Radio broadcast .. (1922-30)

FEBRUARY, 1927 THE SHIELDED DUAL-CONTROL RECEIVER 387 ASSEMBLING THE SHIELD TOPS THERE is a slight trick to fitting the s r shield tops down over the tubes, coils. and variable condensers, and getting the edges to fall well inside of the edges of the shield pans. One precaution must be observed, and that is to leave at least a ^5 to ff-inch space between the front lock collar of the variable condensers and the lock collars of the link motion, in which space the front edge of the shield body must fit. If a shield is taken in the right hand and dropped down into its proper pan, this should be done with the front end inclined downward. Thus, the front end of the shield top can very easily be fitted inside the front edge of the shield pan and pushed down, while the rear end of the shield top stands above the edge of the pan. If the hand is then transferred to the back side of the shield and pushed forward slightly toward the panel, it will be found that this rear side may be slipped down very easily inside the up-standing edge of the shield pan. SELECTIVITY AND QUALITY IN THE "Shielded Six" every possible en- deavor has been made to obtain as perfect quality of reproduction as could be gotten. It is the writer's belief that the design has not FIG. 3 / How to loosen the lock collars on the variable condensers so that the shaft may be re-positioned or the compression on the small spring washer tightened up. Pressure is exerted by the thumb and fingers of the left hand on the rear end of the shaft and on the lock collar respectively radio frequency amplifier tubes. It is therefore sometimes advisable to connect a condenser made of, let's say, two two- foot lengths of ordinary insulated magnet wire tightly twisted together, between the shield and terminal No. 3 of the detector coil socket in the detector stage compart- ment. Of course, one end of each wire should be left free and disconnected in order that the capacity formed by the two adjacent wires may not be short- circuited. THE ANTENNA THE use of an antenna much over 70 feet is not recommended with the "Shielded Six." It has been found in Chicago that a ten-foot antenna would give ample volume on local stations, and will allow the reception of some out-of-town stations. A thirty-foot antenna will give very satisfactory results and will fre- quently bring in West Coast stations with fair loud speaker volume. A seventy-foot antenna will give all that can be desired in the way of re- sults. In practically all cases the antenna switch should be kept turned to the "short" position, CONDENSER ADJUSTMENT THERE is one factor which will affect the selectivity, and, for that matter, the operation of the entire receiver—the condenser, ganging and adjustment. As the condensers recommended for the receiver leave the factory, they have all passed mechanical and electrical inspections which insure that, if re- ceived by the builder undamaged, their uniformity is such that they will operate in any gang control circuit quite satisfactorily. The possibility of their being slightly strained in transit or in assem- bly is|not remote, although every endeavor was put forth in their design to produce a rigid and sub- stantial mechanical assembly. Upon receipt, the condensers should therefore be carefully examined before being placed in the receiver. The link motion should be slipped over the shafts of the three condensers as shown in the photo- FIG. 2 This is the circuit diagram of the "Shielded Dual-Control Receiver" as described in the October, 1926, RADIO BROADCAST. The shielding is indicated by the dotted lines. The volume-control potentiometer may be seen to the left of the diagram between the first and second tubes. The second, third, and fourth condensers are controlled by one knob failed in this matter for this belief has been borne out by the enthusiastic comments of builders of the receiver. Nevertheless, in any radio receiver, a certain amount of extreme selectivity must be sacrificed if thoroughly good quality of reproduction is to be obtained. This is because the radio frequency circuits are es- sentially band selection filters designed to accept and pass a band of frequencies such as would be required for proper transmission of voice and music, and to reject all other frequencies falling outside of the particular band tuned to at the moment. Obviously, if the receiver is designed to be very selective, the band will be so narrow that the higher frequencies in music will be cut off: while, if the band is too broad, several stations operating at different frequencies will be re- ceived. A very considerable amount of time and energy was devoted to this single considera- tion in the design work, and it is believed that the "Shielded Six," giving selectivity sufficient for the most congested broadcasting centers of the country, should be entirely satisfactory to the average builder irrespective of the locality in which it is used. which represents loosest coupling between the antenna and receiver, since in this position of the switch, selectivity will be greatest and re- ception most free from static or other atmos- pheric noises. The "long" position of the switch should only be used when few local stations are operating or with a very short antenna, say, one from ten to thirty feet long. The "Shielded Six" may be used with an indoor antenna of from ten to fifty feet in length, con- sisting of a single wire run around the picture moulding or otherwise disposed of to suit the owner. A water-pipe ground may be used, and if it is desired to dispense with an antenna altogether, another piping system other than that used for the ground connection may be used for the antenna. Thus gas and water-pipe systems, or water and steam-pipe systems, would serve nicely for both antenna and ground. An electric light socket antenna plug may be used. BOOSTING DETECTOR CAPACITY DUE to the fact that there is no radio fre- quency load in the plate circuit of the detector, the effective grid to filament capacity is considerably lower than that of the three graph on page 495 of the October issue, after which they should be adjusted as follows: The rotor plates of all condensers should be inter- ! FIG. 4 Pushing the shaft into a new position. Once the shaft has been located as desired, the whole assembly is re-locked by means of the two set screws visible in the rotor shaft collars. Need- less to say, if this adjustment is made according to instructions in this article, no change in the capacity characteristics of the condensers will