Radio Broadcast (May 1929-Apr 1930)

No. 25 Radio Broadcast's Set Data Sheets June, 1929 THE SPARTON RECEIVERS The Sparton Equasonne Receivers are especially interesting for they use two devices— a band-pass selector in combination with an untuned radio-frequency amplifier — which are to be found in few other radio receivers. A Sparton Equasonne receiver contains three separate sections, a " selector unit," an "amplifier unit" and the "power converter." The selector unit picks out the signals from the station to which the user desires to listen, the amplifier unit amplifies and detects these signals, and the power converter amplifies the detected signal sufficiently so that satisfactory volume may be obtained from a ioud speaker connected to the output of the power converter. This briefly is the way this set works. It differs from ordinary tuned r.f. receivers in the several ways. In a tuned r.f. set the incoming signals are amplified by the r.f. amplifier tubes and the selecting is accomplished by the r.f. transformers connected between the successive r.f. amplifier tubes. The desired signal is, therefore, selected as it passes through the r.f. amplifying system. In the Sparton circuit all the selecting is done at one point and then, after the desired signal is completely separated from all the undesired signals, it passes to the r.f. amplifier unit to be amplified. The r.f. system in an ordinary tuned r.f. set will fail to amplify unless all the stages are tuned to the desired signal. In the Sparton set it is not necessary to tune the T.f. amplifier, for without adjustment it is capable of amplifying any signals (in the broadcast band) that may be impressed on its input. In the Sparton sets one simply tunes the selector to the desired signal and then the amplifier unit automatically does its work of intensifying the particular signal that has been selected. The last tube in the amplifier unit is the detector. It is of the platewletection type and is provided with sufficient plate and grid voitage so that it may supply, without overloading, 20 volts or more at audio frequency to the transformer in its plate circuit. The a.f. output from the detector passes into the primary of the audio transformer, and the secondary of this transformer feeds the grid circui t of the power tubes, which, in many of the models, are connected in push pull. This Sparton receiver, therefore, contains only one stage of audio-frequency amplification in contrast with the two stages ordinarily used in broadcast receivers. It is possible to use one stage instead of two because the r.f. amplifier unit has sufficient gain and the detector has sufficient load capacity so that a single transformer is all that is required to step up the a.f. voltage to a value sufficient to operate the power tubes at their maximum output. The plate circuits of the power tubes contain an output transformer and the secondary of this transformer feeds the moving coil of the dynamic loud speaker used in some of the models. The plate current of the tubes also passes through the field coil of the dynamic loud speaker and the 40 or 50 mA. obtained in this way is sufficient to excite the winding. The preceding paragraphs have described in a general way the operation of these excellent receivers. Let us now examine in more detail the operation of the selector and amplifier units. Before entering a detailed discussion of the selector unit, it is advisable to consider, in a general way, the intimate relation between selectivity and fidelity. The perfect radio receiver would be capable of tuning-in any station without interference from any other stations. This is the ideal condition at which set designers aim. Unfortunately, however, as the selectivity of an ordinary tuned r.f. receiver is increased the fidelity tends to decrease, because the selectivity of the r.f. amplifier tends to suppress partially certain essential parts of the receiver signal. When this occurs the fidelity of the output is effected adversely, being apparent by an absence of high frequencies. The problem Two new cabinet models used for housing Sparton receivers. has been to design an r.f. selecting system that will give adequate selectivity but which will not suppress any essential parts of the incoming signal. In the Sparton receivers this has been accomplished by the use of a "band-pass" filter. The term band-pass is applied to the selector unit because its characteristics are such that it passes a band of frequencies just wide enough to contain all the essential components of the audio signal. The band-pass selector in the Sparton sets is responsible in no small degree for the good quality which may be obtained from the set. The selector unit consists of four tuned circuits. If the connections to these tuned circuits were traced it would be noted that a small coil, is connected in series with two of the tuned circuits. Now it is a characteristic of two circuits each tuned to exactly the same frequency that, when they are coupled together by any means (as for example a small coil), the response curve of the two circuits together is quite different from either circuit alone. Either circuit alone would give an ordinary resonance curve — a sharply peaked curve that cuts sidebands resulting in the loss of some of the higher audio frequencies. Both circuits together, however, produce a curve with a flat top and very steep sides. The flat top effect prevents side DO YOU KNOW— ? What the important features of the new radio receivers are? See page 74. How you can increase profits by selling phonograph pick-up units? See page 77. That you will sell many more tubes during 1929 than you did during 1928? See page 85. What the trends are in radio cabinet design? See page 69. How to arrange most efficiently your service department? See page 87. The important fads about Sparton radio receivers? band cutting and the steep sides provide excellent selectivity. Such a characteristic — flat top and steep sides — results, however, only when the circuits are tuned accurately to the same frequency. With these circuits it is, therefore, quite important that the coils be matched carefully and the tuning condensers be ganged accurately. The entire selector section is contained in a single metal box and if any part of it becomes defective the unit may be removed from the set and replaced by another selector unit, the job of removing the defective unit and substituting a good unit taking not more than fifteen minutes. The amplifier unit which contains five r.f. amplifier tubes and a detector has an overall gain which is considerably more than that of many ordinary tuned r.f. receivers. The circuit of the amplifier is very unusual but no details regarding its operation are available at this time. The amplifier unit like the selector unit can, when defective, be removed from the set and replaced by another amplifier unit. The fact that any of the three sections of the set may be removed quickly and replaced by a good unit makes the servicing of the set a very simple matter. When a dealer gets a call to service a Sparton receiver, the serviceman sent on the job merely needs to determine which of the three units is defective. He then replaces it with a good unit and takes the defective unit back to t he store to repair it at the first opportunity. By means of a simple series of tests it is possible to determine quickly which unit is defective. For example, to detect a defect in the selector unit it is simply necessary to remove the antenna wire from its usual location and connect it instead to the connection between the selector and the amplifier units. With the antenna in this position, signals from all the local broadcasting stations will be heard in a jumble provided the amplifier and the power convertor are in good condition. When signals cannot be heard with the antenna connected to its usual position ahead of the selector the serviceman has a definite indication that the selector is at fault. He then proceeds to remove it and replace it with a good unit. In this way the customer is not deprived of the use of his set while the selector unit is being repaired. It is seldom that one finds in a single receiver as many unusual characteristics as are contained in the Sparton sets. In the preceding paragraphs several of these features have been described in detail. The following is a rather complete list of all the special characteristics of Sparton sets: (a) Sparton receivers contain a band-pass selector (b) An untuned r.f. amplifier (c) A power detector which gives much less distortion than is obtained from the usual type of weaksignal detectors. (d) A single stage of a.f. amplification rather than two stages as used in the conventional receivers (e) A phonograph pick-up jack so that the set may be used in conjunction with a phonograph pick-up unit to permit the electrical reproduction of phonograph records (f) A dynamic loud speaker supplied from tubes with sufficient output to insure highquality reproduction. (g) A tapped power transformer so that adjustment may be made to compensate differences in line voltage, an arrangement permitting the operation of all the tubes at maximum efficiency. • JUNE • 1929 •