Push-Button Station Selectors.—Push button station selectors is primarily an arrangement whereby the process of tuning has been greatly simplified. It is thus possible by means of a mechanical arrangement to choose a selected number of stations each one of which may be tuned in by the method of some control to a pre-determined position.
It is only recently however, that these systems have achieved the measure of popularity that it undoubtedly deserves, and this is probably because of the technical difficulties involved in producing a receiver which has the same capabilities as any ordinary set—the problem being not only to incorporate this additional device, but of maintaining it consistently in operation.
These early difficulties, however, have been largely overcome, primarily by the employment of apparatus of a higher standard of quality than was previously possible, and also due to a better understanding of the problems involved.
Various Systems in Use.—There are many push-button tuning systems in use as well as many different methods of control. Perhaps most common, however, is that of a series of push buttons (one for each station) located on the receiver itself, although sometimes these buttons may be duplicated, one set being mounted on the receiver, and the other at the end of an extension cable of suitable length.
Typical Extension Cable System,—A typical system of this kind is incorporated in the current line of General Electric receivers.
In this system remote tuning and volume control is accomplished by extending the push-button tuning circuits by means of ä cable to the remote control box.
Changes in the volume level are effected through the use of a motor on the volume control shaft as shown in fig. 5. A reversible motor is employed and controlled by two switches on the remote control box.
FIG. 5—Schematic wiring diagram of a General Electric push button remote control system.
H->7 tvr MOTOR
H->7 tvr MOTOR
FIG. 5—Schematic wiring diagram of a General Electric push button remote control system.
The station selector consists of the usual electric motor mechanism with a split stator winding. On account of the split stator winding arrangement, the device is homing, i.e., goes directly to the selected station.
The capacity of the remote tuning system makes thirteen stations available at the remote control box. The arrangement is such that when the button is depressed for any one of the thirteen stations, the power is automatically turned on to the set.
The remote control keys are non-latching in order to avoid any interference with the buttons on the receiver. At present only six of the stations have been extended for the remote control, which is attached to the set by means of a plug on the rear of the set.
To avoid the possibility of keeping the tuning motor running, by pressing two buttons simultaneously, single pole-double throw switches are utilized at both the receiver as well as at the remote control station.
The power to the volume control motor is supplied from the same transformer which supplies the tuning motor.
It is possible to change the volume of the receiver only after the station button at the remote control station has been released on account of the interlocking feature.
Finally a scan switch for rapid manual tuning from one of the bands to another is provided on the receiver. This switch is of the double throW type, normally open, which permits directive operation of the motor.
Again, instead of the usual push-button system a similar effect may be obtained by a mechanism similar to that of the well-known automatic telephone, and as a matter of fact it is perfectly possible to utilize standard telephone parts in the design of such a tuning control system.
Another remote control system in which the previously discussed control cable is being eliminated, and in which the tuning is accomplished by means of tuning pulses oscillations emanating from a dial, is described on page 346.
How the System Works.—Electrically these various systems divide themselves into two main classes, namely:
1. Those in which a large number of pre-set switch selected condensers are used.
2. Those in which an ordinary variable condenser is provided for tuning but can be remotely controlled by means of an electric motor.
Considering the former the basis for a tuned circuit is given in fig. 6.
It may be observed that instead of a variable condenser for^ tuning the coil L, a number of pre-set condensers are provided and the one desired can be selected by means of the switch shown.
It is obvious that each tuned circuit in the receiver must be provided with a similar bank of condensers and switches. With the system under discussion, the switch is set to the first position, and one station is tuned in on the opposite condensers; the switch is then set to the next position and another station is tuned in and so on.
For every station required, it is necessary to provide an extra condenser and switch contact for each tuned circuit.
This particular system has been commonly employed in the past in simple types of receivers. The system has a great merit especially where only two or three stations are required on account of its simplicity.
It is obvious, however, that if a dozen or more stations are required, it begins to be complicated by virtue of the large number of condensers required. There is also a further drawback when it is applied to a selective receiver such as a superheterodyne, and this drawback is that it may not prove stable enough for satisfactory operation.
Where the circuits are flatly tuned as in the case of the local station receiver, small changes in tuning capacities and the input capacities of tubes have very little effect upon the performance of the receiver, but where the set is selective, then these changes do command quite a large effect.
In a super-heterodyne the oscillator is the critical circuit, and it is common experience with ordinary receivers that the tuning drift somewhat, for perhaps a quarter of an hour or so after switching on.
Where systems of this kind are used, therefore, great care must be taken to maintain stability, and the oscillator circuit must itself be designed to this end.
In addition, the layout of components must be carefully chosen so that their temperature remains as nearly as possible constant and the condensers themselves often have to be of special types, with unusually high stability of capacity.
Motor Tuning.—In this type of remote control tuning systems, the use of a standard type receiver with a gang condenser is utilized.
For the purpose of control the tuning condensers are driven through a chain of geass from a small electric motor of the
reversible type. This motor usually operates from a 24 volt supply and the method of operation may readily be understood with reference to fig. 7 but there will be one disc for every station required and at the remote control there will be one push button for every disc.
It will be noted that of the two push buttons shown, A will be out while P is pushed in, so that the contacts of S2 are closed. The circuit is then completed through the ring D, and the motor, revolves turning the variable condenser and also the disc D.
When the insulated segment comes opposite the contact the circuit is broken and the motor stops. The receiver is then tuned to the desired station, for the initial set-up, the discs have been so aligned on the condenser shaft that the insulated segments in every case correspond to the condenser position for the wanted station.
This is a comparatively easy matter and it could for example be imagined that each disc is being held on by its own set screw to the shaft.
To set up any one disc for a particular station, one would tune in that station manually in the usual manner and then twist the disc so that the insulated segment comes opposite the contact and then tighten up the set screw.
It will be seen that upon pressing a button the condenser may start moving away from the desired station instead of towards it. When this happens the condenser goes on moving to minimum or maximum as the case may be, and then trips the automatic reversing switch S3 and comes back to the desired station.
With some of the latest systems this reversing switch is unnecessary, for means are included to insure that the motor always start off with the correct direction of rotation.
It is clear, however, that a system of the kind under discussion would by itself hardly be satisfactory since it would not be possible to guarantee sufficiently accurate tuning for a selective receiver. It is, therefore, that this system is almost invariably associated with an A.F.C. system which most usually takes the form as shown on page 334. Such A.F.C. circuits properly arranged, will give very good control and take out quite large changes in tuning of the medium and long-wave bands, but in general they are not directly applicable to short-wave reception although naturally they can be employed in a double super-heterodyne.
The disadvantage of A.F.C. is that it increases the cost of the receiver, because it increases the number of tubes, and the initial adjustments of the circuit involved is fairly critical. It is therefore generally only found in the more expensive types of receivers. In the less expensive sets it is less often included and a good performance is then secured by paying great attention to stability.
Mechanical Accessories.—It is not within the scope of this discussion to go deeply into mechanics of the actual control circuits because they vary so widely and generally do not effect the principles of operation.
The use of systems which may be known variably as pushbutton or dial tuning is not confined to remote control, and in some cases these controls are mounted instead of on the ordinary tuning dial, on the receiver itself.
They are then often very much simpler and one arrangement consists merely of mounting a telephone type of disc with the usual finger holes on the shaft of the gang condenser.
Again in another system the condenser shaft carries a number of heart-shaped discs, one for each station. One operating key is provided for each disc, and its pressure moves the cams around in the manner shown in fig. 8.
Still another system has a series of control bars mounted on the condenser shaft. One such bar with its actuating lever is shown in fig. 9.
The lever presses against the rounded portion of the bar and so rotates the condenser shaft, until it reaches the flat part.
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