From “How It Works”, Special Section of Vol. IX, Rider’s Manual,
pp. 23-29. Copyright 1938 by John F. Rider.
A number of different factors were responsible for the widespread adoption of tuning indicators of one type or another. Apart from the usefulness of these indicators in reducing distortion and noise background and in simplifying the tuning operation, receiver manufacturers realized that the incorporation of tuning indicators constituted an important item in increasing the attractiveness and salability of their receivers.
Let us investigate some of the early tuning indicators, to examine their construction, the circuit arrangements in which they were used, and their connection with the rest of the receiver circuit.
Relation to A.V.C. System
It was not until automatic volume control had made its appearance that tuning indicators became at all common in commercial receivers. This was no coincidence but occurred because receivers equipped with a.v.c. lent themselves readily to tuning indicator devices with a minimum of added expense. Let us examine the connection between the operation of tuning indicators and the a.v.c system and see the manner in which these are related. In the first place, if we review in a few words the action that takes place in a typical a.v.c. system, then we shall be able to see how an a.v.c.-equipped receiver is adapted for use with a number of different types of tuning indicators. We do not propose to go into any detailed description of a.v.c as this has already been done in “Automatic Volume Control” in the “Hour a Day with Rider” series. For our present purposes we wish merely to present some of the basic ideas as to the factors which change when an a.v.c.-equipped receiver is tuned to a signal.
As you know, a.v.c. operates by feeding a control voltage to one or more of the r-f., mixer, and i-f. tubes and the magnitude of this control voltage depends upon the amount of the signal which reaches the second detector. Generally this automatic control bias is produced by the rectified carrier in the second detector circuit or sometimes by a separate rectifier. All of these different a.v.c. circuits are discussed in the book previously mentioned, but the point which we wish to make here is that the a.v.c. voltage which is produced by the a.v.c. rectifier or by the second detector furnishes a convenient and obvious method of operating a tuning indicator. To understand why this is so, we shall examine the manner in which this a.v.c. control voltage varies as a signal is tuned in.
In Fig. 1 we show in skeleton form a schematic of a receiver incorporating a typical a.v.c. circuit. In accordance with the operation of a.v.c. systems, the rectified voltage across Rl is fed over to the grids of the tubes under control through the first a.v.c. filter resistor R2 and through individual grid filters. Now we want to investigate the changes which take place in this circuit as the receiver is tuned from a point at which no signal voltage is present through a point on the dial where a signal is present. To make the explanation more concrete, we are going to consider what happens when a receiver is tuned to an 800 kc. signal. Suppose that initially the receiver happens to be tuned somewhere in the neighborhood of 600 kc. and that we advance the tuning control to approach 800 kc.
To begin with, the a.v.c. voltage will of course change as the signal is being tuned in. Referring to Fig. 2, you will observe that the amount of control voltage produced is plotted against the frequency to which the receiver is tuned, assuming of course that the signal which is desired is 800 kc. Now when the tuning control reaches 790 kc., the control voltage developed by the a.v.c. rectifier is zero as is shown on the curve. The reason for this is that there is no signal passing through the receiver to develop a control voltage. As the tuning control is further advanced toward the signal frequency the a.v.c. voltage slowly increases from this zero value and at 800 kc. where the signal is tuned in perfectly, the control voltage developed reaches its maximum value. As the correct tuning setting is passed, the control voltage again drops and at 810 kc. the control voltage is substantially zero again.
What conclusions can we draw from the above variation of the control voltage? The most obvious conclusion as far as the problem on hand is concerned is that the variation in automatic control voltage can be utilized to indicate when the receiver is exactly tuned. That this can be done is evident from an inspection of the curve in Fig. 2, since reference to this curve shows us at once that the control voltage is a maximum when the station is tuned in perfectly and hence to insure accurate tuning it is only necessary to tune the receiver so that the control voltage is a maximum.
The connection between the status of the tuning of a receiver and the amount of control voltage developed can be considered as the basic fact which is behind the operation of every tuning indicator which we know of at the present time. It makes no difference whether the tuning indicator proper takes the form of a cathode-ray tube, a neon tube, a d-c. meter, the changing intensity of a bulb, or the change in the color of the dial light, or the movement of a shadow in every case you will find that it is the changing rectified voltage which is the moving force behind the operation.
The Vacuum-Tube Voltmeter Type of Tuning Indicator
One of the most direct and simple types of tuning indicators is that type which uses a simple vacuum-tube voltmeter as the indicating element. The connections for this arrangement are shown in Fig. 3. Note that the grid of the triode is connected to the a.v.c. bus, that is, to the terminal which feeds the a.v.c. voltage to the several tubes under control, and that as the result of this connection the grid voltage of the tube T will vary as the signal is tuned in. This variation takes place in accordance with the curve shown in Fig. 2, and as a result there is a corresponding variation in the plate current. When the signal is tuned in exactly, the negative grid bias on the indicator tube T is a maximum and consequently the plate current of the meter is a minimum. In other words, to tune a receiver with a meter of this type, it is only necessary to adjust the tuning control so that the meter deflection is a minimum.
There are numerous variations of this vacuum-tube voltmeter method. In some cases, the meter is inserted in the cathode circuit of the indicator tube rather than in the plate circuit. The operation of this circuit is of course essentially the same as the case in which the meter is in the plate circuit with the exception that, in the circuit of Fig. 4, the meter is at ground potential rather than at a high positive potential and furthermore, the resistance of the meter acts to supply bias for the tube due to the voltage drop across the meter. Other arrangements of this basic circuit include the use of triode and pentode tubes to produce a more uniform change in the meter reading over a wider range of input signals to the receiver. This facilitates tuning regardless of whether a strong or weak signal is being tuned in.
The R-F. and I-F. Plate Current Meter Indicator
A very widely used tuning indicator circuit is that which is shown in Fig. 5. Essentially this circuit is a further extension and simplification of the method just described, the simplification taking place because of the fact that one or more of the i-f. and r-f. tubes under control is used as the vacuum-tube voltmeter. In this way, the necessity for a separate tube to be used solely in connection with the tuning indicator is done away with. The circuit does not involve any new ideas, since the change in plate current of a tube under a.v.c. control is quite similar to the change in plate current of a tube which acts as a separate vacuum-tube voltmeter.
Referring back to the circuit shown in Fig. 1 you will appreciate that the voltage which is developed across the second detector changes widely as the receiver is tuned through a signal, the changes in grid voltage being of the order of 1 or 2 volts for weak signals, and as high as 20 or 30 volts for strong signals. This varying control voltage is of course that which is responsible for the a.v.c. action in the receiver and at the same time it is also responsible for the change in plate current of the tubes under a.v.c. control occurring when the receiver is tuned through a signal. In other words, it is not necessary to use a separate tube to actuate the meter as we did in the circuit of Fig. 3, but instead the meter can be inserted in the plate circuit of any one of the controlled tubes as is shown in Fig. 5. The deflection of the meter will then be similar to that described in connection with the vacuum tube voltmeter method and consequently tuning is accomplished by adjusting the tuning control so that the deflection of the meter pointer is a minimum.
Inasmuch as the operation of this arrangement is so similar to that of the vacuum-tube voltmeter method previously described, the question arises as to why the other method should be used when an extra tube can be eliminated by placing the meter in the plate circuit of one of the tubes under control. The answer to this question is that the second method is used much more widely than the separate tube method; the only advantage which is obtained by the use of a separate tube is that it is possible to arrange the vacuum-tube voltmeter so that a more uniform action is obtained regardless of the strength of the signal being received. However, where the meter is placed in the plate circuit of one of the controlled tubes, then the operation of the tube as a controlled amplifier is of first importance and the characteristic of the tube as a tuning indicator is of secondary importance.
It might be mentioned here that the meter is sometimes placed in the cathode circuit of the tube under control for reasons previously mentioned. In addition the meter may be arranged to carry the plate current of more than one controlled tube.
The Shadowgraph or Shadowmeter Tuning Indicator
Throughout this discussion as we have occasion to go into the various types of tuning indicators which have been and are being used in receivers, you will note that there has been a constant aim in the minds of the set engineers to design and produce tuning indicators which would have eye appeal and increase the salability of their product. If this were not so, there would be no reason for this text on tuning indicators, since basically the meter type of indicator would be quite as satisfactory as any other type available. This factor of sales appeal accounts in large part for the large number of novel arrangements which have made their appearance in radio receivers from year to year.
One of the devices which has been widely used is the so-called shadowgraph or shadowmeter type of indicator. The basic type of circuit in which this indicator is used is similar to that which we described in the case of ordinary current meters; shadowmeters are generally placed in the plate circuit of one or more of the tubes under a.v.c. control or in the plate circuit of a special indicator tube. The latter arrangement corresponds to the vacuum-tube voltmeter type of meter arrangement.
The actual construction of the shadowgraph indicator is of interest. The indicator mechanism employs a small permanent magnet which is in the form of a circular flat ring having a small air gap. Mounted within this ring so that it pivots on two supports diametrically opposite each other is the moving armature which forms the indicating part of the system. This armature consists of a flat disc of soft iron with a rectangular slit in the center of it. Mounted in the middle of this slit is a thin black opaque vane which is rigidly attached to the iron armature so that any movement of the armature is accompanied by a corresponding rotation of the vane. Surrounding the permanent magnet is a coil of wire placed so that the magnetic field of the coil of wire due to the current flowing through the coil is at right angles to the plane of the permanent magnet.
This description completes the electrical part of the tuning indicator. Let us now examine the manner in which the shadowgraph functions. The first important point is that the magnetic field of the permanent magnet tends to keep the armature in a horizontal plane. This is due to the fact that the magnet does not form a closed ring, so that the leakage flux acts to penetrate the soft iron of the armature and, by a well known property of magnetism, the armature will tend to assume that position which enables the maximum amount of the leakage flux of the permanent magnet to pass through it. In other words, the permanent magnet is essentially a control mechanism and really is an ingenious device which takes the place of the coil spring which is used in meters to return the pointer to zero.
The force which deflects the armature and therefore the vane, is due to the magnetic field created when a current flows through the coil L surrounding the armature. This field is at right angles to the field which tends to keep the armature in a horizontal position and therefore the effect of current flow through the coil L is to turn or rotate the armature and the attached vane. The greater the current which flows through L the greater is the magnetic field created by the current, and consequently the greater the angle through which the armature and vane are rotated.
As we previously mentioned the circuits in which these shadowmeters are used show a marked similarity to those employing a conventional type of meter indicator. The reason for the similarity of circuits is that the shadowmeter is itself essentially a meter but instead of using an ordinary pointer as an indicator it uses an optical system arranged so that the pointer is represented by the width of the shadow formed on the screen.
A typical circuit arrangement which employs the shadowmeter tuning indicator is that shown in Fig. 6 in which the current flowing through the winding is the plate current of both the r-f. and i-f. stages. You will note that the shadowmeter coil is shunted with a 2000 ohm resistance so that the full plate current of these tubes does not flow through the shadowmeter. In the event of a complete burnout of the shadowmeter coil, the receiver will still be operative, since the plate current has a path through the shunt resistor. In cases where the shadowmeter is not shunted, then the burnout of the shadowmeter coil results in a completely inoperative receiver since there is no plate voltage on one or more of the controlled tubes.
As far as the significance of the shadow width with respect to the tuning condition, this can easily be understood from the following consideration. In the first place, when the station is exactly tuned in, the tubes under control receive the maximum amount of automatic control voltage and consequently the current flowing through the shadow-meter coil is likewise a minimum. Under this condition, the deflection of the vane is a minimum and consequently the shadow cast on the screen has its minimum width when the station is exactly tuned in. Similarly, the plate current of the tubes under control is large when a signal is not tuned in and therefore the shadow width is broad.
There are a number of other circuit arrangements employing these shadow type tuning indicators. Among the arrangements which are quite popular is the type of circuit which uses a separate tube to actuate the shadowmeter. These circuits are similar to those shown in Figs. 3 and 4 with the exception that the plate current flows through the shadow-meter coil rather than through a conventional meter. No further comment is required since the action is the same as that described in connection with meter type indicators.
The circuit shown in Fig. 7 incorporates the same basic idea as the preceding circuits, with the exception that a long cutoff, variable-mu pentode tube is used rather than a triode tube. In some receiver layouts the use of the pentode tube furnishes a more desirable variation of plate current with tuning, so as to make the tuning meter effective for weak signals as well as for strong signals.
All the circuits which we have shown up to the present time have not had any separate tuned circuits associated with them, apart from the tuned circuits through which the signal passes. However, in some cases, there is an additional tuned circuit for the purpose of sharpening the selectivity of the tuning meter channel. In these cases it is of course important that this circuit be aligned at the same frequency as that at which the i-f. amplifier is peaked. If the tuning meter transformer is aligned at a frequency different from the i-f. peak, then it is quite possible that incorrect tuning will result in spite of the shadowmeter being adjusted for minimum width. These same remarks apply to all types of tuning indicator circuits which employ separate tuned circuits for this circuit.
An example of this type of arrangement is that shown in Fig. 3 which is the circuit used in the Philco Model 201 Receiver. The last i-f. transformer you will observe has three windings which perform the following functions. The primary winding Ll is closely coupled to the secondary winding L2 so as to provide a broad bandpass effect for the signal channel and to prevent the attenuation of the outer sidebands representing the higher audio frequencies. In this connection you may be familiar with the fact that the remaining i-f. transformers in this receiver are also of the three winding type, with a variable resistance in the tertiary tuned circuit to provide variable selectivity. The third winding L3 is loosely coupled to the primary winding so as to provide an increased selectivity for the tuning meter channel.
The i-f. voltage appearing across L3 is rectified by one of the diode sections of the 75 tube. (The remaining elements of this tube function as the second detector and the first a-f. stage.) The rectifier circuit is somewhat unusual and we might point out that Rl is the diode load across which the rectified voltage is developed. This voltage is filtered by means of the R2-C2 combination so that the voltage actuating the grid of the 37 tube does not contain any a-f components. The shadowmeter is placed in the plate circuit of this tube.
The incorporation of an additional tuned circuit for the tuning meter channel, as in this receiver, is especially desirable in the case of high fidelity receivers which have a flat-topped i-f. amplifier. In this case the additional selectivity makes the action of the tuning indicator much sharper and makes it possible to set the tuning condenser so that the signal comes through at exactly the center of the bandpass and not at a point near the edge.
Adjustment of Shadowgraph Indicators
In order to obtain a symmetrical shadow which is properly centered on the translucent screen, it is necessary to properly locate the filament of the bulb. Ordinary bulbs having a U type filament are not satisfactory as reference to the figure will show you that a bulb of the type having straight filament is desirable. Use of a bulb having a different or irregularly formed filament will result in a fuzzy and indistinct shadow. It is also important that the bulb itself be mounted in its socket so that the incandescent portion of the filament is in the same plane as the vane. In addition to the proper orientation of the bulb in the socket, it is necessary that the bracket holding the bulb be adjusted so that the filament is centrally located over the hole through which the light casting the shadow is admitted. Failure to have the bulb properly adjusted will result in the shadow being off center on the screen and possibly in a faint and indistinct shadow.
If it becomes necessary to readjust the shadow tuning meter due to any of the above reasons, the initial step should be to remove the rectifier tube and to turn on the power switch. The rectifier tube is removed because it is desirable to make the adjustments while there is no current flowing through the coil of the indicator so that the vane is in a central position. Under this condition, the shadow should be a rather sharply defined line in the center of the screen. If the shadow shows the need of adjustment, then the procedure is to adjust the position of the bulb bracket and the bulb itself as explained above.
Faulty Operation of Shadowmeter
Failure of the shadowmeter to operate may be due to a number of different causes. It is convenient to classify the method of procedure in accordance with whether or not the shadowmeter is shunted by a resistor, is not shunted, or is in the plate circuit of a separate tube.
If the shadowmeter is shunted by a resistor and the receiver is in operating condition, then there are several possibilities for failure of the shadow to change with changes in tuning. Among these are an open circuit in the shadowmeter coil and a jammed or sticky vane. If the receiver is not in an operative condition, then failure of the tuning meter to operate is not generally to be attributed to the tuning meter itself but rather to some defect in the r-f. and i-f. end of the receiver which is preventing the proper change in plate current with changes in tuning. Thus if the shadow broadens in the usual manner after the set has warmed up but does not narrow when a signal is passed, then you can conclude that there is no signal voltage reaching the second detector and consequently the first step is to restore the operation of the receiver rather than to look for trouble in the shadowmeter circuit.
If the shadowmeter is not shunted by a resistor, and the receiver is dead, then one of the first things you should check is the shadowmeter coil. The reason for the importance of this check is that an open circuit in the shadowmeter coil will make the receiver dead and at the same time result in lack of operation of the shadowmeter indicator. If the receiver is operative and the shadow remains fixed, then it is possible that the failure is due to the vane being jammed. Before removing the tuning indicator to examine the condition of the vane, it is desirable to insert a milliameter in series with the shadowmeter to make certain that the current passing through the shadowmeter is changing in accordance with changes in tuning. We might mention here that manufacturers generally do not recommend that shadowmeter indicators be taken apart for repairs because of the difficulties of repair and re-assembly. In the case of defective units, the entire assembly should be replaced.
In the case of receivers of the type shown in Fig. 8, failure of the tuning meter to function properly or at all may be due to lack of alignment of or a defect in the tuned circuit associated with the tuning meter channel. Thus in Fig. 8, an incorrect setting of the trimmer C3 would result in insufficient change of shadow width and if the circuit were badly out of alignment there would be no noticeable change in the shadow width with changes in tuning. The proper method for adjusting this trimmer is to feed a signal into the grid of the first detector at a frequency equal to the i-f. peak and to adjust the trimmer so that the shadow width is a minimum. This indicates that the shadow-meter tuned circuit is peaked at the intermediate frequency. Under this condition the signal passes through the i-f. amplifier at the i-f. peak when the tuning control is adjusted for minimum shadow width. In this connection lack of sharp variation in the shadow width is very often a sign that the receiver needs realignment. This remark applies with equal force regardless of the type of indicator and the circuit in which it is used.