Radio Broadcast (May 1928-Apr 1929)

OCTOBER, 1928 RADIO BROADCAST 369 Radio Broadcast's Service Data Sheets on Manufactured Receivers The Bosch Model 28 Receiver THIS a seven-tube re■ ceiver employing three stages of tuned radiofrequency amplification, a detector and a two-stage trans former-coupled alu d i o amplifier, the second stage of which is push-pull. Type 226 tubes are used in the r.f. and first a.f. stages, a type 227 tube in the detector stage and two 171A type tubes in the push-pull stage, and a type 280 rectifier in the B-power unit. The four tuning condensers are ganged to a single panel control — an illuminated drum dial. The additional controls on the front panel are a volume control and a " clarifier," which is actually a variometer with a pretty name. It tunes the antenna circuit to resonance. The volume control is a high-resistance potentiometer Ri. This receiver is designed for operation on 60-cycle a.c. lighting circuits at voltages from 100 to 130 volts, a tapped transformer primary serving to adjust the receiver for correct operation on any voltage between these two limits. A plug is used to connect to that particular tap on the transformer which gives best results. Moving the plug to the right (to the "115" or "125" socket) decreases the voltage on the tubes. It will, therefore, prolong the life of the radio tubes to operate the receiver with the switch plug in a position as far to the right as is consistent with satisfactory reception. For example, if the maximum line voltage measured is 115 volts, try the switch plug not only in the "115" socket, but also in the "125" socket as well. If the latter gives good results it will be of decided advantage to use this position, as it will increase the useful life of the tubes and reduce to a minimum the danger of burning out tubes. Do not, therefore, move the switch plug to the left in an effort to secure louder reception. Model 28 is housed in a table mounting cabinet Models 28A and 28B are console types. All models have self-contained power units. TECHNICAL DISCUSSION 1. Operation. The receiver is operated by three controls. The main tuning control serves to turn all the tuning condensers and tune in any desired station. The control is graduated into 100 divisions and after a station has been recorded, it can always be picked up at the same reading of the dial. Changes in the length of the antenna will not affect the station logging. The Clarifier is actually a variometer across Gnd October, 1928, 4. Volume Control. This control consists of a high resistance potentiometer, Ri, connected across the input to the first r.f. tube. In this position it functions to .regulate the signal energy entering the r.f. amplifier and therefore prevents tube overloading. 5. Filament Circuits. The filament leads to the various tubes are twisted pairs of wires connecting between the transformer and the various tube sockets. The type 226 r.f. tube filaments are supplied with approximately 1.4 volts, the 227 type detector with about 2.4 volts, the first a.f. tube — a type 226 — with 1.4 volts, and the power tubes with 5 volts. Resistances, R2, R3,and Ri, are connected across the various filament circuits for hum balance. CABINET MODEL 28 the antenna and ground and is an auxiliary tuning device which brings this circuit of the receiver to the point of exact resonance. After tuning in a station with the main dial, the Clarifier should be adjusted to the point of maximum volume. The volume control adjusts the volume to any desired level. 2. Tuning System. The r.f. amplifier consists essentially of three stages of tuned and neutralized amplification, with a variometer across the input circuit to permit the accurate tuning of this circuit to resonance. The circuits are all matched at the factory by means of the small condensers, Ci and C2, connected across the second and third tuning condensers. The sensitivity and selectivity of the set is improved distinctly by the use of the variometer in the input circuit. 3. Detector and Audio System. Grid leak and condenser detection is used and a type 227 tube is used as the detector. The audio stages are transformer coupled, the second stage being push-pull. The push-pull amplifier with 171A type tubes can deliver all the volume required for home use, without any distortion due to overloading. The detector is supplied with 40-50 volts, the first a.f. amplifier with about 100 volts plate potential and a C bias of 7 volts. The push-pull tube ; receive about 150 volts and a C bias of 35 volts. The push-pull tubes feed into the output transformer, Ti, the loud speaker being connected to its secondary terminals. 6. Plate Circuits. The plates of the r.f. tubes in this receiver are supplied with approximately 100 volts and the grids with 7 volts. The detector is supplied with the same voltage as the r.f. tube, but this voltage is reduced to about 50 by the resistance, R5, in series with the plate circuit. The first a.f. tube has 100 volts on the plate and 7 volts on the grid. The power tube receives about 130 volts and the grid bias is about 35 volts. The filament circuits of the r.f. tubes are bypassed to ground by O and O, each with a value of 0.5 mfd. The plate circuits are bypassed by the 1.0-mfd. condenser, C6; the detector bypass condenser, C, also has a capacity of 1.0 mfd. Re reduces the maximum voltage to 100. 7. Grid Circuits. Grid biases for the various tubes are obtained by connecting resistors between the center tap of the filament circuits and B minus, which corresponds to ground. Seven volts for the r.f. and first a.f. tubes is supplied by R? and 35 volts approximately for the grid of the power tube is obtained from Rs. There is zero bias on the grid of the detector. 8. Power Supply. The transformer, T2, in the power supply unit supplies low voltage to the filaments of all the tubes and the dial light and also high voltage to the rectifier system, which uses a type 280 tube in a fullwave system. A single section filter is used consisting of Co L and C7. The maximum output voltage from the filter is supplied to the output audio stage. The primary of the power transformer is tapped to pe'rmit the operation of the receiver on line voltage from 100 to 130 volts. CIRCUIT OF THE RECEIVER AND POWER SUPPLY r rwo"ooooo'ffooooodooooi)vWi T T 7 "~ no v.6o