A TYPICAL ELECTROMAGNETIC DEFLECTION CENTERING AND FOCUSING CIRCUIT
Section 1 illustrated and described the circuits for the control of be am center in g in both horizontal and vertical directions in the case of the electro static type of picture tube. We described the methods of focusing and deflecting the electron beam in an electromagnetically controlled picture tube. The circuits used for the control of focusing and centering of electromagnetic tubes were purposely not shown at that time, since they are interrelated with the action of sweep and deflection circuits and with the peculiar voltage waveform requirements of electromagnetic deflection.
Figure 73 shows the schematic diagram of the picture tube control circuits in a popular type of receiver design. It has been drawn in slightly different form from the presentation of service manuals in order to clarify circuit study. The circuit has been simplified to combine electrolytic capacitors in one unit at each point on the diagram where they occur as a single by-pass value. In the actual receivers, these units are often m separate cans in order to provide radiating area for the high ripple currents encountered.
FOCUS CONTROL ACTION. In section 1, we described the action of the focus coil, L1 of Figure 73, was described and illustrated. By controlling the current through the coil, as determined by the setting of the variable resistor R1. the electron beam paths are brought together to form a very small, concentrated spot at the surface of the fluorescent screen. In the circuit illustrated. a high current coil of relatively low resistance (250 ohms) has been used.
This coil, together with the ion trap electro magnet coils. is placed in the negative return lead of the power supply system and carries a large portion of the total "B" current. In this example, the average current through the coil is approximately 115 ma. Other receiver de signs may use focus coils having more turns of finer wire, hence higher resistance, in other parts of the supply circuit. Although the circuit shows R1 as a single variable resistor, this control is often part of a network of series and parallel resistors to achieve smoother control action.
As will be noted in Figure 22, the focus coil is mounted in such a manner that its axis can be rotated relative to the axis of the picture tube. We will see later that this serves two purposes. the positioning of the spot so that it is centered on the fluorescent screen and also, in some receiver designs, a fixed control of the range of action of the "back mounted" centering controls.
ION TRAP CIRCUIT: Ion trap action was de scribed and illustrated on pages 16 and 17. In this particular circuit, the ion trap magnet coils (L2) are shunted by resistor (R2). This shunt is used because the ion coil current for proper operation is less than the focus coil current (approximately 105 ma.). Some of the latest receivers appearing on the market are employing permanent mag nets of the "Alnico" type rather than electro magnets. for the ion trap action. The permanent magnet ion trap assembly i::; independent of the power supply circuits of the receiver and is another example of simplification in television receiver design.
VERTICAL CENTERING ACTION; The vertical centering circuit, shown in simplified form in inset A, Figure 73, consists of a series arrangement of the vertical deflection coils (L3), the vertical output transformer secondary (L4), and a source of DC voltage from the potentiometer R5. R5 is provided with a center tap, and it will be seen that, as the con tact arm passes this center point. the voltage introduced into the series circuit changes polarity. In this way it is possible to produce a DC polarizing current through the deflection coils in either direction, c au sing a steady magnetic field bias to exist in the deflection system. The sawtooth scanning current operates about this bias as a center point, and the picture can be moved up or down on the face of the tube by adjustment of R5.
It is interesting to note that the by-pass capacitor (C1), across the centering potentiometer, has an unusually large value ( 1000 mfd). It serves a dual purpose: (a) to keep any residual hum originating in the filter system of the power supply from entering the vertical scanning system and thereby causing interference with the 60 cycle scanning action: and (b) to keep the combination sawtooth-pulse voltage wave of vertical scanning from interfering with the operation of the video and audio amplifier systems through introduction of ripple into the "B" supply circuits. While this capacitor has been rated in microfarads as a matter of convenience, the design engineer specifies its measurement in terms of impedance over the frequency range ,which it must by-pass. This fact is mentioned because the actual value in microfarads may vary greatly as measured by a bridge or capacitance tester, and yet have sufficiently low impedance to do an adequate by-passing job.
HORIZONTAL CENTERING ACTION: Horizontal centering circuits in pre-war receivers were usually of the same type as that just described for vertical centering (a center-tapped wire-wound control carrying high current in the "B" return). In post-war receivers this has been abandoned in favor of an expedient which allows circuit simplification.
Inset B, Figure 73, shows, in simplified form, that portion of the large diagram which governs horizontal centering. As before, we find a series circuit consisting of the horizontal deflection coils (L4), the secondary of the horizontal output transformer (L6) with its shunt "width" control coil (L 7), and the source of centering voltage, potentiometer R6. In this circuit, however, the DC bias current through the horizontal deflection coils cannot be made to reverse in direction as was possible in the case of the vertical centering system.
To obtain the equivalent of control in both directions, use is made of the fact that positioning of the focus coil, in its mounting, can exert a magnetic field bias on the electron beam. In practice, when initial receiver installation is in process, the following steps are taken:
1. The horizontal cent e ring control, potentiometer R6, is first set in the middle of its range.
2. The position of the focus coil is adjusted in its mounting to center the "raster" in the picture area.
3. Adjustment of R6 will now serve to make minor correction of the picture in the horizontal direction (right or left). The focus coil position has provided a magnetic field bias which is equivalent to the introduction of an opposing current to that flowing in the series deflection circuit, due to the initial setting of potentiometer R6.
There appears to be a trend, as evidenced by the latest receivers appearing on the market, to dispense entirely with the use of potentiometer type centering controls.
In these sets, mechanical adjustment of focus coil position has been improved by the use of four corner adjustment screws. These screws have been provided with spring backing to assure that their adjustment will not change.
Access to these screws is provided from the rear of the receiver without the necessity of removing the cabinet back panel. It is possible to make either horizontal or vertical centering corrections by these purely mechanical adjustments.
HORIZONTAL SWEEP DAMPING CIRCUITS The horizontal output circuit of Figure 73 will be recognized as the high voltage supply system described, which utilizes the collapse of magnetic energy in the horizontal output transformer and horizontal deflection yoke. No explanation was given at point of the action of the 5V4G "damping" or "reaction scanning" tube. Its function, as well as the action of the horizontal linearity control (inductor LB), and horizontal width control (inductor L7), will now be considered. This tube, together with its associated circuit components, is used to stop or "damp out" oscillations in the system, and help produce the required linear current sawtooth through the deflection coils.
Referring again to Figure 65, which illustrated the timing requirements of the horizontal sawtooth current wave, it will be seen that retrace must be accomplished in the extremely short time of 7 microseconds. Since the deflection coil system is predominately inductive, it is necessary to employ a different method of operation than that of the lower frequency vertical system. To obtain the rapid reversal of current through the horizontal deflection coils, the output transformer and deflection coil circuit is tuned to a frequency of approximately 71 kilocycles by the associated circuit capacitances. This frequency is used because one half cycle of oscillation is equal to the required retrace time of 7 microseconds.
The current through the deflection coils is at its maximum value at either the extreme left or extreme right of the picture frame, the axis or zero point occurring at the center.
Fig. 73. A Typical Electromagnetic Deflection System.
When the right hand end of the trace is reached, the horizontal output tube is conducting high plate current and a maximum of magnetic energy is stored in the deflection coils. At this instant, a negative pulse arrives at the grid of the horizontal output tube from the plate circuit of the horizontal discharge tube, and the output tube plate current is suddenly cut off. The magnetic field in the transformer and deflection starts to collapse at a rate determined by the resonant frequency of the system ( 71 kc). This will shock-excite the circuit into a state of damped oscillation, which if allowed to continue would produce a wave form of current as shown in Figure 74.
HORIZONTAL COIL CURRENT TIME
Fig. 74. Horizontal Deflection Coil Oscillation Which Would Occur If Damping
Tube Were Not Used
Continuation of this oscillation would cause serious distortion of the left hand side of the picture. An "absorption" or damping device is used to "kill" the oscillation during the 10 microsecond period in which the picture tube is "blanked". This function is provided, in the circuit of Figure 73, by the 5V4G damping or reaction scanning tube.
Immediately, as the output tube plate current is cut off by the negative scanning pulse, the induced voltage, caused by the collapsing magnetic field, becomes negative in polarity at the plate of the damping tube. The tube will not conduct and there is no load imposed upon the circuit, allowing it to oscillate for one half cycle at its resonant frequency of 71 khz (approximately 7 microseconds). This causes the current through the deflection coils to reverse to a maximum in the other direction and accomplishes the rapid return trace. The circuit would continue to oscillate for many cycles if it were not for the damping tube which comes into action during the second half cycle.
As the current reverses through the deflection coils to start the second half cycle of oscillation, the voltage of self induction also reverses and the polarity becomes positive at the plate of the damping tube. This tube starts conducting and acts as a load across the terminals of the deflection coil system, preventing any further tendency toward oscillation. The current through the tube decays at a linear rate determined by the circuit constants. This linear current starts the next active scanning wave for the visible or "unblanked" trace.
If no additional current were supplied to the circuit from the horizontal output tube, the electron beam would come to rest at the center of the picture tube as the current through the damping tube decreased to zero. The latter portion of the current decay curve departs from the desired linear sawtooth form. To overcome this non-linearity, conduction of the horizontal output tube is so timed by the scanning generator and discharge tube, that it starts contributing current to the deflection coils before the original current has completely decayed.
Fig. 75. Current Waveforms In Horizontal Reaction Scanning
This current contribution from the horizontal output tube, which deflects the beam from the center to the right hand side of the screen, is so shaped at its start to correct the non-linearity of the original decay current wave. As shown in Figure 75, the two current waves overlap at the center of scanning action, and the combination produces a coil current which is linear.
To summarize the circuit action to this point, note:
1. The damping tube, sometimes called a reaction scanning tube, allows one half cycle of natural resonant oscillation to occur in the deflection circuit, after which it loads the circuit and prevents further oscillation.
2. The first half cycle of oscillation accomplishes beam retrace in the required 7 microseconds.
3. Decay of current through the damping tube produces the first half of the 53 microsecond active trace.
4. The horizontal output tube starts to contribute power to the system before final decay of the damping tube current, and produces the final half of the active scanning wave.
5. At the end of the active scanning cycle, a negative pulse causes plate current cut-off of the output tube, and starts a new oscillation to repeat the cycle.
CONTROL FUNCTIONS IDENTIFIED WITH HORIZONTAL SWEEP DAMPING CIRCUITS: Three adjustable circuit components appear associated with the damping tube of Figure 73.
These are the horizontal linearity control ( inductor L8), the horizontal width control ( inductor L 7), and the linearity adjustment (resistor R9).
HORIZONTAL LINEARITY CONTROL. The network comprised of L8, C3, and C4, per forms two functions in this circuit. It provides a means of operating the horizontal output tube at a higher plate voltage than that of the power supply, and, at the same time, functions as an adjustable means for making small corrections in the shape of the active sweep section of the sawtooth current wave.
Since these two functions are inter-related, an explanation of the voltage addition action will precede the description of the control operation. It will be noted that the plate voltage of the horizontal output tube is supplied by a series circuit consisting of the primary of the output transformer, the inductor L8 and the cathode-to-plate conducting path of the 5V4G damping tube. The damping tube is con ducting over the major portion of the sawtooth as shown in Figure 75. Capacitors C3 and C4 are charged during this conduction period and then discharge while the damping tube is not conducting. In this manner, the plate current of the horizontal output tube is maintained over the portion of the cycle when the damping tube is not conducting.
The charge on these capacitors is greater than the "B" supply voltage because the rectified surge or "kickback" of the deflection coils is added to the charge placed upon them by the "B" supply. In a typical receiver, employing this circuit, the output tube plate volt age is approximately 50 volts higher than the "B" voltage.
It will be noted that capacitors C3 and C4 do not seem to be high enough in value to act as storage capacitors. This fact is a clue to the action of the circuit as a linearity control. During the first half of the trace period, the voltage across C3 rises due to the rectified deflection coil "kickback", and during the second half of the period, the voltage falls due to the current demand of the horizontal output tube which is conducting at that time. In this manner, a ripple voltage of the same frequency as the sawtooth is impressed on the plate supply of the horizontal output tube. By shifting the phase of this ripple with respect to plate current requirements of the output tube, it is possible to modify the shape of the output current wave of the tube. Changing the value of inductor L8, by adjustment of a powdered iron core, controls the phase of the ripple current and permits minor adjustments of horizontal linearity. other controls which affect horizontal linearity are, the horizontal drive control (to be described later), the linearity adjustment (R9), and the horizontal width control (L7). While the major effect of certain of these controls is for another purpose, they are inter de-pendent and the adjustment of one of them often necessitates re-adjustment of the others.
LINEARITY ADJUSTING RESISTOR. Resistor R9 is known as a damping resistor and is provided with a series of taps rather than being made continuously variable. This adjustment is primarily intended as a factory means to compensate for manufacturing variations in the deflection yoke and output transformer. It controls the linearity of the trace on the left hand side of the picture only.
HORIZONTAL WIDTH CONTROL. Variable inductor L7 shunts a portion of the secondary of the horizontal output transformer (L6). This controls the output voltage and hence the extent or "width" of the sweep.
Since it also has a minor effect on the phase relations in the plate of the output tube, it causes slight changes in the linearity of the right hand side of the picture, as width changes are being made.
Fig. 76. Use Of Triode As Horizontal Damping Tube
HORIZONTAL DRIVE CONTROL. The horizontal drive control is a variable resistor which constitutes the "pulse" forming element in the plate circuit of the horizontal discharge tube. An explanation of its action appears on page 60. Figure 34 shows its application to the circuit under discussion.
The value of this resistor determines the ratio of the negative pulse amplitude to the sawtooth amplitude impressed on the grid of the horizontal output tube. It can, therefore, control the point on the trace at which the output tube conducts. Increasing its value causes a greater picture width. crowds the right side of the picture and stretches the left side. Ad1ustment may require a new setting of the horizontal "width" control.
USE OF THE TRIODE FOR HORIZONTAL DAMPING: In the circuit just discussed, a power rectifier type of diode (5V4G) was used to damp the oscillation produced in the horizontal deflection coil system, by cut-off of out put tube plate current. In some receivers, a triode is used as the damping tube, since the presence of a control grid gives the designer another opportunity to modify circuit action to obtain better sweep linearity.
Figure 76 illustrates a circuit application of the 6AS7G, low-mu, dual, power triode, as a damping tube. To act effectively as a damping tube, the plate resistance must be very low during the conduction period. The 6AS7G was designed especially for voltage regulator and television work and meets this requirement.
The circuit conditions at the instant of plate current cut-off in the horizontal output tube are similar to those Just described for the 5V4G. The natural resonant frequency of the deflection circuit is approximately 71 khz.
The first half cycle of oscillation finds the plate of the tube negative and no plate conduction can occur. During this time (7 microseconds), horizontal retrace takes place.
On the next half cycle, the plate of the tube becomes positive, causing the following series of events:
1. Plate current immediately starts to flow, since no charge exists on C1 and the grid is at cathode or zero bias. The tube damps the oscillation.
2. Discharge of the energy, stored in the vertical deflection coils, through the tube be gins to initiate the next active retrace.
3. At the start of conduction, capacitor C1 has been charged through the internal grid to cathode path of the tube. Resistors R4 in the grid circuit limit the grid current to a safe value.
4. The time constant of the discharge path, capacitor C1 and resistors R1 and R2, can be varied from 2 to 15 microseconds by the adjustment of R2.
5. The changing bias on the grid of the tube makes its plate resistance vary as scanning progresses, and allows the shape of the decay part of the curve of Figure 75 to be altered by adjustment of R2.
With this type of horizontal "linearity" control, the action is independent of the horizontal output tube plate circuit. Some inter action with the horizontal "width" control (L2 of Figure 76 occurs, however, and the setting of one may require the readjustment of the other.
The horizontal ''width'' control (L2) varies the proportion of the horizontal trans former output which reaches the deflection coils, thus controlling the sawtooth current strength and the width of the picture.
VERTICAL OUTPUT AMPLIFIER AND DEFLECTION CIRCUITS: The vertical deflection system of Figure 73 is much less involved in its action than the horizontal system shown.
A type 6K6G, triode-connected, is used as the output amplifier. Transformer L4 matches the plate impedance of the tube to the resistance of the vertical deflecting coil assembly.
Any tendency for shock-excited oscillations to occur following vertical retrace is nullified by the damping resistors (R4) which are connected across the vertical deflection coils (L3). The proper shape of the combination sawtooth and pulse wave of voltage is supplied to the vertical output tube grid from a blocking oscillator having a "peaking" C-R circuit in its output.
The only circuit element of this system which we have not covered is the vertical ''linearity" control (R7).
VERTICAL LINEARITY CONTROL. The grid voltage versus plate current characteristic of the triode -connected 6K6G is not a straight line over its entire range. The cathode bias resistor (R7) is made variable to shift the point of operation of the tube. As the operating point is shifted along its curve, changes in shape of the output scanning wave occur.
These variations are sufficient to correct any lack of vertical linearity.
Since the gain of the tube varies as the bias is changed, this linearity adjustment will also affect height, and readjustment of the vertical "height" control may be required after linearity correction.
TYPICAL DEFLECTION CIRCUITS AS USED IN COMMERCIAL RECEIVERS
An analysis of the sawtooth forming deflection circuits employed in twenty-two of the most popular current television receivers shows that the basic types of vacuum tube saw tooth generators described on pages 40 through 50 have been used in many differing combinations. The type of deflection system chosen for a particular design depends upon a number of factors. Of these design considerations two are of primary importance in determining the choice, namely:
1. The method of beam deflection (electrostatic or electromagnetic) determines whether a sawtooth of voltage or a sawtooth of current is required. The electrostatic requirement is readily accomplished in the generator circuit and we therefore find less diversity between sets using electrostatically controlled picture tubes.
2. The type of synchronizing system or method of using the "sync" pulses to "lock in" the sweep generator is also a determining factor in the choice of deflection circuit. While the details of separating the pulses from the signal and utilizing them for sweep control will be taken up in detail later, at this time it will be necessary to note that there are two basic types of synchronizing circuits used in television receivers at present. These are known as "triggered" sync and A. F. C. (automatic frequency control) or "flywheel" sync. Triggered sync may use any of the types of sawtooth generators· previously discussed while A. F. C. sync usually employs the sine wave generator discussed briefly.
In the analysis of present day receivers, mentioned above, it was found that two basic deflection system s have been used in sets employing electrostatic be am control ; the cathode-coupled multivibrator, and the blocking oscillator.
Fig. 77. Typical Vertical Deflection Circuit For An Electrostatic Picture
Tube
TYPICAL COMMERCIAL ELECTROSTATIC DEFLECTION SYSTEMS: The requirements for electrostatic scanning, discussed here, consist of the production of a sawtooth wave of voltage and its application to the deflection plates by a push-pull output circuit.
Two methods of meeting these requirements have appeared to date. The first, and most widely used, consists of a pulse controlled, cathode-coupled, multivibrator feeding a phase-inverted push-pull amplifier. The second method is an unusual adaptation of the blocking oscillator in which a single tube and its associated circuits fulfill all of the scanning requirements.
Fig. 78. Typical Horizontal Deflection Circuit For An Electrostatic Picture
Tube
ELECTROSTATIC DEFLECTION CIRCUITS USING THE CATHODE-COUPLED MULTIVIBRATOR. The majority of electro static type television sets on the market, at present, employ the cathode-coupled multivibrator with a phase inverted amplifier for both horizontal and vertical systems. Since a number of differences in circuit constants and arrangement exist between the vertical and the horizontal systems, typical circuits of each will be discussed separately.
A Typical Vertical Deflection Circuit.
Figure 77 shows a vertical deflection circuit which is typical of many receivers using the 7GP4 picture tube. Its action is as follows:
1. The vertical synchronizing signal, consisting of a series of pulses spaced as shown in Figure 66 (page 53), of negative polarization with respect to ground , passes through the network comprised of R1, C1, R2 and C2. This is known as an "integrating" circuit and its action is to add up all of the small "serrations" of the vertical signal as a voltage across capacitor C2, until the negative potential on the grid of T1 reaches a value corresponding to plate current cut-off. This "trips" the multivibrator circuit as described here.
2. The time constant, determined by the values of capacitorC3 and the sum of resistors R4 plus R5, controls the "free running" frequency of the multivibrator. R5 can readily adjust the frequency to "lock in" with the vertical sync pulse at 60 cycles per second and is called the vertical "hold" control.
3. During the interval between the "triggered" conduction pulses of T2, the saw tooth wave forming capacitor C4 is linearly charged from the '' B'' source through resistor R6. Firing of T2 by the "sync" pulse discharges C4, causing retrace.
4. Control R7 adjusts the input voltage impressed on the p has e inverted, push-pull amplifier consisting of tubes T1 and T2 and since this control varies the output of the sys tem it is known as the vertical "size" or "height" control.
5. The circuit associated with these tubes is of the same type as used in the audio system of many broadcast receivers and requires no further mention except to note:
a. The coupling capacitors and load resistors are of such values that the circuit gain remains flat to the low frequency of vertical scanning (60 cycles).
b. The plate decoupling capacitor (C5) has a higher value than would ordinarily be required for an audio sys tem. This is necessary to keep the vertical sawtooth ripple out of the video and audio supply circuits.
This circuit has no "linearity" control.
Such a control is not necessary because the sawtooth produced across C4 is sufficiently linear to provide good pictures.
A Typical Horizontal Deflection Circuit.
Figure 78 illustrates the horizontal system employed in the same receiver as the vertical system of Figure 77. It is similar to the vertical system with the following exceptions:
1. The horizontal synchronizing signal, which consists of short time pulses (5 micro seconds) at the end of each horizontal picture line as shown in Figure 65, are impressed upon the grid of T1 through the network comprising R1, C1 and R2. This is known as a "differentiating" circuit . Here the grid "triggering" voltage is taken across the resistor rather than across the capacitor as is done is the vertical circuit. It should be noted that the circuit sharpens the pulse and delivers a negative '' stab" of voltage on the grid to "lock in" the oscillator at the proper time.
2. The time constant of "free running" oscillation, which is determined by the product of the value of C2 times the sum of R4 plus R5, is much shorter (1/262.5 of the vertical time) than the vertical time constant just discussed.
This accounts for the lower values of capacitance and resistance as compared with those of Figure 77. Again, as in the vertical circuit, R5 can be adjusted to accomplish lock-in with the repetition rate of the "sync" pulses and is known as the horizontal "hold" control.
3. The size or width of the picture is adjusted by a voltage control in the plate circuit of T2 (R7 in series with "B" plus). This type of size control is sometimes used in both the vertical and horizontal circuits. It controls the charging voltage impressed on the sawtooth wave forming capacitor (C3) and hence the height of the wave.
4. The phase inversion circuit is a bit unusual as compared with familiar audio practice. Since the fundamental frequency of horizontal scanning is 15,750 cycles, the capacitance as well as the resistance balance must be considered. This accounts for capacitor C4.
5. A word or two of explanation is in order with regard to the use of the choke supply to the plates of T3 and T4. Many of the receivers on the m ark et employ resistance coupling in this circuit as shown in the vertical circuit of Figure 77. Chokes can be employed in this case since the high frequency (15,750 cycles) allows an economical design in which the impedance of the chokes is high enough to have negligible shunting action across RB and R9 as far as the horizontal line frequency is concerned. The advantages from the use of chokes are that the plate voltage, and consequently the output, is higher and also that their series impedance acts as an excellent isolation of the system from the audio and video supply.
ELECTROSTATIC DEFLECTION CIRCUITS USING THE BLOCKING OOCILLATOR. An interesting combination of circuits , de signed around the blocking oscillator, have recently made their appearance in a table model, electrostatic ally deflected receiver.
The vertical and horizontal systems are shown in Figures 79 and 80 respectively.
VERTICAL DEFLECTION CIRCUIT US ING THE BLOCKING OOCILLATOR. Figure 79 combines a number of the circuit arrangements which we have covered separately in our consideration of sawtooth generators and deflection means. The circuit is conventional with regard to the blocking oscillator tube T1 and its associated circuit elements. The unusual features are found in the phase inverted, push-pull amplifier.
As in the cathode-coupled multivibrator just described, an "integrating" network (R1, C1, R2, C2) "sorts" the vertical sync pulse from the picture signal. In this case, however, the polarity of the pulse is positive with respect to ground. Control of the oscillator frequency by the pulse occurs as described here.
The adjustment of the oscillator "free running" frequency is accomplished by control of the time constant of the C -R network in the grid circuit of the blocking oscillator tube T1.
R5 in this circuit constitutes the vertical "hold" control.
Vertical "size" or "height" of the picture is controlled by adjusting the charging voltage impressed on the sawtooth wave-forming capacitor (C6) by means of the vertical "size" control (R7). The phase inverted, push-pull amplifier (T2-T3) is a 6SL7GT high-mu twin-triode. The voltage division, to supply the grid voltage of T3, is by means of a capacitance voltage divider, consisting of C7 and C8, rather than the usual resistance divide r found in the audio systems of broadcast receivers. "Contact" bias of both tubes is derived by grid current through the 10 megohm grid resistors (R8 and R9). The plate supply voltage of approximately 900 volts (from a bleeder across the high volt age supply) feeds the tubes through the 4.7 megohm plate resistors (R10 and R11). The actual voltage at the plate is approximately 450 volts. With this supply, it is possible to derive a plate swing of approximately 700 volts.
As in the case of the cathode-coupled multivibrator circuit, the linearity of the saw tooth voltage wave produced by this circuit is sufficiently good to assure satisfactory pictures. Therefore, no "linearity" control is required.
Fig. 79. Blocking Oscillator--Vertical Electrostatic Deflection Circuit
Fig. 80. Blocking Oscillator--Horizontal Electrostatic Deflection Circuit
Horizontal Deflection Circuit Using the Blocking Oscillator. In the horizontal system employing the blocking oscillator, the deflection sawtooth voltages (balanced to ground) are developed in the oscillator itself. No amplifier stage is required. With a DC plate supply of only 250 volts this circuit delivers a balanced linear sweep of 1200 volts, peak to peak, to the deflection plates of the picture tube. This circuit differs from the customary blocking oscillator in that the cathode is not grounded.
The plate-to-ground and cathode-to-ground circuits are symmetrically arranged with respect to ground or "B" minus. This accounts for its balanced or push-pull output.
In explanation of the action of the circuit (see Figures 80A, B, C and D) it will be instructive to follow, in sequence, the steps in the production of a sawtooth voltage wave.
1. Let us assume that we are at point "a" of Figure 80D, and that a positive sync pulse through C6 (Figure BOA) is starting to "fire" the tube. This causes plate current conduction, and provides a low resistance path for the discharge of capacitors C4 and C5 through the tube and the primary of the feed back transformer. (See Figure 80B.)
2. The plate winding of the transformer is the inductive branch of a tuned circuit. This tuned circuit consists of L1 with a group of tuning capacitances. These are: C1; C5, the reflected capacitance from the secondary, or grid winding; and the series combination of C4 and C5. This tuned circuit is resonant to approximately 71 khz, corresponding to a frequency in which a half cycle is equal to the horizontal "flyback" time of 7 microseconds.
(See Figure 65.)
3. When the tube conducts, as controlled by the positive sync pulse which has arrived at its grid, an oscillation starts in the plate circuit. This continues for the half cycle ''a'' to "b" of Figure 80D.
4. At the end of this half cycle, the grid has been driven to the point of plate current cut-off. The oscillation is now stopped for two reasons:
a. The tube, acting as a switch, has removed the main tuning capacitors C4 and C5.
b. Damping resistors R1 and R2 (Figure 80A) have suppressed any tendency toward further oscillation.
5. The swing of voltage, due to the half cycle of plate current oscillation, has caused a condition wherein the polarity of the charge on capacitors C4 and C5 has been reversed during the retrace period. We now find that at time "b" (Figure 80C), capacitors C4 and C5 are oppositely charged and the "high" side of C4 is negative with respect to ground. The cathode, or low side of C5, is positive with respect to ground.
6. Since the tube is now cut off, capacitors C4 and C5 have no other path, for charge or discharge, than the windings of the output chokes L3 and L4. These choke windings are shown on the same core in Figure 79A, as in the actual receiver. This construction is not essential for the operation of the circuit, and we have shown them as separate chokes in the partial circuit of Figure 80C. The inductance of these choke windings is high, and when such a high impedance choke is connected across a charged capacitor, the current flow is constant and the voltage across the capacitor changes linearly with time.
7. During the time interval from "b" to "c" of Figure 80D, capacitor C4 is being charged linearly by the voltage of self induction of choke L3. Current starts flowing from the "B" source to sustain the charging cycle.
8. During the same time interval, the charge in capacitor C5, due to the resonant swing of voltage during the "firing" cycle, discharges through choke L4 and produces the sawtooth voltage wave shown in Figure 80D. It is seen that the sawtooth waves across C3 and C4 are symmetrical and of opposite polarity. Hence, the requirements for balanced electrostatic deflection have been fulfilled.
Another method of explaining the sweep interval (time ''b" to ''c", Figure 80D) is to consider the chokes and their parallel sawtooth wave -forming capacitors as individual tuned circuits. These circuits are resonant at approximately 1/ 10th of the horizontal line frequency, or about 1600 cycles. The portion of a sine wave where the curve passes through zero and reverses voltage is very close to a straight line. At time "b", an oscillation starts in these tuned circuits, but it is allowed to continue only for about 1/10th of a cycle.
The wave produced is still on the essentially linear portion of the sine curve when the next firing interval occurs and starts a new wave.
The free running frequency of blocking action is controlled by the time constant of the grid circuit. Therefore, resistor R4 functions as a "hold" control.
Resistor R5 functions as a "width" or ''size'' control by adjusting the value of voltage applied to the plate circuit and consequently controls the amplitude of the sawtooth wave.
Coupling capacitor C6 serves to intro duce the horizontal sync pulse which "trips" the proper time for synchronization with the transmitted signal.
TYPICAL COMMERCIAL ELECTROMAGNETIC DEFLECTION SYSTEMS: When we analyze the circuits of electromagnetically deflected television sets, which have appeared on the market to date, we find much greater diversity of design than in electrostatic systems. The chart of Figure 81 shows the result of such an analysis. The combinations selected by the design engineers for the horizontal and vertical systems respectively have been arranged in the order of the frequency of use in current production models. From this diversity of design, we can infer that the art is in a state of flux, and that as new models appear, many un usual circuit arrangements will be employed.
The ultimate aim of the research and design departments is toward simplification and economy.
The combinations shown in the chart are each representative of receivers of several manufacturers. Example No. 1, for instance, has been employed with minor design differences by at least six companies. It will be instructive to examine the typical combinations and determine the operating principles of the various parts of the circuits.
ELECTROMAGNETIC SYSTEM--EXAMPLE 1. The deflection system outlined, Example 1 in the chart of Figure 81, has been used in more models than any other to date.
We have shown examples of its application in Figure 34 of page 29 and Figure 73.
Let us examine the circuit of a different receiver than that used previously.
Vertical Deflection System. Figure 82 shows the details of the vertical deflection sys tem of this receiver. It combines a number of the circuit elements which we have previously discussed such as: the vertical pulse-integrating circuit, the blocking oscillator, the discharge tube, and the series C-R circuit which forms the combination sawtooth and pulse wave required for electromagnetic deflection.
The network of resistors and capacitors R1-C1, R2-C2, and R3-C3 comprises the "integrating" circuit which shapes the vertical sync signal and applies it in series with the grid circuit of the blocking oscillator tube (T1). The pulse arrives at the grid at the proper time to "trigger" the plate current pulse as described and illustrated on pages 54 and 55.
This insures that retrace of the vertical scanning from bottom to top of the picture occurs at the correct instant as shown in Figure 66.
The action of the blocking oscillator circuit of tube T1 has been covered in pages 47 through 49. In the present circuit its application is conventional. The "free running" frequency is controlled by the time constant of capacitor C4 and its discharge resistors R4 and R5 in series. R5 is variable and acts as the "hold" control to adjust the oscillator frequency to "lock" with the repetition rate of the vertical sync pulses.
Fig. 81. Chart of Electromagnetic Deflection Systems as Used in Present-day
Receivers
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HORIZONTAL SYSTEM
1. A. F. C. Sync Discriminator Reactance Tube Sine Wave Oscillator Pulse Forming Stage Output Amplifier Damping Tube
2. A. F. C. Sync Discriminator D. C. Amplifier Cathode-Coupled Multivibrator Output Amplifier Damping Tube
3. Blocking Oscillator Output Amplifier Damping Tube
4. A. F. C. Sync Discriminator D. C. Amplifier Cathode-Coupled Multivibrator Output Amplifier Damping Tube
5. A. F. C. Sync Discriminator Reactance Tube Sine Wave Oscillator Pulse Forming Stage Current Oscillator Output Stage
6. A. F. C. Control Stage Blocking Oscillator Output Amplifier Damping Tube
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VERTICAL SYSTEM
Blocking Oscillator Output Amplifier Asymmetrical Multi vibrator Output Amplifier Blocking Oscillator Output Amplifier Blocking Oscillator Output Amplifier Cathode-Coupled Multivibrator Output Amplifier Asymmetrical Multivibrator Output Amplifier
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Fig. 82. Electromagnetic Deflection Systems--Example 1. Vertical Circuit
The second section of the dual triode acts as a discharge tube to "short" the network consisting of C5 and R8, which forms a voltage wave consisting of a linear sawtooth followed by a pulse as described on page 60. The charging voltage, supplied to net work C5-R8, is controlled by the series combination R6 and R7 connected to the "B" supply. R7 is made adjustable to act as the "height" or "size" control.
Tube T3 is a triode-connected output amplifier and serves to increase the amplitude of the "sawtooth-pulse" to the proper level for action of the vertical deflection coils. The only feature of special interest · in this part of the circuit, is the variable cathode bias resistor (R9 and R10 in series). Adjustment of the operating point on the grid voltage-plate current curve, by the setting of R10, serves to introduce the proper amount of distortion to correct any departure from linearity of the sawtooth scanning wave. This control is known as the vertical "linearity" control.
In practice, adjustment of R7 (vertical "height") and R10 (vertical "linearity") are somewhat interdependent. In this type of circuit, the vertical "size" or "height" control primarily affects the lower half of the picture, while the vertical "linearity" control has its major effect upon the upper half of the picture.
Figure 82B and 82C show the form of voltage waves appearing between ground and points A and B respectively.
Horizontal Deflection System. Figure 83 shows the horizontal deflection system of the receiver which has been selected as typical of Example 1 (Figure 81). It consists of a combination of circuit elements which have been treated separately in our discussion to this point, namely: the sine wave oscillator (con trolled by the sync pulse through an A. F. C. system and "reactance" tube), pulse shaping circuits and tubes, followed by a horizontal output tube and a triode damping tube.
The sync pulses are separated from the signal and applied to the grid of a "sync amplifier" T1 through network C1-R1. The plate circuit of this tube supplies the vertical amplified pulses directly from the plate to the "integrating" network of Figure 82. The amplified horizontal pulses are taken from a tap on the voltage divider R3-R4, and applied through C2 to the A. F. C. sync discriminator or '' phase detector." This circuit, employing a dual diode (6AL5), will be recognized as the "Seeley-Foster" discriminator used for FM detection. Its use in this circuit is to compare the repetition rate of the sync pulses with the frequency of the horizontal oscillator, and to produce a DC output voltage for control of the reactance tube (T4). The reactance tube automatically adjusts the frequency of the horizontal oscillator and keeps it in step with horizontal scanning. The action of this circuit will be described in more detail later.
Tube T5 is employed as an electron coupled oscillator. The screen is at ground potential, for the oscillator frequency (15,750 cycles), due to the bypass capacitor C7. Feed back, to sustain oscillation, is furnished by the "hot" cathode tap on the transformer secondary L2. In this circuit, there are two adjustable frequency controls: the iron core tuning "slugs" of transformer windings L1 and L~, and the grid resistor combination R10 and R11.
The transformer adjustments are pre-set, or semi-fixed, to assure that the action of the horizontal "hold" control (R11) will cover the proper frequency range on either side of center of its rotation.
Figure 83B shows the sine wave voltage appearing across the oscillator grid circuit.
This voltage is electron-coupled to the plate circuit of tube T5. In this circuit, the first shaping of the wave from its sine form to the required pulse occurs as shown in Figure 83C. This action takes place when the plate swing reaches the region of plate current cut-off during part of the cycle.
Fig. 83. Electromagnetic Deflection Systems--Example 1. Horizontal Circuit
The coupling network C8- R12, C9-R13, acts as a "differentiating" circuit and further sharpens the wave-shape as shown in Figure 83D.
The voltage wave, of Figure 83D, is impressed on the grid of the horizontal "forming" tube (T6). This tube acts in much the same manner as the discharge tube described here. The positive voltage peaks on the grid cause plate conduction to discharge a sawtooth capacitor C10 in its plate circuit. This produces the wave form (Figure 83E) which is used to "trip" the horizontal output tube (T7). R14 and R15, in the plate circuit of tube T6, control the charging voltage of C10, and therefore, the variable portion (R15) acts as a horizontal "size" or "width" control.
The horizontal out put and horizontal damping circuits are similar to those described in pages 62 through 67. In this case, how ever, a number of new circuit features and additional controls appear:
1. An "inverse" or "negative" feedback circuit consisting of the resistance divider ( R1 7 and R18), across part of the horizontal output transformer secondary, and the capacitance divider (C11 and Cl2) in the grid circuit of T7, feeds back a small "corrective" signal to the input of the stage. This aids in correcting departure from linearity of the part of the sawtooth current wave contributed by tube T7.
The expedient of employing a small amount of negative feedback to improve linearity is used in other deflection circuits and the service technician should be able to recognize it. Often this feedback takes the form of coup ling from the cathode circuit of the output tube to a capacitance divider in the input of the same tube.
2. The screen voltage of the horizontal output tube (T7) has been made variable by means of resistor R21. This control serves to adjust the amplitude of plate current contribution by tube T7, to the scanning wave. In this case, the control is a "semi-fixed" adjustment and has no descriptive name. It is an auxiliary "width" adjustment for the right hand half of the picture.
3. The dam ping tub e circuit, shown in Figure 83A, is similar to that described on pages 66 and 67, with the addition of a second "linearity" control (R23). In this case, "linearity" control No. 1 (R20) is used as described on page 67 and its function is to adjust the center area of the picture. "Linearity" control No. 2 (R23) is used to expand the left hand side of the picture.
As in other deflection circuits discussed, these controls, "width" (R15), "linearity" No. 1 (R20), and "linearity" No. 2 (R23) are interdependent and the adjustment of any one may necessitate readjustment of the others.
ELECTROMAGNETIC SYSTEM--- EXAMPLE 2. Example 2 in the chart of Figure 81, employs a grounded cathode asymmetrical multivibrator in the vertical circuit, and a cathode-coupled version in the horizontal system. Figure 84 shows the details of the vertical system, and Figure 85 illustrates the horizontal circuit.
Vertical Deflection system. The vertical deflection circuit of Figure 84, employs elements which have be en previously covered: the sync amplifier, the asymmetrical multivibrator and the vertical output tube.
A circuit not previously discussed, known as a "sync clipper", is included since its plate circuit components are an essential part of the deflection circuit under consideration. The "sync clipper" operates at very low plate volt age, (approximately 15 volts in this circuit), and derives its bias by grid rectification of the video signal. Plate current conduction occurs on the peaks of the video signal only. These peaks actually are the sync pulses. Thus tube T1 ''clips" the sync pulses and passes them on to the sync amplifier tube (T2). (See Figure 84B.)
Fig. 84. Electromagnetic Deflection Systems--Example 2. Vertical Circuit.
Although we have labeled T2 as a sync amplifier (its common commercial name), in reality it should be considered as a coupling tube and isolation stage. Since the load of the tube, resistor R10, is in the cathode circuit, the voltage gain of the stage is less than unity.
The output is taken from the cathode and the voltage pulse which this tube delivers to the grid of the multivibrator tube (T3) is negative with respect to ground. This is the requirement for "tripping" the sweep action, as described here. The shape of the negative pulses which appear at the grid of T3 is shown in Figure 84C. The coupling network between the "sync clipper" (T1) and the "sync amplifier" (T2) has several unusual features:
1. Resistor R6 and capacitor C3 constitute an "integrating" circuit to accept the vertical sync signals and reject the horizontal sync signals. To make use of this circuit, an unconventional grid biasing method becomes necessary.
2. It will be noted that the grid circuit of T2 is connected to "B" plus through resistors R6, R4 and R5 and to ground through resistor R3. This group of resistors acts as a voltage divider, across "B" plus. The positive voltage impressed on the grid causes a plate current through the cathode bias resistor (R7). This bias is opposed to the positive bias from the plate circuit and, as a result, the actual grid bias with respect to ground is approximately plus 3 volts. The positive bias assures a condition of plate current conduction until cut-off action is initiated by the negative pulse from the horizontal sync signal.
The asymmetrical multivibrator, consisting of tubes T3 and T4, actually a single dual triode 6SN7, is the same circuit as covered in Figure 59. It will be noted that the time constant of tube T4 grid circuit is approximately ten times that of tube T3. The short time constant of T3 corresponds to the retrace period, while the longer time constant of T4 controls the active trace portion of the sawtooth wave.
As in multivibrator circuits previously discussed, the free running frequency is controlled by the time constant of capacitor C9 and the series combination of R11 and R12.
R12 is made variable to act as the vertical ''hold'' control.
Fig. 85. Electromagnetic Deflection Systems.
An interesting circuit variation is the use of capacitor C12 as a combined sawtooth generator and coupling means to the vertical output tube (T5). The bias voltage of tube T5 is determined by a balance of the cathode voltage drop across R15 and R16, and a positive voltage from the "B" supply through R14 and R13. The net bias voltage of the tube is approximately plus 7 volts. Capacitor C 12 is charged from the ''B" source through R14 and R13. R14 acts as the vertical "size" or ...
Example 2. Horizontal Circuit.
... "height" control. It will be noted that R14 controls not only the plate voltage of T4 but also the screen voltage of the vertical output tube (T5). Since the amplification of a pentode is partially dependent on the screen voltage, this control exerts a dual influence on the overall gain of the vertical system and hence the height of the picture.
The adjustable cathode bias resistor (R15) serves as a "linearity" control. A slight amount of inverse, or negative, feed back occurs in this stage due to the connection of C12 to the cathode of the tube, rather than to ground. This feedback assists in improving the linearity of the sweep applied to the deflection coils.
Two new circuit elements, capacitor C13 and resistor R1 7, appear in connection with the output transformer. As in the circuit of Figure 73 and its explanation on pages 62 and 64, the secondary of the vertical output transformer (L2), and the vertical deflect ion coils, are tuned to resonate at a frequency such that the first half cycle corresponds to the vertical re trace time (500-750 microseconds). (See Figure 66) When the plate current of vertical output tube is suddenly cut off by the negative swing of voltage on its grid, as shown in Figure 64D, the "ringing", or oscillation, of the output circuit causes a rapid retrace of scanning. Any tendency of this circuit to continue in a state of oscillation is suppressed by the damping resistor ( R17). Horizontal Deflection System. The horizontal deflection system of this general class (Example 2, Figure 81) is similar in some respects to that described in Example 1, since it employs "flywheel", or A. F. C., control of the scanning oscillator by the repetition rate of the sync pulses.
This circuit, Figure 85A, employs the same "sync clipper" tube as was shown in Figure 84A. In this case, however, the grid of the horizontal'' sync amplifier'' is coupled directly to the plate of tube T1 by means of capacitor C 2. This allows the short time (5 microseconds) pulses to be passed directly to the A. F. C. circuit through amplifier T2.
The sawtooth generation is accomplished by a cathode-coupled multivibrator rather than a sine wave oscillator. A "sample" of the scanning wave voltage is taken from the output transformer secondary L5 and injected into the series diode discriminator, or '' phase detector", where it is compared with the repetition rate of the horizontal sync pulses.
The direct current output of discriminator tubes T3 and T4 is applied to the grid of a DC amplifier. It will be noted that a high-mu twin triode ( 6SL7 -GT) has been employed as a dual -diode by connecting the grids and plates of the respective sections to each other. The plate resistance of amplifier tube T5 is part of the discharge resistance network which determines the operating frequency of the cathode coupled multi vibrator.
The function of this DC amplifier tube (T5) is similar to that of the reactance tube (T4) of Figure 83A. The resistor discharge network comprised of R12, R13, R14 and the plate resistance of tube T5 controls the time constant of the multivibrator grid circuit tube (T6). One of these resistors, R13, is made variable to adjust the free-running frequency, and therefore, acts as the horizontal "hold" control. The tuned circuit in the cathode return of tubes T6 and T7, consisting of capacitor C6 and inductor L3, is an additional control of the free-running frequency of the multivibrator.
This is a service or ''semi-fixed'' adjustment, and is used to set the range of the "hold" control so that it operates symmetrically about its mid-position.
The operation of the remainder of the circuit, consisting of horizontal output tube (T8) and damping tube (T9) needs no further explanation at this time since it has been described on pages 62 through 67.
Figures 85B, 85C and 85D show the waveforms found at various points in this circuit. The wave of Figure 85B requires explanation since it differs from the series of single horizontal pulses delivered to the grid of T2 by the "sync clipper" (T1). It consists of a positive "pip'' followed immediately by a negative "pip". The reason for this action lies in the fact that the plate load of tube T2 consists of the primary of the discriminator transformer (L1). When a square wave pulse of plate current passes through the inductance, magnetic flux is generated during the rapid rise of current at the leading edge of the pulse, and the rapid fall of current at the trailing edge. Since these current changes are in opposite directions, they produce the waveform of Figure 85B.
ELECTROMAGNETIC SYSTEM--- EXAMPLE 3. Example 3 in the chart of Figure 81, uses blocking oscillators in both the vertical and horizontal deflection systems. This combination has been employed mainly in table models using the 7DP4 or the 10FP4 picture tubes. Synchronization of both oscillators is caused by "triggering" action of the "clipped" and "shaped" pulses. Fi gure 86 shows both the vertical and the horizontal deflection circuits of a typical commercial receiver employing this combination.
Vertical Deflection System. The circuit employs the following elements: a "sync amplifier", a diode "clipper" or "sync leveler", a "sync separator", an "integrating" circuit, a blocking oscillator, and a vertical output tube.
The video signal, with its vertical and horizontal sync pulses , is impressed on the grid of the "sync amplifier" tube (T1), Figure 86, through a coupling network comprised of C1 and R1. This stage is a normal voltage amplifier, biased so as to amplify the signal without change in waveshape. The polarity of the video signal applied to the grid of T1 is such that the sync pulses are in the positive direction in its plate circuit.
Diode T2 is connected, by coupling capacitor C2, across the plate load resistor (R2) of tube T1. The diode load resistor (R3) also acts as the grid resistor of '' sync separator'' tube (T3). The resistor is returned to a negative bias supply of -20 volts and this bias performs two functions:
1. It causes tube T3 to act as a "detector", since it places operation beyond plate current cut-off. Under this condition of operation, only the most positive portion of the input signal (the synchronizing pulses) causes plate current flow. Thus the tube T3 can separate or "clip" the sync pulses.
2. This negative voltage also acts as delay bias for the diode "clipper" or "leveler" tube (T2). When the level of the amplified pulses, appearing across plate load resistor (R2), exceeds this delay bias, rectification occurs in diode T2 causing a negative voltage to appear across the diode load resistor (R3). This voltage adds to the normal bias and is in effect an A. V. C. voltage for the control of sync separator tube (T3). This 'levels" the line of sync pulses so that each recurring pulse in the output of tube T3 is of an equal amplitude.
Fig. 86. Typical Electromagnetic Deflection System--Example 3
The series combination of resistors R5 and R6, in the cathode circuit of T3, acts as the output load of the sync separator, and feeds the separated pulses to both the vertical and horizontal systems. The integrating network, for acceptance of the vertical sync signal, is com posed of capacitors C3, C4, C5, C6 and C7 together with resistors R7, RS, R9 and R10. Control of the blocking oscillator tube (T4), by the sync pulses, is accomplished through series injection in the grid circuit as covered on pages 54 and 55. The action of the blocking oscillator is the same as that described and illustrated on pages 47 and 48, with the exception of the addition of peaking resistor (R18) in series with the sawtooth forming capacitor (C12). Vertical ''hold" is controlled by R12 in the grid circuit and vertical ''height" by R14 in the plate circuit, as previously covered.
The action of vertical output tube (T5), with its cathode bias control of vertical "linearity", has been covered previously.
Horizontal Deflection System. The horizontal deflection system is similar to that described on pages 61--64 and illustrated in Figure 73, with the substitution of a pulse "triggered" blocking oscillator for the A. F. C. controlled sine wave generator. The elements of the circuit are: a blocking oscillator, a discharge tube, a horizontal output tube and a damping tube.
Horizontal sync pulses are fed to the grid of the blocking oscillator tube (T6) from the voltage drop across resistor R6 in the cathode circuit of the sync separator tube (T3). The circuit consisting of resistor R6 and the secondary L6, with its distributed capacitance, accept the horizontal pulses and reject the vertical group. Blocking oscillator tube T6 is thus controlled by the sync pulses and proper scanning occurs.
The grid of horizontal discharge tube (T7) is directly connected to the grid of the blocking oscillator tube (T6) and functions as described here. The sawtooth generating and peaking circuit, consisting of Cl6, R26 and R27, is discharged or triggered by plate conduction of tube T7 as controlled by the oscillator tube (T6). Resistor R26 is made variable as a horizontal "drive" control. The action of this control was discussed here.
The action of the remainder of the circuit (horizontal output, damper, and high voltage rectifier) have been covered previously.
Figure 87 shows a commercial variation of the blocking oscillator which has several unusual features. The feedback action involves the use of both sections of a high-mu twin tri ode (6SL7GT). The input section ( T1) operates with cathode bias due to its plate current flow through R2. This establishes a negative bias as shown in Figure 87B. T1 is coupled to the cathode of a '' grounded grid'' triode T2. The plate of T1 feeds the cathode of T2 through C3.
The unusual feature of this circuit is the incorporation of a tuned circuit comprised of L3 and C4 in the plate return of T1. The combination of L3 and C4 is tuned to a frequency slightly lower than the horizontal scanning rate of 15,750 cycles per second and is shock ex cited by the pulse action caused by feedback through transformer windings L1 and L2. The sudden positive pulses shown in Figure 87B have an action similar to the "push" given a pendulum in a "grandfather's clock" by the mechanism. The mechanical pulses are correctly timed to sustain the simple harmonic motion. The action of the electrical pulses results in a condition of sustained oscillation in which the voltage wave across L3 is practically sinusoidal. (See Figure 87F .) The C-R circuit in the cathode of T2, consisting of C3, R3 and R4, has a variable time constant of from 15 to 30 microseconds and by adjusting R3 it is possible to effect small changes of the free running frequency. Resistor R3, therefore, functions as a "hold" control. It will be noted from the waveform of the cathode to ground voltage (Figure 87D) that the combination of pulse action and capacitor discharge produces a sharply peaked input to T2.
Fig. 87. A Variation of the Blocking Oscillator.
The plate current conduction pulses of T2 (see Figure 87E) "trip" the charge on the sawtooth and pulse forming circuit C6-R7.
The path of this discharge action includes transformer primary L 1, the internal plate to cathode path of the tube T2, and parallel circuits from cathode to ground comprised of R3, R4 and C3, R5, C4.
This oscillator is very stable in action due to the "flywheel" effect of the tuned circuit (L3-C4). It is not readily tripped by random ·noise pulses which arrive at times other than the synchronizing pulses.
ELECTROMAGNETIC SYSTEM -- EXAMPLE 4. In the combination example 4, Figure 81, the horizontal circuits are identical to those described under Example 2, and illustrated in Figure 85A. The vertical system, however, exhibits several interesting features and merits description.
Vertical Deflection System. The elements of this system include: a sync "clipper", an integrating circuit, a vertical sync amplifier, a blocking oscillator and a vertical output tube.
Figure 88 shows the circuit detail s of this combination.
Tube T1, a sharp cut-off pentode (6SH7), operates at low plate and screen potentials and is self-biased by grid circuit rectification of the video signal impressed through C1 and R1. Under these conditions, only the positive peaks of the input signal cause output plate current. Thus the sync pulses are "clipped" from the video signal.
The sync signals are passed to the vertical sync amplifier tube (T2) through a three stage integrating circuit (R5-C4, R6-C5, R7-C6). The sync pulses, having been inverted in polarity through tube T1, arrive at the grid of tube T2 as a negative voltage swing which drives the tube to cut-off. This results in positive "going" pulses in the plate circuit which are of the proper polarity for the control of the blocking oscillator circuit. It will be noted that tube T2 is given a small initial positive bias from the "B" plus supply through a one megohm resistor (R8). Since this tube is a high-mu triode (6SL7GT) these operating conditions result in a large swing of plate volt age under control of the sync pulse. It will be noted that the entire swing of the plate voltage of tube T2 is applied to the secondary (L2) of the blocking oscillator feedback transformer.
The blocking oscillator is conventional in its circuit features. As in the systems previously described, grid circuit resistor (R12) acts as the vertical "hold" control and plate supply resistor (R15) serves as the "size" or "height" control.
The vertical output stage (T4) employs a power pentode (6L6GT) with the feedback method of linearity correction from the cathode circuit. Variable bias resistor (R18) controls vertical "linearity".
Fig. 88. Electromagnetic Deflection Systems--Example 4. Vertical Circuit
Fig. 89. Oscillating Horizontal Output Stage
ELECTROMAGNETIC SYSTEM--EXAMPLE 5. In the deflection system listed as Example 5 of chart Figure 81, the only element which we have not previously considered is the "oscillating output stage" or "beam relaxor". Circuits of this general type, in which the generator itself supplies the deflection current, are also called "current oscillators". Figure 89 shows a version of this type output stage which has been used in a number of models now on the market. In this circuit a beam power pentode ( 6L6) has its control grid connected to the secondary (L3) of the output transformer, with proper polarization to cause oscillation. The screen grid of this pentode is coupled to the plate of a sync pulse clipper and can inject a voltage pulse at the correct time to synchronize the frequency of oscillation with the rate. of the horizontal sync pulses. Since the action of this circuit differs considerably from any which we have previously considered, a step by step analysis of its operation follows:
1. As a starting point, let us assume that the plate current of the tube has just been cut off, by the negative sync pulse at the screen or by the oscillator action itself. This cut-off occurs at the instant that retrace of scanning starts a new line. The magnetic flux stored in the transformer collapses inducing a high negative voltage on the grid. This collapse of magnetic flux, and its resultant voltage change across the horizontal deflection windings, produces the change in polarity necessary for retrace of the scanning wave.
2. A series circuit consisting of R7 (1.8K ohms), L1 (800 microhenries), and C5 (.008 MFD) is connected across this winding.
This network critically damps out any tendency for continued oscillation. Following retrace, the grid potential grows less negative until plate current starts to flow.
3. The rate at which the plate current will flow is determined by:
a. The plate resistance of the tube , which is controlled by the cathode bias network. Since we are now considering the trace or linear part of the cycle, the influence of the variable portion of the cathode circuit resistance (R4) can act as a horizontal "hold" control.
b. The inductive load in the plate circuit.
This consists of the plate to "B" plus part of winding L2 with the reflected effect of secondary grid winding L3.
4. The time constant of a circuit consisting of inductance and resistance is determined by the ratio of L to R. When an inductance is suddenly disconnected from a source of direct current and allowed to discharge its stored energy through a resistor, a change of current, with time, will occur which is identical in shape to the change of the volt age wave discussed in connection with capacitor sawtooth wave forming circuits.
With an effective plate circuit inductance of 60 millihenries and an AC or dynamic plate resistance of 500 ohms , the L/R ratio corresponds to the required time constant for 15,750 cycle scanning. These conditions are met in the circuit of Figure 89.
5. As the sawtooth current wave builds up in the plate circuit, it induces a similar wave, by transformer action, in winding L3, which supplies the horizontal deflection coils and the grid of the ''beam relaxor''.
6. As the current wave increases in amplitude, a point is reached at which the grid voltage has become sufficiently negative, with respect to the cathode, to produce plate current cut-off. It will be noted, by examining the plate current versus plate voltage curves of the 6L6 beam pentode, that there is a sharp "knee" in the curve after which the plate current reaches saturation. In this circuit, the action takes place by sweeping rapidly from the reg ion below the "knee" of the curve to the saturation region. The saturation or flat region corresponds to the active sweep. The low current region below the "knee" corresponds to the retrace period.
In a "beam" pentode, a negative pulse of voltage applied to the screen will cause a sharp increase of applied plate voltage. The pulse is supplied in this circuit, by the pulse clipper tube ( T1). This initiates the retrace cycle and assures "lock-in" with the video signal.
Fig. 90. Automatic Pulse Width Control of Blocking Oscillator
ELECTROMAGNETIC SYSTEM--- EXAMPLE 6. The circuits of the combination outlined as Example 6 (Figure 81) are similar to those of Example 2, with the exception of a different type horizontal oscillator sync control. While, for simplicity, it has been listed as an A.F.C. control, in reality "pulse width control" would be a more accurate description.
Figure 90 shows the essential details of this system, which include a sync clipper, a control tube , and a blocking oscillator. The other elements of the system have been fully covered.
The pulse output of the clipper ( T1) which appears across the cathode resistor (R2) is positive with respect to ground. This pulse is applied to the grid of control tube (T2) through the differentiating network consisting of C1, C2, and C3. The action of the remainder of the circuit involves simultaneous events which can best be described in sequence:
1. The control tube (T2) fulfills a number of functions, namely:
a. Since a portion of its cathode circuit, resistor R6, is common to the grid circuit of the blocking oscillator tube (T3), it is able to affect the frequency of oscillation by influencing the time constant of the circuit comprised of C10, R11 and R6 (paralleled by C8 and C9).
b. The voltage drop across R6, due to the plate current of tube T2, can be controlled by potentiometer R8, which adjusts the plate voltage. R8 constitutes the horizontal "hold" control.
c. Tube T2 serves as a means of automatic frequency control of scanning by "mixing" the original pulse from tube T1, a signal fed back from the output of the blocking oscillator tube (T3), and a pulse from the horizontal deflection coils through R15 and C12.
2. The pulse voltage derived from the deflection coil is that which occurs at the instant of retrace. It is "sharpened" by network R15 and C12. This pulse is negative with respect to the original sync pulse.
3. A small portion of the sawtooth voltage in the plate circuit of the blocking oscillator tube (T3) is also fed back through resistor R14 to the grid of control tube (T2). This voltage is positive (the same polarity as the original sync pulse) and is combined with the sharp negative pulse ??? m the deflection coil. The result of this combination produces a sawtooth ·which has a very sharp or vertical return.
4. The combination voltage just described is applied to the control tube grid. It will coincide with the original sync pulse only if the oscillator is in exact step with the video sync pulses.
5. As exact synchronism is reached, the control tube grid pulse, which consists of the original sync pulse added to the fed back pulse, will be narrow and of high amplitude. If the fed back pulse is slightly fast or slow it will not add to the original sync pulse, but instead will have the effect of widening the original pulse.
6. From this combination of variable width and height of the voltage pulse on the grid of tube T2, a very precise timing is achieved.
7. The plate current pulse of tube T2, flowing through cathode resistor R6, adjusts the grid circuit time constant of the blocking oscillator tube T3 and produces the required exact synchronization.