complete circuit. The condenser "Neut. Cond." was a SilverMarshall type 340 balancing condenser. Coils, R.F.C., are Samson r.f. chokes. The following parts are indicated on the wiring diagram: L — Plug-in coils, C — Pacent 0.000135mfd., Ci — Sangamo 1.0-mfd., C2 — Pacent 0.0001 3 5-mfd., C3 — 0.0001-mfd. Micadon, C4 — Cardwell o.ooo5-mfd.,C5 — Sangamo 1.0-mfd. Without a doubt an electro-static shield would be desirable in many broadcast receivers. R. Wm. Tanner, Berkeley, California. STAFF COMMENT HpHE problem of constructing a satisfactory * short-wave r.f. amplifier involves the difficulty of obtaining stable operation. Our contributor seems to have overcome the difficulties to a satisfactory extent, although we doubt that the completed receiver is very efficient. The problem of short-wave receiver stabilization should be relatively a simple matter using a screen-grid tube. This department will be interested in receiving data from readers who have experimented along these lines. Wave Trap Tuned Antenna Combination OPERATING a six-tube neutrodyne (Crosley "Bandbox") the writer is able to obtain more volume in case of weak distant stations during daytime reception by connecting in series with the outdoor antenna, 75 feet of bell wire wound on a if" cardboard tube plus a (Steinite) wave trap in parallel connection across this coil and receiver. This arrangement is sketched in Fig. 4A. A. Klingbeil Astabula, Ohio. STAFF COMMENT The improvement secured by Mr. Klingbeil is due to the tuning effect of the wave trap on the antenna circuit. In some cases similar results will be obtained with the very simple connection shown in B, Fig. 4. Additional Amplification for Phonograph Pick-Up I HAVE found that insufficient volume is ob * tained with the some phonograph pick-ups when using high grade low-ratio audio amplifying transformers and a loW-mu power tube such RADIO BROADCAST as the 171 type. The conventional phonograph pick-up arrangement plugs into the detector socket, inputting the output of the pick-up into the audio amplifier. The volume can be increased by connecting the pick-up to the grid of the detector tube. Additional amplification, due to the detector, will then be obtained. R. T. Anderson, Shreveport, Louisiana. STAFF COMMENT HP HE RE are several ways in which this can be * done, two convenient connections being shown in Fig. 5. Drawing A shows the more simple arrangement. The plug which ordinarily is placed in the detector socket, is disconnected from the flexible braid leading to the pick-up unit. One of the two wires in the lead is connected to the grid prong on the detector socket and the other to the negative terminal on the A battery (or the negative A binding post on the set, but not to negative A on the socket). The arrangement shown in sketch B is preferable because of the better impedance relationship between the pick-up and the input to the tube. Almost any amplifying transformer can be used at T. The pick-up wires are connected to the primary, and the secondary is wired to the detector tube — one lead to the grid prong and the other to the negative A post on the set. A simple switching arrangement can be devised by the thoughtful user to throw the receiver from radio to phonograph pick-up. To Stop That Whistling IN SOME cases a receiver of the tuned radio' frequency type will cause trouble by oscillating (whistling) so badly, especially on the lower JULY, 1928 Wave-trap 75 Bell wire 1 W diam 0.00025 /I mfd Ant. Gnd. -o O Receivel (B) FIG. 4 Unusual wave-trap circuits which may be found effective in increasing volume on distant stations (A> DET. TUBE FIG. 5 Phonograph pick-up circuits utilizing the detector tube as an additional amplifier wave bands, that it is impossible to obtain satisfactory reception of broadcast programs. In a set having the radio-frequency transformer coils mounted directly in back of and parallel to the variable condensers, the writer recently stopped such oscillations by a very simple expedient, i. e., by moving the three coils slightly closer to their respective condensers. It was really surprising how quickly the unwelcome oscillations were reduced and finally completely stopped. This result is caused by a slight additional loss introduced in the coils due to their closer proximity to the metal end-plates of the condenser. These losses are not large enough to make any appreciable difference in the operation of the receiver, and the owner should have no qualms in applying this method. If, for some reason, it is desired to make the receiver oscillate more easily, it is only necessary to move the coils further away. Jack L. Baker, Cooper, Texas. STAFF COMMENT IT IS not always a simple matter to affect the 1 mechanical rearrangement suggested by our contributor. The same results can be obtained with less labor by cutting a narrow strip from a sheet of tin, brass, or copper, and bending it so that it can be clipped around one of the radio frequency coils, preferably that one preceding the detector coil. This strip should not be more than one-half inch wide and should extend only about three-quarters of the way around the coil. The degree of stabilization can be varied by changing the size of the strip. The more metal in the strip the greater will be the stabilization attained in the circuit. Many methods of stabilization are in effect " losser" systems, as is this one.