Radio broadcast .. (1922-30)

Notes on the Shielded Dual-Control Some Trouble-Shooting Hints on the Receiver Described in the October Issue —Hou; to Distin- guish Between the Two Types of Coils Employed —Where to Expect Trouble in the Volume-Con- trol Resistance—What Kind of Antenna to Use By McMURDO SILVER GETTING THE SIX READY FOR OPERATION In conjunction with a Balkite trickle charger, a storage battery, a line power-supply device, and a Yaxley automatic switch. "Zero left" dials were specified in the original article, October, 1926, so as to have the dial readings increase with increasing frequency. If the builder prefers to tune by wavelength and have the long wavelength stations tune-in on the upper instead or the lower part of the dial, counter clockwise dials should be procured THE data presented herewith are intended primarily for those readers of RADIO BROADCAST who have built the receiver described in the October, 1926, issue, under the title of "A Shielded Dual-Control Receiver." This present article deals particularly with methods for insuring that the best operation of the outfit is obtained, and, in a measure, it might very well be termed a symposium, for it presents in condensed form the information and suggestions gleaned from several thou- sands of builders who have already con- structed the "Shielded Six." While some of these home constructors have com- mented unfavorably on the receiver, the majority has been very satisfied with the results obtained. The author has one of these receivers, built from standard store parts, in opera- tion in his home in Chicago. This set is situated about two miles from KYW, one and one-half miles from WMAQ, three miles from WEBH and WQJ, a thousand yards from the Moody Bible Institute station WMBI, and, in addition, in the center of the Chicago hotbed of some twenty stations. So situated, it is possible to tune-in Schenectady, Pittsburgh, De- troit, Davenport, Fort Worth, Cincinnati, Denver, and many other stations on al- most any evening. Station KFI, Los Angeles, has been heard with a ten-foot wire as an antenna. One of the first points to come up in con- nection with the building of the receiver was the fact that the four inductance coils, or radio frequency transformers, look very much alike, and once they have been removed from their cartons carrying the different type markings, many builders have found it impossible to tell them apart. This is unfortunately a serious condition, for if the coils are improperly inserted in their sockets, that is, if the an- tenna coil is placed in one of the radio frequency stages, there will be a short- circuit which will burn out one of the windings on the antenna coil. It is very im- portant that the njA coils be placed in the three right- hand coil sockets of the receiver, as viewed from the front. The i i6A coil should be placed in the left-hand (first r. f. stage) or antenna compart- ment of the receiver. Reference to Fig. I will explain how to tell the ujA from the i i6A. THE VOLUME-CONTROL RESISTANCE IT IS very important that the volume-control resistance, or 25,ooo-ohm Hi-Pot, be exactly as specified—a Carter 25,ooo-ohm No. 25. It FIG. The difference between the 115 A coil at the right and the type n6A coil at the left may easily be told. The differentiating characteristic is the slot at the bottom of the winding form through which light is visible in the left-hand coil, whereas, in the right-hand coil, the slot is completely filled with the wire turns. These coils must not be interchanged in the receiver, for the 116 A coil, if placed in the wrong socket, will cause damage.- At the present time, long-wave coils, to adapt the "Shielded Six" for reception between 500 and 3000 meters (600 and loo kc.), are not obtainable, but the writer will furnish data for such coils to anybody writing for it. The set will not function well below 200 meters (1500 kc.) should be carefully tested with headphones and battery, and clicks should be obtained on all con- tacts. It will be found that a comparatively weak click will be gotten between terminals Nos. i and 3, whereas a strong click will be obtained between terminals Nos. i and 2, if the contact arm is turned around close to terminal No. i. As the contact arm is turned away from No. i, and toward No. 3, the strength of the click will decrease. This is correct, and the same condi- tion will apply between terminals Nos. 2 and 3. Should a 5OO,ooo-ohm standard potentiometer be used in the receiver instead of the 25,ooo-ohm one specified, trouble is bound to develop. The receiver will not be sensitive for distant stations, will be very unstable and prone to oscillate con- tinuously. Every Carter No. 25 Hi-Pot is supplied with two insulating washers having small projections which fit inside the actual panel and chassis holes intended to receive the shaft bushing of this resistance. One washer should be placed on the inside of the chassis and one on the out- side of the panel. If this is done, there is no danger of a short-circuit between the chassis and volume-control resistance, providing also that the three arms of the Hi-Pot carry- ing the binding posts are bent up and away from the chassis when the resistance is in place in the receiver assembly. In other words, none of the metal parts of the Hi-Pot should touch the chassis er the front panel. THE 2OO-OHM BALANCING RESISTANCES THE 2oo-ohm balancing resistances, connected between terminals No. 3 of the radio frequency coil sockets and the grid posts of the radio frequency tube sockets, need not be exact in their resis- tance values, and may vary at least five per cent, either way. These resistances should be tested before the receiver is put in operation with head phones and bat- tery, a click being obtained between the mounting clips when they are touched with the battery lead and phone cord tip. Either Carter or Yaxley resistances can be used in this position, but it is impor- tant that they be wire-wound resistances and not graphite, carbon, or deposit types of resistances, for the inductive effect provided by the wire-wound resis- tances assists in the operation of the receiver.