Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

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724 <strong>Radio</strong> <strong>Broadcast</strong> .0005 OPTIONAL HI- FIG. The schematic circuit of the reflex receiver. The peculiar connection to the grid and plate circuits are especially to be noted. The constructional details are contained in the accompanying article. The radiofrequency transformer, shown in the diagram is completely illustrated in Fig. 3 I transformer should have a ratio of the order of 10:1 to work best with the crystal. A small fixed condenser, "3" should be placed across the secondary of this transformer to bypass the radio-frequency currents. Its capacity should be .00025 or .0005 mfd. The condenser "4" is fixed, and may be as large as .005 mfd., and is placed across the two outside terminals of the double circuit jack, as a further by-pass. The second audio transformer should be of about 4:1 ratio for good quality. The tuning unit used here is the result of considerable experimental work. It has a low distributed capacity and at the same time the three coils have a high degree of coupling. In designing this unit the following conditions had to be met. The circuit must be able to oscillate over the entire range of broadcast wavelengths, with a fixed winding radiofrequency transformer in the plate circuit of the tube. By means of the balancing condenser and part of the secondary coil, these oscillations must be balanced out. At the same time, a high degree of selectivity must be maintained. Thetuningcoilisofthefixedcouplertypewith an untuned primary and a tuned secondary. The primary has one tap to be used with a long antenna. The tuned secondary has a tap so placed that one part of the coil is included in the grid circuit, while the other part acts as a balancing-out or compensating When coil. used in conjunction with the balancing condenser "CB", all regeneration can be controlled. The coil winding data for the tuning unit is outlined below: Size of Tubing:--2" long x if" diameter. Coil No. i consists of 21 turns tapped at the i6th. Inside turn connects to grid i6th to LA and 21 st to SA. Coil No. 2, 35 turns, outside lead to C2, inside lead to A. Coil No. 5, 25 turns, outside to A, inside to Cl. All the coils are wound with No. 26 d. s. c. wire on a form slightly greater in diameter than the tubing. The form has 19 spokes f" in diameter producing a coil about iV wide. The winding style is over two and under two spokes. BALANCING CONDENSER A PHOTOGRAPH of the balancing condenser is shown. It will be noted that this condenser is designed to have an extremely small minimum capacity. Unless this minimum capacity is very small, the circuit will be of the sensitive only over the middle portion broadcast wavelengths. The small commercial condensers available were found to be unsuitable for this purpose, so that a special condenser had to be designed. Those who desire to make their own can do so with old parts on hand. The only conditions to be met are a low minimum capacity and a maximum capacity of about fifty micromicrofarads. "C" is a variable condenser whose capacity may be as low as .00029 mfd. and should be of fairly good design. This capacity is equivalent to 13 to 17 plates depending upon the make of the condenser. A FIXED crystal THE DETECTOR is used for the detector. Right here it may be advisable to point out a great weakness of the variable crystal detector. A given setting of the detector introduces a certain resistance into the circuit. If the circuit is tuned and adjusted to a certain wavelength and the variable detector is then
How to Build a One-Dial Reflex 725 INSIDE The " works." As can be seen, the wiring for this receiver is not difficult and the layout parts quite easy to duplicate reset, a different resistance will be introduced necessitating a retuning of the entire circuit. This is particularly annoying if the station that is being received is a distant one. This difficulty is entirely eliminated when a fixed type of crystal detector is used. THE RADIO-FREQUENCY TRANSFORMER D EFERRING to Fig. i once more it will 1^- be noted that the radio-frequency transformer used, is of unique design and is especially built to give maximum amplification over the broadcasting wavelengths when used between a tube and a crystal detector. It has a step down ratio and will not function between two tubes as is the case with the ordinary radio-frequency transformer. Its construction is as follows: On a spool if" in diameter and f" wide, having a slot \" wide and \" deep are wound 170 turns of No. 35 wire. First, 70 turns are wound in the slot and a tap taken off which connects to G, then too turns are wound over this, the end connecting to P. The beginning of the winding connects to F and B. See Figs, i & 3. WIRING THE SET FIGURE 2 is a picture drawing of the exact layout of the interior of the set and shows each piece of apparatus in relation to the other parts. It also clearly indicates the wiring. The panel size is 7 x 24" and is drilled according to the layout in Fig. 4. There are ten terminals at the back of the cabinet. Reading from left to right they are LA, SA, GND, A, +A, B, + B, + B, C+, C . These terminals should be mounted on a bakelite strip and so supported that the terminals do not come in contact with the wood. For the sake of simplicity, the wiring will be divided into four stages, namely, the filament wiring, the antenna-grid wiring, wiring and the audio-frequency wiring. the reflex REFLEX TUNER U N 11 PUSH-PULL AMPLIFIER UNIT FIG. 2 A picture layout of the complete receiver
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What Reflex Means How One Tube is M
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Page 29 and 30: The Listeners' Point of View 687 SI
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Page 49 and 50: Who Owns Our Broadcasting Stations
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Page 57 and 58: . "You Our Busiest Dav 7'5 Follette
Page 59 and 60: THE LOSS STABILIZING R. F. CIRCUITS
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Page 63 and 64: In the R. B. Lab. 721 This resistan
Page 65: How to Build a One-Dial Reflex Rece
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Page 73 and 74: THE ANTENNA AT STATION 5IT, BIRMING
Page 75 and 76: What We Are Doing With Broadcasting
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Page 79 and 80: A Winder for Small Inductances How
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Page 85 and 86: Problems of Receiver Design 743 sho
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Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

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