Radio Broadcast (Nov 1926-Apr 1927)

MARCH, 1927 THE R.G.S. INVERSE-DUPLEX RECEIVER 481 sets the conventional arrangement somewhat, placing the second r.f. tube behind the detector. The two audio transformers are purposely widely separated to prevent annoying audio howls. The power audio tube is placed near this third audio transformer to give a short grid lead. This places the tube somewhat out of line. The r.f. filter coil and r.f. grid choke may be placed in line, as shown in Fig. 1., with no resulting trouble, because their magnetic fields are very small. All wires, except those connecting the apparatus with the grid circuits, and the leads to the socket grids themselves, should be run under the base panel in the shortest possible manner, irrespective of crisscrossing. By keeping all of the grid wires above the panel, sufficient separation is obtained to prevent oscillation and extraneous pick-ups. The bypass condensers, with the exception of those on the detector socket, are mountedon thetuningcoilframes. OPERATING INSTRUCTIONS TH E operating instructions are next to be considered. The R.G.S. receiver is very simple in this regard, considering its exceptional performance. Extreme selectivity and distance-getting ability have always been associated with delicate adjustments. Not so with the R.G.S. receiver. This is due to the exceptionally efficient long-wave amplification that the circuit produces. But the operator must follow certain suggestions if the ultimate is desired. No two circuits require the same tuning adjustments, and the new Inverse Duplex is no exception to this. It has its own simple operating peculiarities which are fully discussed here. In the first place, it is necessary to understand the correct operation of the antenna switch. The selectivity, distance, and tone quality will depend a great deal on the proper setting of this switch. No other adjustment is nearly so important. This switch connects with a series of taps on the antenna primary coil. The first tap connects with two turns only, the second with four turns, the third with eight turns, the fourth with sixteen turns, and the fifth tap with thirtytwo turns. As the taps are increased, it is as if the antenna were being lengthened and, as the lower taps are used, it is as if the antenna had been shortened. The antenna tap switch acts like an electrical reel by means of which the antenna can be electrically lengthened or shortened. It is this lengthening and shortening of the antenna that is so important in order to secure best reception under all conditions. If you understand just what effect a long or a short antenna has on receiving conditions, you will understand fully the effect of the antenna tap switch. A short antenna always makes a receiver more selective and at the same time brings in the short-wave stations with more volume. A long antenna broadens the tuning of a set somewhat, especially at the short waves, but brings in the long-wave stations with greater efficiency. Still another function of the antenna switch must be considered. W hen receiving some powerful local station, the radio energy can easily overload the detector tube. By cutting down the antenna somewhat (dropping to a lower tap), the radio energy can be reduced and the detector will then deliver an undistorted output. An important thing to remember in adjusting this antenna switch is that the correct tuning position of the variable condenser associated with this coil will change slightly whenever a change is made in the tap switch setting. As the length of the antenna is increased, the setting of the tuning condenserdrops below the settingof the double condenser, while shortening the antenna will bring the proper setting of the single condenser back to approximately the setting of the double condenser for any special station. To make this antenna switch operation clear, a few examples will be given. Assume that a 570-kc. (526-meter) dx station is to be tuned-in. 2nd. R.F Coil The next R F Filter Coil 5V£ft.of No. 36 D S C Nickle-Chrome per slot. FIG. 2 The coils and chokes for the R.G.S. receiver are wound in accordance with the specifications given here. Note that the secondary for T3 is wound with only 90 turns of wire while the other two tuning coils have a 92-turn secondary Under ordinary conditions tap 5 (32 turns) would give best results, and the single condenser dial might read 85 while the double condenser would read 92. But near-by local stations being heard in the background might make more selectivity a necessity. Hence tap 4 (16 turns) is used. Probably nothing would be heard upon dropping to tap 4, until a readjustment of the single tuning condenser is made. On tap 4, the single condenser would probably read 88. Upon shortening the antenna, the tuning position will be found to have risen slightly. Some locations demanding super selectivity might force the antenna setting down to 3, which is pretty low for a long-wave dx station around 600 kc. (500 meters). Distant stations between 1000 and 750 kc. (300 and 400 meters) usually come in best with the antenna switch on position 3, although, here again, selectivity may not be best until the setting is dropped to tap 2 and the single tuning condenser reset. On distant stations between 1500 and 1000 kc. (200 and 300 meters), either tap 2 or 1 will be best. In operating the set for the first time, tap 3 is suggested, as it is a very good compromise for all wavelengths. Even then, you may be required to drop this to a lower tap if a local station is choking the detector tube. important control is the volume regulator. This controls the audio energy at its source. By means of this volume control, the audio is kept within the limits of the tube so that quality may be had with good volume. If this adjustment is set too high, distortion will first take place in the ux-171 power amplifying tube. If it is boosted still higher, overloading of the resistance stage will occur, producing a choking noise in the loud speaker. Merely reducing the volume will restore the set to normal. In practice it is best to leave this control about a quarter or one-half the way on at most, unless more volume is desired on the weaker stations. When shifting from one station to another, always keep the volume control down. It has no effect on tuning so no tuning readjustment is necessary when operating this knob. The 2-ohm rheostat regulates the current in the filaments of all four tubes, and when turned to the "off" position, acts as the cut-off switch. By using a low resistance, formed of many turns of large wire, a smoothly operating control of the tubes is obtained. The total variation in the rheostat only changes the voltages on the tubes from four to six, while this two-volt change offers just the right amount of variation for the new ux-200-A detector tube. The rheostat has, therefore, practically no effect on the amplifying tubes, but mainly controls the detector tube for best sensitivity and freedom from noise. For normal performance, the rheostat should be turned slightly more than half way on, placing about 5 volts on the filaments. A schematic diagram of the R.G.S. receiver is given in Fig. 3, while Fig. 5 is a detailed wiring STUDY THIS PHOTOGRAPH IN CONJUNCTION WITH FIG. In this back-of-panel view the manner of running the under-base wiring through holes in the base is shown