The peculiarities of power requirements in the several parts of representative television systems make necessary the use of power supplies more complicated than those normally found in other radio and electronic devices. In general, it is necessary to provide for at least two separate and basically different load conditions.
1. A low-voltage, high-current system to power oscillator, amplifier, and similar stages where applied potential does not exceed approximately 450 volts. In addition, some receivers power modulation and deflection systems from this source.
2. A high-voltage, low-current system to supply the accelerating anode potential, and, in some applications, DC deflection potentials, for the cathode-ray or picture tube.
LOW-VOLTAGE, HIGH-CURRENT POWER SUPPLY: What might be termed the signal reception portion of the receiver, which includes sound and video amplifier or control tubes, presents a power requirement not greatly different in voltage range from that of other radio devices, and, therefore, this portion of the television receiver power supply system is very similar to those used in large radio sets.
In general, the voltage requirement is no more than the conventional 450-volt value. The current required, however, is frequently much greater than that necessary for radio operation: and, in addition. there is also a necessity for rather good supply regulation. This need is occasioned by the operation of ''sawtooth'' oscillators for deflecting the electron beam of the cathode-ray tubes from this source. These oscillators tend to produce currents in the power supply system, which , if not properly filtered, would appear as serious modulation hum in the beam control and sound circuits.
!t is not unusual to encounter power supply currents for the low-voltage application of as much as 300 milliamperes. Television receivers produced to date may employ from one to three rectifiers for this low-voltage, high-current supply, the number, of course, dependent upon the current requirement and the designer's preference as to voltage and current distribution. Thermionic and selenium rectifiers have been used up to this time in circuits which largely duplicate those for equivalent radio set power supplies, including full-wave transformer types and transformerless half wave doubler designs.
Figure 29 provides schematic diagrams of representative low--voltage , high-current supply systems.
Fig. 29. Left--Low Voltage Transformer Type Supply. Right--Half-Wave Doubler
Low Voltage Supply
HIGH-VOLTAGE, LOW-CURRENT POWER SUPPLIES: The high-voltage supply differs considerably from the supply just discussed in that the current requirement is very small, usually in the neighborhood of 300 micro- amperes, while the voltages employed may range to extremely high values, especially in television receivers using projection-type cathode-ray tubes. Receivers currently in production may use accelerating potentials from 3,000 to 30,000 volts.
SAFETY PRECAUTIONS: Since these high voltage power supplies represent extremely dangerous shock hazards, perhaps it might be well to consider normal precautions to be followed in working with them.
First, locate your bench well away from metal objects, or wiring, which might provide an accidental contact to ground or a voltage source. Don't use metal bench tops. ff you have a concrete floor, it is essential that it be covered with a substantial rubber mat, or other good insulating material, in an area sufficient to preclude any possibility of stepping off the mat during normal service operations. The mat isn't a bad idea even if you don't have a concrete floor.
Second, don't attempt high-voltage measurements unless they are actually necessary.
It is the general opinion, on the basis of experience to date, that resistance measurements are adequate to identify power supply troubles in at least 90% of all failures.
In the event that it is necessary to measure high potentials, do so in the approved manner. With power switch off and plug disconnected, hook up the test lead to the ground or low potential side of the circuit. ff a clip lead is to be used for connection to the high potential side of the circuit, make this connection with power off, using one hand only, so that if a residual charge remains there will be no possible circuit through the body. It is usually advisable to make sure that the high-voltage supply is completely discharged before making any connection to the television receiver.
If a probe-type instrument is to be used with the power supply operating, always use one hand only for placement of the probe to the test point. Keep the other hand in the pocket so that there is no possibility of contact with the result of establishing a circuit through the heart.
In other service operations, such as alignment, etc., it is advisable to disable the high-voltage circuit.
Try to detect the fault by individual component diagnosis. There are nine mental operations to one manual operation in the testing of any electronic circuit. Think--then act! You owe it to yourself and your family to take all necessary precautions.
TYPES OF HIGH VOLTAGE SUPPLIES: At the time of going to press, there are four basically different types of high-voltage power supplies being used in commercial television receivers:
A. The conventional, or "brute force," 60-cycle half-wave power supply system.
B. The r-f power supply system which uses an r-f oscillator as a high frequency volt age source, steps up this voltage through a suitable RF transformer, and the n rectifies it for application to the accelerating anodes.
C. The "horizontal flyback" or "kick back'' type high--voltage supply employing the pulse voltage generated by the collapsing field in the primary of the horizontal deflection transformer. This pulse voltage is stepped up, rectified, and supplied to the accelerating anodes.
D. The pulse-type power supply using a blocking oscillator whose pulses then trigger the plate current of a power tube. This sudden change of plate current is fed to a transformer, thence to a rectifier or a series of rectifier-doublers or triplers to produce the high voltage.
"BRUTE FORCE" POWER SUPPLY: Almost all pre-war and some post-war television receivers use the half-wave "brute force" power supply. In theory and operation, it is not essentially different from power supplies commonly used in ordinary radio receivers with the exception that the power transformer must be adequately insulated for high voltages. The half-wave circuit is employed for the obvious reason that through its use the high voltage transformer may be held to the lowest possible number of turns and smallest physical size.
The fact that the filter must be effective for a 60-cycle hum or ripple , instead of the 120 cycle ripple of the full-wave supply, is not particularly bothersome since the current requirement is relatively small.
Fig. 30. "Brute Force" Type High Voltage Power Supply
A schematic diagram of a representative "brute force" power supply appears in Figure 30. Note that this system employs one trans former to supply all voltages for receiver operation. Winding ''A'' supplies the heater current for the type 2X2 rectifier. Any tube used in this application must be capable of withstanding extremely high inverse voltages.
Winding '' B'' supplies plate potential for the 2X2 rectifier and ultimately the accelerating anode voltages. Capacitor C1 is the input filter unit for the nigh voltage supply, resistor R1 the filter resistor, and C2 the output filter capacitor. Through a bleeder network, the various voltages are picked off for tube control functions, such as centering, focusing, and brilliancy. The remainder of the power supply represents conventional full-wave design practice with the exception that the outputs of all branch supplies are additive.
Figure 31 shows a variation of the brute force power supply to provide approximately 12,000 volts DC through the use of a half-wave doubler circuit. A pair of 2X2 rectifiers is used in a conventional doubler system fed by a high-voltage winding of the power transformer.
This application uses a separate high-voltage supply transformer.
RF POWER SUPPLY SYSTEM: The RF Oscillator High-Voltage Power Supply is frequently used, especially where electrostatic deflection is employed. It is compact, requiring only two tubes, and is independent of deflection system of the receiver. Power is generated by an r -f oscillator operating at frequencies ranging be tween 50 and 500 KC, the r-f output is stepped up through transformer action to several thousand volts, and then rectified. Due to the low current drain on the power supply, the output voltage is nearly equal to the peak voltage applied to the rectifier.
Fig. 31. High Voltage Supply Employing
Doubler Circuit
The oscillator usually employs a power output type tube which is capable of generating 10 to 15 watts of r-f energy and is normally connected as a tuned -plate oscillator with tickler feedback. The plate circuit is tuned to the natural resonant frequency of the high-volt age winding, providing a minimum of load on the oscillator circuit. One of the features of this power supply is the fact that any change of capacitance in the circuit, which would result if a hand were inadvertently placed near it, will reduce the output of the oscillator and lessen the danger of a high-voltage shock. Nevertheless, all aforementioned precautions should be taken when making any voltage measurements and care should be taken not to get too close to the cap of the rectifier tube as a severe r-f burn may result.
The 8016 or 1B3GT tube, which is used as the rectifier, requires only one quarter watt of power for heating the filament and since this is only a small percentage of the power generated by the r-f oscillator, the filament voltage is taken from an additional secondary winding of one or two turns on the oscillator coil. Obtaining the filament voltage in this manner eliminates the need for a large iron-core transformer with high-voltage insulation.
Care should be taken that the position of this winding is not changed because any change in coupling would result in an increase or de crease in filament voltage. Since the voltage applied to the filament is r-f and there is no practical method of measuring its heating efficiency, a reasonably accurate check may be made by a visual comparison of the brilliance of the heater on a similar tube with its filament connected across a 1.6 volt dry-cell battery.
The filament of the 8016 tube is quite easily paralyzed by a momentary overload and it is suggested that another tube be tried if trouble is suspected in the rectifier circuit.
Due to the frequency of the r-f voltage and the low-current drain (approximately 200 to 400 microamperes) on the power supply, a very small value of capacitance is required for filtering the output voltage. Likewise a large value of resistance up to 500K ohms may be used as a filter, further lessening the chance of a lethal shock.
This power supply has good regulation and there is no need for additional voltage regulation circuits. Even with a varying current from 0-200 microamperes, occurring as the electron beam in the tube is modulated, there is less than 5% fluctuation in voltage, which is a permissible range.
A schematic of an RF High-Voltage power supply is shown in Figure 32. A type 12A6 tube, connected as a triode , is used as the oscillator, feedback being obtained from a tickler winding in the grid circuit. The voltage is stepped up in the secondary winding and fed to the plate of the rectifier tube, rectified and filtered. This particular receiver has an additional secondary winding which is connected to the plate of the 6X4 rectifier tube, providing additional "B" plus voltage for use in the vertical and horizontal sweep generators.
Although most receivers employ a tickler winding for obtaining a feedback to sustain oscillations, another method in use eliminates the need of a separate winding on the trans former. Feedback is obtained through capacitive coupling to the plate of the rectifier by placing a spring around the rectifier tube at the exact position giving correct feedback. The position of this spring is critical and instructions for positioning it should be followed closely.
Fig. 32. RF Oscillator-Rectifier High Voltage
Supply System
Fig. 33. Typical RF High Voltage Supply (Shield Cover Removed)
RF power supplies should be shielded to prevent radiation into the receiver. Such radiation causes "birdies" in the sound channel and r -f bars to appear on the screen. This shield also gives protection against shock.
Figure 33 shows a typical complete RF high-voltage power supply system.
HORIZONTAL FLYBACK HIGH VOLTAGE SUPPLY SYSTEMS: This method of obtaining high voltage makes use of the high-voltage pulse created in the plate circuit of the horizontal amplifier during retrace time. In using this system, relatively few additional components are required beyond those that would normally be necessary, since all magnetically deflected receivers make use of a matching transformer between the horizontal output and the horizontal deflection coil.
Another reason for employing this system is that it guards against modulation of the video signal by stray energy from the high-voltage supply, since the screen is blanked out during the retrace time.
The addition of two windings to the horizontal matching transformer makes possible the use of this system. When the quick collapse of plate current through the horizontal deflection amplifier takes place, due to the sawtooth of voltage on its grid, the primary winding, which is part of the plate circuit of the horizontal deflection amplifier, will have produced across it a relatively high pulse voltage be cause of self induction.
By adding an auto-transformer winding to the primary of this deflection transformer, the pulse voltage may be stepped up to any desired value. This high voltage is then fed to the plate of a hot cathode type rectifier, rectified and filtered , and becomes the high accelerating potential for the picture tube.
The second additional winding on this transformer consists of one or two turns, which provide the filament power for the rectifier tube. This is possible since the current requirements of the particular tube developed for this purpose is low.
Figure 34 shows a representative Horizontal Flyback System.
The transformer itself is of unique design employing pressed powdered iron in the form of a shell about the windings. It is possible to use powdered iron for this transformer since the horizontal scanning frequency is 15,750 cycles per second. The time of one cycle is, therefore, approximately 63 microseconds. Of this, 53 microseconds are used up in the forward scan and the remaining 10 are employed for flyback and starting the next horizontal line.
The transformer resembles a design which one would expect for the handling of power at low radio frequencies. The windings are of the universal type and are well impregnated.
The parts of a typical horizon t al flyback transformer are shown in Figure 35. Since the frequency is high and the current drain low, the filtering required on this high-voltage supply is very small. Actually, the output filter capacity is often realized by using the capacity between the outer and inner layers of aquadag coating on the picture tube.
Fig. 34. Horizontal Flyback High Voltage System
Fig. 35. Typical Horizontal Flyback Trans former ( Photo from Sample Courtesy
RC A)
Fig. 36. Typical Horizontal Flyback High Voltage Assembly. (Photo from Sample
Courtesy Emerson Radio and Phono. Corp.)
Fig. 37. Pulse Type High-Voltage Power Supply
Fig. 38. Horizontal Flyback High-Voltage Tripler.
Figure 36 is a photograph of a complete Horizontal Flyback High-Voltage System.
PULSE TYPE HIGH-VOLTAGE POWER SUPPLY: A second method has been developed for generating the high voltage during the horizontal retrace time. This system uses essentially the same type of transformer and rectifier as discussed under "Horizontal Fly back High-Voltage systems , " except the transformer has no secondary deflection coil winding. It derives its pulse voltage from a blocking oscillator, which is triggered by the horizontal flyback decay. These oscillations are then amplified and fed to the primary of an auto-transformer whose output is connected to the plate of the rectifier. Voltage regulation is usually obtained by controlling the amplitude of the pulse fed to the rectifier. A typical schematic of this type system is shown in Figure 37.
In this application, a portion of the high DC voltage output is fed to the grid of half of the 6SN7, which has a type VR105 in its cathode circuit. The plate current of the 6SN7 is drawn through the 15K ohms screen-dropping resistor of the type 807 amplifier stage. A change in plate current will change the screen voltage of the 807, thus regulating the amplitude of the pulse supplied to the auto-transformer.
HORIZONTAL FLYBACK SYSTEM FOR PROJECTION TELEVISION USING VOLTAGE TRIPLERS: In the systems previously described, a single rectifier tube was employed to obtain voltages ranging up to 10,000 volts.
Projection tubes require potentials of between 25,000 and 30,000 volts, and such voltages cannot be obtained readily by single rectifiers due to limitations of the tubes themselves. To overcome this difficulty the power supply systems for projection tubes employ a principle of voltage multiplication in which a number of capacitors are individually charged to the peak voltage of the system through respective rectifier tubes associated with the capacitor.
Figure 38 shows a power supply of this type.
The transformer is similar in design to the one already described and illustrated in Figure 35, but differs in that it has three filament windings for three individual type 8016 high-voltage rectifier tubes.
Another difference in this system is the use of two horizontal output tubes, type 6BG6G, connected in parallel to provide the additional energy required.
The diagram shows a "ladder" arrangement of rectifier tubes, condensers and resistors to accomplish this voltage multiplication. This circuit is somewhat different from the familiar "common line" type of voltage doubler or tripler, in that the individual capacitors need a voltage rating no greater than the peak supply voltage, whereas, in the familiar multiplier circuits, the voltage ratings increase in each stage.
The voltages shown on the diagram are measured from ground. The operation of the circuit is as follows: A pulse produced across the primary of the transformer is applied to tube V1, and the rectified current of this tube charges capacitor C1 to approximately peak value of the pulse.
In the interval between pulses, capacitor C1 discharges into capacitor C2 through resistor R1. Since rectifier tube V2 is conductive in the proper direction, capacitor C3 is charged by capacitor C2. Capacitor C3 can then charge capacitor C4 through resistor R2. The final step of the multiplication consists of the charge of capacitor C5 through rectifier tube V3 from the charge existing in C4.
This series of events will require a number of cycles of operation of the horizontal oscillator for each of the capacitors to assume their final charges. When a steady state condition is reached, the charge across each of the capacitors in this group will be approximately the peak voltage of the supply system. Capacitors C1, C3 and C5 in series provide the output voltage used for the accelerating voltage of the projection tube.
Another pulse-type high-voltage supply, which was developed in Holland and recently introduced in this country, is shown in Figures 39 and 40.
Fig. 39. Pulse Type High-Voltage System.
(Hallicrafters-Norelco) Photo Courtesy North American Phillips Company, Inc.
It is an entirely self-contained unit measuring 8-1/2x7x4-1/ 2 inches, which can be mounted adjacent to the projection tube and supplied with heater and plate voltages from the television chassis by a three-lead cable.
While a total of five vacuum tubes is used in this system, only two of the tubes can be seen upon removal of the perforated cover.
The other three tubes are specially designed, mi nature -size rectifiers and are hidden from view in the sealed, oil-filled, transformer assembly. The circuit components comprising the assembly are s how n enclosed by dotted lines in the schematic diagram Figure 41. The photograph in Figure 40 shows this unit with the can removed.
Fig. 40. Transformer -Rectifier Assembly with cover removed. (Hallicrafters-Norelco)
Photo Courtesy of The Hallicrafters Company
Fig. 41. Schematic Diagram of Hallicrafters-Norelco High Voltage Power Supply
The circuit shown in Figure 41 comprises a blocking-type oscillator employing the triode section of a 6SR 7 to produce a "sawtooth" voltage whose frequency is approximately 1000 cycles per second. This waveform, represented by Vg in Figure 42, is coupled to the grid of the 6BG6G power amplifier, which is biased be yond cutoff by a combination of cathode self bias and an additional voltage from an automatic regulating circuit to be described later.
Plate current, Ip in Figure 42, in the 6BG6G tube flows only for a short portion of the positive peak of the sawtooth wave applied to the grid. This positive excitation is made sufficiently great to cause plate current pulses almost equal to the maximum emission of the tube.
These pulses of plate current flow through part of the primary of an iron-core transformer.
The current in the transformer is represented by iL in Figure 42. Note that the current wave form across the transformer at the time of conduction of the 6BG6G contains no oscillatory component. However, when the tube is cut off, the field generated by the plate current in the transformer collapses. Since the tube is now cut off, there is no heavy damping, due to plate current flow, and the coil will start oscillating at its natural resonant frequency. This resonant frequency is governed by the inductance in the transformer and the distributed capacity in the transformer and associated circuits. The resonant frequency of this particular supply is approximately 30KC. These 30KC oscillations, shown in the iL waveform of Figure 42, will continue oscillating until plate current is drawn through the winding. This heavy current flow completely damps out the oscillations and an other cycle is started. Note that at the time the oscillatory circuit is free running, the oscillations are damped. This is due to the loading of the rectifier filament windings and the losses in the transformer.
The voltage is stepped up by auto-transformer action and applied to the rectifiers. The waveform of this volt age is shown in Vo of Figure 42. Since the high voltage is additive to the B+ supply, represented by Vb, Vmax. is equal to the peak voltage plus Vb.
The first positive oscillation peaks, which are approximately 8,500 volts, charge the capacitor C1, through V1, to peak voltage. When the oscillation peaks are negative, the voltage across C 1 (negative on the lower plate) is additive to the peak voltage of the source, and C2 is charged to two times the peak voltage through V2. On the next positive peaks, the conductive path will be through C1, the source, C2 and V3. Since the voltage across C1 is equal to the source and of opposite polarity, C3 will charge to a peak voltage equal to that of C2. It can now be seen that voltage across C1, which is peak voltage, and the voltage across C3, which is two times peak voltage, are of the correct polarity to be additive and result in an output equal to three times peak voltage.
The additional feedback winding, mentioned previously, provides an ingenious method of improving the voltage regulation of the system. Normally, a power supply system is improved in regulation by reducing its internal resistance and providing more storage of energy by increasing the capacity of the filter capacitors. Both of these expedients tend to increase size, weight, and cost, and in a high voltage system has the additional disadvantage of making the device more dangerous from the standpoint of electrical shock.
The voltage from this fourth winding is fed back to the diode plates of the 6SR7 blocking oscillator tube. The rectified current from these diodes passes through a filter network and is added to the bias of the 6BG6G amplifier tube. Thus, when the voltage of the system tends to drop due to greater output load, the bias of the amplifier tube is made less negative and the duration of the pulse is increased. This tends to increase the output.
Fig. 42. Voltage and Current Waveforms -- Pulse Type Power Supply
With constant input supply voltages to this power pack, the output voltage is 25,500 volts plus or minus 2,000 volts at no load, and drops less than 600 volts at a load of 60 micro-amperes and less than 1,200 volts at a load of 125 microamperes.