TRANSMITTER TEST PATTERNS FOR RECEIVER ADJUSTMENT
For a period of from fifteen to thirty minutes preceding each television broadcast, the station transmits a test pattern or chart which usually carries the stations call letters or a distinguishing insignia. This same test pattern is frequently broadcast for longer periods of time when no regular television pro gram is scheduled. The purpose of the test pattern is to provide the television service technician with a useful "tool" to assure pro per adjustment of the various receiver controls which effect a correct presentation of the television picture. Another purpose of the test pattern is to give the user an opportunity to adjust the front panel controls for the best picture before the regular program starts.
Fig. 111. Normal Transmitted Test Pattern
Fig. 112. R. M. A. Standard Transmitter Test Chart. (Courtesy R. M. A. Data Bureau)
Such controls as horizontal and vertical hold, horizontal and vertical centering, linearity in both directions, and sharp focus can all be precisely adjusted by the use of the test pattern.
In this section we will present a review of the use of those controls which have been discussed up to this point and show their effect upon the appearance of a transmitted pattern.
Figure 111 shows the appearance of a typical transmitted test pattern as received on a correctly-adjusted modern television receiver. The various characteristics of the pattern are similar in application to those of a more complex test chart which has been developed as a standard by the television transmitter committee of the Radio Manufacturers Association engineering department. This R. M. A. standard resolution ch art is used in testing the performance of both television transmitters and receivers and a study of its features will serve to explain the use of less complicated television broadcast station test patterns such as Figure 111.
Figure 112 is a reproduction of the R. M. A. television resolution chart, with the addition of explanatory letter symbols for some of its salient features. The chart will be seen to consist of a series of geometric forms and a number of tones ranging from black through a series of gray steps to white. The gray scales are of value in determining whether all elements of the television system are preserving the correct ratios of light intensities (as video modulation) to accurately reproduce the televised scene at the receiver picture tube.
The "fan shaped" wedges in both horizontal and vertical directions are composed of lines whose width gradually decreases as the lines approach the center. By observing the point at which the lines are no longer distinguished from one another, an estimate of the "resolving power" of the television system including the receiver under test, can be determined. The standard test chart shows by numbers placed beside the horizontal and vertical wedges, the corresponding numbers of lines which are being reproduced when the individual lines of the fan are just distinguish-able from one another. It should be noted that the vertical fans of the test picture are used to determine the performance of the horizontal system of the receiver and conversely the horizontal fans of the test pattern are used to determine vertical receiver performance.
Tests for vertical and horizontal linearity, as well as other requirements, such as interlace, are also provided by this chart and are described in the captions accompanying the chart. Their application in the simplified type of pattern represented by the station trans mission of Figure 111 will be discussed in greater detail, as we consider the effect on the reproduced pattern caused by maladjustment of controls or malfunctioning of the various circuits.
Fig. 113. "Operating" or "Front Panel" Controls
Fig. 114. "Non-Operating" or Pre-Set Controls (Not on Front Panel)
CONTROLS--THEIR USES AND ADJUSTMENT In our study of the operation of the circuits employed for the production of the scanning raster, the control of this scanning by signal pulses and the modulation and focusing of the cathode-ray beam, we have covered the action of a number of variable adjustments known as controls. Controls can be generally classified in two distinct groups:
1. "Operating" or Front Panel Controls.
These controls which are located on the front panel of the receiver are operated by the user.
They normally include the sound level volume control with an associated switch to turn the receiver on and off, a station selector to allow the set to be tuned to the desired tele vision broadcasting station and a group of controls to adjust the appearance of the picture.
The picture control group permits the user to adjust the brilliance, contrast, clarity and stability of the image.
2. "Non-Operating" or Pre-Set Service Controls. The circuit controls which require adjustment only during original installation or at infrequent intervals are located in such a position that they are not readily accessible to the user. Most television manufacturers, realizing that the proper adjustment of these "service type" controls determines the satisfactory operation of the receiver, insist that these controls be accessible only to well qualified, authorized television service technicians.
The number and complexity of these pre -set or semi-fixed adjustments differ greatly between the designs of individual manufacturers.
The Radio Manufacturers Association through its engineering standards group are attempting to standardize the names of controls and their functions. Due, however, to the accelerated pace of receiver production in its initial stage and the enthusiasm of sales and advertising departments, controls have been called by a variety of names.
An analysis of the receivers of leading manufacturers, including electrostatic deflection, electromagnetic deflection and projection types, reveals the fact that as yet, no design pattern has evolved to determine which controls should be placed on the front panel or relegated to a position within the receiver.
PLACEMENT OF ADJUSTABLE CONTROLS: A representative group of television receivers which from our analysis covers all of the design variations, has been studied to determine the placement of controls. The results of this study are presented in the form of two charts shown as Figures 113 and 114.
In these charts, the control function is first described by the name which has been suggested for radio industry standardization and the other names following it are those which are still in use in the service literature of many companies.
In the chart of Figure 113, the operating or front panel controls are classified in order of frequency of use in modern receiver design.
It will be seen that all television sets employ at least three of the front panel controls in common with one another. These are: station selector, volume, and contrast. In addition to these three basic operating controls, others are employed in the frequency shown in the chart. The number of front panel controls varies from a minimum of three to a maximum of eight. Five percent of the receivers had three front panel controls, ten percent had five, forty percent used six, another forty percent employed seven and the remaining five percent had eight controls.
Figure 115 shows typical examples of the layout of receivers employing from five to eight front panel controls. The functions of the various controls are "called-out" on the pictures. It will be noted that concentric control knobs are employed in certain designs for reasons of styling and simplicity of operation.
The functions of the station selector, volume, and contrast controls are properly a part of the study of RF and Video amplifiers and will be covered later in the course. The other controls listed in the chart of Figure 113 will be discussed in common with the "non operating'' or pre-set controls analyses in chart shown as Figure 114.
The controls to be adjusted by the service technician and listed in Figure 114 are much more varied in number and diversity than the front panel group. In the receivers analyzed, the range of usage of these types varied from a minimum of five in a table mod el to a maximum of fifteen in a large projection receiver.
The pre-set controls are mounted on any surface or plane of the chassis as determined by the particular mechanical design. In general, the controls which will most likely require occasional readjustment are located in such a position that they will be accessible without removing the ch ass is from the cabinet. In some designs certain of these controls are made available by knob or screw driver slot through the back panel and do not require back panel removal. Figure 116 shows a variety of designs indicating the location of many of the controls listed in the chart of Figure 114.
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Receiver example employing six front panel controls. (Photo from a sample receiver courtesy Sears Roebuck & Co.) Receiver example employing eight front panel controls. ( Photo from a sample receiver courtesy General Electric Company.)
Receiver example employing five front panel controls. (Photo from a sample receiver courtesy the Philco Corporation.)
Receiver example employing seven front panel controls. (Photo from a sample receiver courtesy R. C. A.)
Fig. 115. Typical Examples of Operating or Front Panel Controls focus
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An example of service controls available without removal of the back panel. (Photo from sample receiver courtesy the Hallicrafters Co.)
An example of service controls at the back of the chassis with top chassis and tube controls available by removing protective screen. ( Photo from sample receiver courtesy R. C. A. Mfg. Co.)
An example of service controls avail able at back of chassis. Back panel removable for servicing picture tube.
(Photo from sample receiver courtesy Emerson Radio and Phonograph Corp.)
An example of service controls avail able on top, back and side of chassis.
(Photo from sample courtesy the Motorola Co.)
Fig. 116. Typical Examples of Non Operating or Pre-Set Controls
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The controls of a television receiver may be classified into four main groups:
1. Those which adjust the operating conditions of the cathode-ray picture tube.
a. Adjustment of ion trap position and current to return the beam to screen.
b. Adjustment of the deflection yoke to position scanning raster correctly.
c. Focus to obtain sharp definition.
d. Adjustment of C-R tube operating voltages to establish proper ''black level" and highlight brightness.
e. Control of scene brightness.
2. Those which establish the correct "lock in" or hold of the horizontal and vertical scanning oscillators.
a. Horizontal hold which sets the free running frequency of the horizontal scanning oscillator.
b. Vertical hold which sets the free running frequency of the vertical scanning oscillator.
c. Horizontal sine wave oscillator frequency adjustment, in A.F.C. systems.
d. Horizontal discriminator phase control to establish discriminator balance.
3. Those which adjust the dimensions and position of the picture.
a. Width control adjusts horizontal size.
b. Height control adjusts vertical size.
c. Horizontal centering which moves the entire picture in the horizontal plane.
d. Vertical centering which moves the entire picture in the vertical plane.
4. Those which determine the shape of the scanning voltage waves to produce an un distorted picture.
a. Horizontal linearity controls the shape of the horizontal scanning wave.
b. Horizontal drive determines the ratio of pulse to linear sawtooth for the voltage wave in magnetic deflection.
c. Vertical linearity controls the shape of the voltage wave from the vertical scanning oscillator.
The e ff e ct of mis adjustment of these controls will be shown in the order outlined.
Fig. 117. Focus Coil and Ion Trap Misaligned
FOCUS COIL AND ION TRAP ADJUSTMENT: Figure 117 shows the received test pattern when the focus coil (Figure 17) and the ion trap (Figures 20-21) are not in correct position on the neck of the picture tube. The ion trap rear magnet poles should be positioned so that they are approximately over the little "flags" which are attached to the ion trap cylinder.
The picture tube must be mounted in such a position that these ion trap flags are in a horizontal plane when looking down upon the tube. When this has been done, and the tube secured in position, the ion trap can be moved slightly back and forth along the tube neck and at the same time rotated slightly around the tube, until the brightest raster is obtained on the screen. The trap adjustment screws should then be tightened sufficiently to hold the trap in position but still allow further adjustment.
The focus control setting (Figure 73), the focus coil position, and the ion trap magnet position are interdependent, and in the original installation procedure an adjustment of one may require readjustment of the others. The shadowed corner, as well as the poor vertical positioning shown in Figure 117, indicates that the electron beam is striking the neck of the tube. To correct this condition the focus coil should be adjusted in its mounting until the picture is properly centered.
If no raster can be obtained on the picture tube screen it is an indication of improper mounting of the ion trap magnet assembly. An inverted mounting from top to bottom or from front to back can cause such a condition.
Fig. 118. Deflection Yoke not Properly Adjusted. (Rotated); Fig. 119. Focus Control
Misadjusted
DEFLECTION YOKE ADJUSTMENT: Figure 118 shows the effect on the test pattern of improper mounting of the deflection yoke assembly. The action of the deflection coils in moving the electron beam was covered and illustrated in Figures 18, 18 and 22.
If the lines of the raster are not horizontal and squared with the edge of the picture mask, it is an indication that the deflection yoke, made up of both the horizontal and the vertical deflection coils, is not correctly positioned. To correct this condition the adjustment screws which hold the yoke should be loosened, and the coil assembly rotated about the axis of the tube until the raster is properly lined up with respect to the edges of the picture mask. The yoke adjustment screws or wing nuts should then be securely tightened.
The position of the deflection yoke along the picture tube neck will affect the deflection sensitivity (amount of scanning voltage for a given deflection).
FOCUS CONTROL AND FOCUSING ADJUSTMENTS: Figure 119 illustrates the appearance of the received test pattern when the electron beam is "out of focus". The image is not sharply defined as in the normal picture of Figure 111.
The theory of focusing the electron beam in electrostatic ally controlled picture tubes has been covered on pages 5 through 8 and the circuit elements were illustrated in Figures 8 and 11. The variable control takes the form of a carbon type of voltage divider.
Electromagnetically focused picture tubes require two separate service adjustments. The first is the mechanical positioning of the focus coil as covered under "Focus Coil and Ion Trap Adjustments", and the second is the variation of the current through the focus coil by means of a variable resistor. The act ion of the focus coil has been covered. (Figures 17, 22 and 73.) The best focus adjustment is made by sliding the focus coil back and forth along the neck of the picture tube while adjusting the focus control and watching the test pattern for the sharpest picture. In some designs two variable controls (coarse and vernier) are used.
BRIGHTNESS CONTROL: As indicated in the chart of Figure 113, the brightness control is usually made one of the front panel group and is a "user" operated control. It is employed in conjunction with the contrast control to obtain the best possible picture quality. It is possible to cut off the beam with this control, in which case no picture is seen and the tube remains dark. Conversely, too high a setting of the contrast control will result in a light, "washed out" picture as shown in Figure 120. In this case the shadows and lower key half tones have disappeared and the vertical retrace lines have become visible.
The circuit function of the brightness control is to establish the operating control grid bias of the picture tube. Two methods of accomplishing this bias control have been described on pages 107 and 108. Figures 110B and ll0C show the circuit arrangements employed.
Fig. 120. Brightness Control Misadjusted.
(Brightness too High)
Fig. 122. Horizontal Hold Control Misadjusted
Fig. 121. Horizontal Hold Control Misadjusted
Fig. 123. Vertical Hold Control Misadjusted
HOLD ADJUSTMENTS: Hold adjustments, both horizontal and vertical, enable the television technician, or user, to adjust the free running frequency of the two receiver scanning systems so that a stable condition of synchronism or lock-in with the transmitted sync pulses is obtained.
In "flywheel" or A. F. C. sync systems, the hold control is of the same type, but it is placed in the grid circuit of a sine wave oscillator. Its function is to control the phase of the oscillator with respect to the sync pulses.
In the case of "triggered" sync systems these controls are variable resistors in the scanning oscillator circuit.
HORIZONTAL HOLD ADJUSTMENT. Figures 121 and 122 illustrate the effect on the receiver test pattern of two degrees of misadjustment of the horizontal hold control. The actual appearance of the image cannot be reproduced in a printed illustration since the image is in continual motion until a stable lock-in has occurred. When the hold control is adjusted to such a position that the oscillator is nearly in sync with the signal pulses, the image will first appear as a series of diagonal bars similar to those of Figure 124, which is an illustration of horizont al oscillator frequency misadjustment. As sync hr on is m is approached more closely, the image will appear as in Figure 121 and then start to lock in as in Figure 122.
VERTICAL HOLD ADJUSTMENT. Figure 123 shows the effect of misadjustment of the vertical hold control. The effects on the picture are similar to those discussed for "Horizontal Hold" except that, in this case, the motion of the image, before lock-in occurs, is from the bottom to the top of the picture rather than from left to right.
Careful adjustment of vertical hold is essential to avoid "pairing" of horizontal lines of alternate fields which would reduce the vertical definition of the picture.
HORIZONTAL OSCILLATOR FREQUENCY ADJUSTMENT (A. F. C. SYSTEMS). In the flywheel or A. F. C. system of horizontal sync control, described on pages 101 through 108, the major control of the free running frequency of sine wave oscillation is seen to be the tuning of the circuit by adjusting an iron core in the inductance (see Figure 105). With reference to this figure, it will be noted that the horizontal hold (R7) and the discriminator phase control (adjustment of primary L1-L2) also exert an effect on the oscillator frequency. For this reason, the adjustment of the horizontal oscillator frequency must be re-checked if it is found necessary to change the discriminator phase control. The hold control should be set at the middle of its range while making these adjustments.
The service manuals of the television receiver manufacturers contain explicit instructions concerning the order in which these adjustments are to be made in the particular A. F. C. circuit design.
The final setting of this control should be such that, with the hold control at either end of its range, the scanning system will lock-in to signal sync. To test for this condition, tune to a signal while the control is in its mid position. The hold control is then turned to its extreme position in either direction. Next the signal is removed by detuning the receiver.
Upon retuning the system should pull into sync.
The same check should then be made at the other end of the control range. If the receiver will not pull into sync at both ends of the hold range, the horizontal oscillator frequency.
should be readjusted until this is accomplished.
HORIZONTAL DISCRIMINATOR PHASE ADJUSTMENT. As described on pages 101 108, the discriminator stage compares the sync pulse rate with the horizontal oscillator rate or frequency and delivers a DC voltage for the control of the oscillator. If the voltage at the plates of the two diodes, impressed by the oscillator, is equally divided, the DC output will not be zero at the correct time for retrace at the end of the horizontal line. This off-balance condition will result in the received test pattern shown in Figure 125.
In this case, the picture is locked-in and steady but retrace has occurred at the wrong time in the scanning cycle. The black band at the right side of the picture is the blanking period during which horizontal retrace should have occurred.
The adjustment of this control, for re trace at the proper instant, also affects the setting of the horizontal oscillator frequency adjustment as previously described. The service technician should follow the service manual of the receiver manufacturer for the proper sequence of adjustment of these controls.
Fig. 124. Horizontal Oscillator Frequency Misadjusted. (A. F. C. System);
Fig.
125. Horizontal Discriminator Phase Misadjusted
PICTURE SIZE AND CENTERING CONTROLS. The group of controls which are used to fit the picture to the mask or frame are, in most instances, mounted on the back or side and are not "user" operated. Included in this group are: width, height, horizontal centering, and vertical centering.
Fig. 126. Width Control Misadjusted
WIDTH CONTROL. The width control adjusts the voltage applied to the horizontal deflection plates or the deflection coil. The effect on the received test pattern, when this control is not properly adjusted, is shown in Figure 126.
Width control can be accomplished by regulating the output of the horizontal oscillator, as described earlier, or by controlling the output of either the discharge tube or horizontal amplifier. In many instances the width control is not a single adjustment, s inc e the effect of changing other controls, such as horizontal linearity and horizontal drive, may require a readjustment of the width control.
HEIGHT CONTROL. The height control serves a similar function to that of the width control but in the vertical direction. In this case, the vertical oscillator or vertical amplifier is controlled in output.
Figure 127 shows the effect of an in correct adjustment of the height control on the received test pattern. In this case, as well as that of the width control (Figure 126), the picture is shown as "too high", or "too wide", but symmetrical with respect to the center of Fig. 127. Height Control Misadjusted the picture. Often the picture is found to be both incorrect in size and "off center". It will be necessary to adjust the size controls (width and height) simultaneously .with the horizontal and vertical centering controls.
The methods and circuits employed to accomplish height control are similar to those described for width. The same page and Figure references apply.
Fig. 128. Horizontal Centering Control Misadjusted
HORIZONTAL CENTERING. Misadjustment of the horizontal centering control will cause an effect on the received test pattern similar to that shown in Figure 128.
As shown in the chart of Figure 114, two distinctly different methods of centering the picture are employed. The mechanical mounting of the focus coil may be provided with screw adjustments to accomplish both horizontal and vertical centering. Electrical circuit means for accomplishing centering have been described for the electrostatic type, and for the electromagnetic type.
When centering is accomplished by positioning the focus coil, it may be found necessary to make readjustments of the ion trap position simultaneously with the movement of the focus coil, as explained.
VERTICAL CENTERING. Figure 129 shows the effect of misadjustment of the vertical centering control. As in the case of the horizontal centering control, just discussed, vertical centering is accomplished e it her by mechanical means ( adjustment of the focus coil position) or by electrical means (voltage bias of the plates in an electrostatic picture tube or current bias in the case of an electro magnetic tube). The same text references mentioned under horizontal centering apply also to the methods employed for vertical centering.
Fig. 129. Vertical Centering Control Mis-adjusted
CONTROLS WHICH AFFECT SCANNING WAVE SHAPE: In the discussion of the saw tooth scanning raster (pages 51 through 60) it was pointed out that the electron beam must trace at a linear rate in both the horizontal and vertical planes to avoid distortion of the picture. The shape of voltage wave (electrostatic) or current wave (electromagnetic) required to produce the linear beam motion has been covered on pages 56 through 60. In the discussion of beam deflection systems (pages 61 through 84) the various circuits and the controls employed to achieve linearity of scanning are discussed in detail.
The output voltage wave of scanning oscillators of the cathode-coupled multivibrator or blocking oscillator type can be made sufficiently linear to satisfactorily operate the electrostatically deflected type of picture tube and, for this reason, linearity controls are not required on sets using such tubes. In the case of magnetically deflected picture tubes, the scanning current wave consists of the combi nation of a linear sawtooth and a pulse. In the generation of such a wave, it is necessary to incorporate variable controls to achieve the proper time and amplitude relationship of the various sections of the wave shape. These include linearity controls, which may include as many as three for the horizontal scanning, and drive controls which adjust the amplitude of the pulse portion.
Fig. 130. Horizontal Linearity Control Misadjusted
HORIZONTAL LINEARITY CONTROLS.
Figure 120 shows the effect of the misadjustment of horizontal linearity. It will be noted that the circle of the test pattern has been rendered elliptical or "egg shaped." The picture has been cramped in the middle.
Correction of distortion of various parts of the picture, in the horizontal direction, are accomplished by controls affecting the circuit elements which contribute these portions of the scanning current wave. In the case illustrated by Figure 130, correction could be made by horizontal linearity control (L8 of Figure 73) or a similar adjustment for correcting the center of the picture without affecting the sides.
Additional linearity controls, such as resistor R9 of Figure 73, may be employed to correct the right hand side of the picture.
Since the number and circuit function of linearity controls differ between receiver designs, the television service technician is advised to study the service manual issued by receiver manufacturer for the particular linearity adjustment and the interaction of the control adjustment with other linearity and width controls are invariably covered in the service literature.
Fig. 131. Horizontal Drive Control Misadjusted
HORIZONTAL DRIVE CONTROLS. The horizontal drive control adjusts the ratio of the pulse to the linear portion of the horizontal sawtooth scanning current wave. This controls the point on the scanning trace at which the horizontal output tube conducts. The effect of its misadjustment is shown by Figure 131. In effect it is seen to be an auxiliary linearity and width control, since a clockwise rotation will increase the picture width, crowd the right hand side of the picture and stretch the left.
Feedback is often employed in the output stage to provide a negative pulse from the horizontal output transformer to the grid of the output tube. In this case, the drive control is a voltage divider across a winding on the output transformer. The voltage pulse is fed back in series with the output tube grid return.
Fig. 132. Vertical Linearity Control Misadjusted
VERTICAL LINEARITY CONTROL.
Figure 132 shows the effect on the transmitted test pattern of the misadjustment of the vertical linearity control. The type of linearity control most often employed in the vertical circuit is an adjustment of the operating point of the vertical amplifier. This usually takes the form of an adjustable cathode bias resistor, such as R7 of Figure 73. Curvature of the plate current versus grid voltage characteristic is employed to produce a ''counter-distortion'' which corrects any curvature of the voltage wave applied to the grid of this output tube.
In receivers which employ more than one linearity control, the secondary or additional control to that described, is made a variable resistor in the peaking circuit which constitutes the plate load of the vertical discharge tube. This control acts in a manner similar to the horizontal drive control previously discussed.
In the adjustment of linearity controls. of either the horizontal or the vertical type, it should be noted that the result of adjustment is interdependent with the effect of the size (width or height) controls. This interlocking action may necessitate readjustment of one of these controls if it is necessary to adjust the other.
Photographs of transmitted test patterns showing the effect of receiver mis-adjustments courtesy of the National Broadcasting Company and the R. C. A. Service Company, Inc.