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How electronic things work Electronic equipment ''on the blink''? Don''t junk it or pay sky-high repair costs - fix it yourself! Here''s a guide to understanding and repairing electronics equipment written for people who would ordinarily ''call the shop''. With this fully illustrated, simple-to-use guide, you will get a grasp of the workings of the electronics world that surrounds you - and even learn to make your own repairs. And you may even start enjoying it! Whether you want to pocket the savings on repair bills, give your beloved equipment the best possible care, or merely understand how it all works, this book will show you how in easy-to-understand language and clear illustrations - and you don''t need any technical experience. Written by a technician who has fixed virtually everything that plugs into a wall, this handy do-it-yourself introduction to home and office repair delivers: clear explanations of how things work, written in everyday language; easy-to-follow, illustrated instructions on using test equipment to diagnose problems; guidelines to help you decide for or against professional repair; tips on protecting your beloved equipment from lightning and other electrical damage; and, lubrication and maintenance suggestions. This is an ''Electronics 101'' for true beginners. Next time your equipment acts up, don''t get mad. Get it working - with a little help from this book. This book features how to understand (and fix): color TVs, DVDs, wireless cellular phones and PDAs, radios, speaker systems, audio/video tuners, CD players, monitors, camcorders, copiers, and fax machines.
COLOR TV RECEIVER OPERATION 129 4 10 8 5 9 7 6 3 1 2 L505 8aH Horiz. sweep & HV transformer FIGURE 4-12 The Sencore’s OVM meter has input protection to measure the Horizontal Osc. Driver Output Horiz. syn. input B+ power supply To ringer Flyback transformer FIGURE 4-13 The ringer test hook-up for finding shorts and open conditions P-P high-voltage pulses in this horizontal sweep and high voltage circuit. in a flyback transformer. A “bad” reading, less than “10” rings, may be caused by a circuit connected to the fly- back that is loading down the ringer test. Disconnect the most likely circuits in the follow- ing order: 1 Deflection yoke. 2 CRT (picture tube). Unplug the socket connection. 3 Horizontal output transistor collector. 4 Scan-derived supplies. Retest the flyback after you have disconnected each circuit. If the flyback now rings “good,” it does not have a shorted winding. If the flyback still checks out bad after you have disconnected each circuit, unsolder it and completely remove it from the circuit. If the flyback primary still rings less than 10, the flyback is defective and must be replaced. Testing the high-voltage diode multipliers During normal TV/monitor operation, a large pulse appears at the collector of the horizontal output transistor. The output connects to the primary of the flyback transformer and the pulses are induced into the flyback’s sec- ondary. The pulses are stepped up and rectified to produce the focus and high voltages. These voltage pulses are rectified by high-voltage diodes contained in the flyback package or in an outboard diode multiplier package. Because these are high-voltage components, it is often difficult to determine dynami- cally if the diodes will break down under high-voltage conditions. The Sencore analyzer has a special test to determine if these diodes are good or bad. HIGH-VOLTAGE PROBLEMS It is only necessary to do this test if all of the following conditions are met: 1 The high voltage or focus voltage is low or missing. 2 The B+ and peak-to-peak voltages at the horizontal transistors are normal. 3 The horizontal sweep (flyback) transformer passes the ringer test. With the analyzer, feed a 25-volt peak-to-peak horizontal sync drive signal into the pri- mary winding of the flyback transformer. The step-up section of the transformer and the high-voltage diodes should develop a dc voltage between the second anode and high-voltage resupply pin on the flyback transformer. Measure this voltage with a dc voltmeter. Look up this voltage on the schematic to determine if the high-voltage diodes are good or bad. HORIZONTAL OSCILLATOR, DRIVER, AND OUTPUT PROBLEMS If the horizontal yoke, flyback, multiplier, horizontal output transistor, and B+ supply have tested good, but the TV still lacks deflection or high voltage, the horizontal driver circuit might be defective. A missing or reduced-amplitude horizontal drive signal could prevent the TV 130 HOW COLOR TVS AND PC MONITORS WORK from starting and operating properly. Use the Sencore analyzer’s horizontal drive signal to iso- late problems in the horizontal drive circuit. Refer to the signal injection points in Fig. 4-14. TV start-up problem 1 Before injecting into the horizontal drive circuit, test the flyback and yoke, the high- voltage multiplier, the horizontal output transistor, and the +3 - V supply. 2 When injecting at the output transistor, disconnect the secondary winding of the driver transformer from the base. Inject the horizontal drive signal into the driver circuit. Watch for horizontal deflection on the picture tube. If it returns, you are injecting after the defective stage. If nothing hap- pens, inject the horizontal drive signal at the base of the horizontal output transistor. Refer to Fig. 4-15 for these injection points. COLOR TV RECEIVER OPERATION 131 Horizontal stages Inject horizontal test pulses into horizontal driver circuit Horizontal sync. Osc. Driver Output To horizontal output stage FIGURE 4-14 The injection points for test pulse injection into the 50 51 52 53 Horizontal Inject horizontal sync. H orizontal sync. Osc. Driver Output B+ Power supply Yo k e Multiplier Flyback 1 FIGURE 4-15 Horizontal drive signal test injection points. The base of the horizontal driver circuit. output driver transistor is a good injection point. How to measure the TV’s high voltage The picture tube (CRT) requires a very high dc voltage to accelerate the electrons toward the screen of the CRT. This voltage is developed in the secondary winding of the flyback transformer and is amplified and rectified by the integrated diodes in the flyback, or by a separate multiplier circuit. Measuring the high voltage at the second anode of the picture tube lets you know if the output circuit, sweep transformer, high-voltage multiplier, and power-supply regulation circuits are working properly. Additionally, some TVs and computer monitors have adjust- ments for setting the high voltage and focus voltage correctly. Use extreme care when measuring and adjusting any voltage around the picture tube and high-voltage power supplies. Blurred, out-of-focus picture symptom For this problem, first measure the picture-tube high-voltage capacitor with a high-voltage probe. Compare these readings with the HV readings shown in the schematic. Also, if these voltages are OK, check the focus voltage and suspect that the CRT is weak. Switching transformer checks Switching transformers are used in power-supply cir- cuits to step voltages up or down. They are one of the most common components to fail in switch-mode power supplies. Open windings are easy to find with an ohmmeter, but shorted turns are nearly impossible to locate with conventional test methods. The Sencore video analyzer’s ringer test helps you to locate switching transformer with open or shorted windings. For this test, the switching transformer must be removed from the TV’s circuit. To perform this test, connect the Sencore analyzer ringer test leads across a winding on the switching transformer. A reading of 10 rings or more will show that the winding does not have a shorted turn. Perform this same test on all windings of the switching trans- former. THE VERTICAL SWEEP SYSTEM In my feedback from many electronic technicians, most say that the vertical sweep sys- tems are among the most difficult circuits in a monitor or TV to troubleshoot. Even the most small change in a component can cause reduced sweep deflection, nonlinear deflection, or picture fold-overs. These symptoms can be caused by a small circuit part or an expensive vertical yoke. Thus, you must think carefully of a strategy to take the guesswork out of iso- lating vertical sweep problems. How vertical deflection works Knowing how the vertical sweep deflection circuits oper- ate requires an understanding of picture tube beam deflection. The electron beam travels to the face of the picture tube striking the phosphor surface coating to produce light on the front of the picture tube. 132 HOW COLOR TVS AND PC MONITORS WORK If the stream of electrons travels to the face plate of the tube without any control from any magnetic or electrostatic field, the electrons will strike the center of the screen and produce a white dot. To move this dot across the face of the picture tube screen requires that the electrons be influenced by an electrostatic or magnetic field. In video display tubes, a magnetic field is produced by the vertical coils of a yoke mounted around the neck of the tube. The yoke is constructed with coils wound around a magnetic core material. When current flows in the vertical yoke coil windings, a magnetic field is produced. The yoke’s core concentrates the magnetic field inward through the neck of the picture tube. As the electrons pass through the magnetic field on the way to the tube’s face plate, they are deflected (pulled upward or downward) by the yoke’s changing magnetic field. This causes the electron stream to strike the picture tube face plate at points above and below the screen center. To understand how electrons are deflected requires a review of the interaction of mag- netic fields. As you refer to Fig. 4-16A and 4-16B, you might recall that an individual elec- tron in motion is surrounded by a magnetic field. The magnetic field is in a circular motion surrounding the electron. As electrons travel through the magnetic field of the yoke, the magnetic fields interact. Magnetic lines of force in the same direction create a stronger field, but magnetic lines in opposite directions produce a weaker field. The electrons are then pulled toward the weaker field. The direction of the current in the yoke coil determines the polarity of the yoke’s mag- netic field. This determines if the electron beam is deflected upward or downward. How far the electrons are repelled when passing through the yoke’s magnetic field is determined by the design of the yoke and the level of current flowing through the ver- tical coils. The higher the current, the stronger the magnetic field and resulting electron deflection. A requirement of vertical sweep deflection in a TV or monitor is that the current in the coils of the vertical yoke increase an equal amount for specific time intervals. This linear COLOR TV RECEIVER OPERATION 133 Time Yoke current current + 0 Electron beam Electron beam magnetic field Time Yoke current current + 0 Electron beam deflection path Deflection yoke magnetic field A B FIGURE 4-16 The yoke mounted on the CRT neck produces the magnetic field, resulting in electron beam deflection. current change causes the deflection of the electron beam from the top to the center of the picture tube faceplate. The waveforms shown in Fig. 4-16A and 4-16B represent a current increasing and decreas- ing in level, with respect to time. Figure 4-16A shows the current increasing quickly and then decreasing slowly back to zero. This would cause the electron beam to quickly jump to the top of the picture tube screen and then slowly drop back to the center. Figure 4-16B shows the current increasing slowly in the opposite direction and then decreasing quickly back to zero. This would cause the electron beam to slowly move from the center to the bottom of the picture tube faceplate and then return quickly to the center. During normal TV or monitor operation, the yoke current increases and decreases (Fig. 4-16A and 4-16B). The current changes directions alternating between the illustra- tions at approximately 60 times per second. The alternating current moves the electron beam from the top of the picture tube faceplate to the bottom and quickly back to the screen’s uppermost area. How the vertical drive signal is developed The vertical circuit stages of the TV are responsible for developing the vertical drive signals. This signal is fed to an output ampli- fier, which produces alternating current in the vertical deflection yoke. The vertical section consists of four basic circuits or blocks (Fig. 4-17). These include: 1 Oscillator or digital divider. 2 Buffer/pre-driver amplifier. 3 Driver amplifier. 4 Output amplifier. The circuitry for these stages can be discrete components on the circuit board or might be included as part of one or more integrated circuits. 134 HOW COLOR TVS AND PC MONITORS WORK Oscillator Buffer predriver Driver Output amp Vertical size Vertical hold Vertical linearity Feedback Cs Rs Vertical yoke coils FIGURE 4-17 The vertical section of a TV receiver consists of an oscillator, buffer, driver, and output amplifier. The vertical oscillator generates the vertical sweep signal. This signal is then fed to the amplifiers and drives the yoke to produce deflection. Vertical oscillators can be free run- ning or the more modern digital divider generators. These free-running oscillators use an amplifier with regenerative feedback to self gener- ate a signal. More common types are RC (resistance-capacitance) oscillators associated with ICs or discrete multivibrator or blocking oscillator circuits. A digital divider generator uses a crystal oscillator. The crystal produces a stable fre- quency at a multiple of the vertical frequency. Digital divider stages divide the signal down to the vertical frequency. You will usually find most of the digital divider oscillator circuitry located inside an integrated circuit. The output of a vertical oscillator must be a sawtooth-shaped waveform. A ramp genera- tor is often used to shape the output waveform of a free-running oscillator or digital divider. A ramp generator switches a transistor off and on, alternately charging and discharging a capacitor. When the transistor is off, the capacitor charges to the supply voltage via a resistor. When the transistor is switched on, the capacitor is discharged. The vertical oscillator must then be synchronized with the video signal so that a locked- in picture can be viewed on the picture tube. The oscillator frequency is controlled in two ways. 1 A vertical hold control might be used to adjust the free-running oscillator close to the vertical frequency. 2 Vertical sync pulses, removed from the video signal, are applied to the vertical oscilla- tor, locking it into the proper frequency and phase. If the oscillator does not receive a vertical sync pulse, the picture will roll vertically. The picture will roll upward when the oscillator frequency is too low and downward when the frequency is too high. Several intermediate amplifier stages are between the output of the vertical oscillator and output amplifier stage. Some common stages are the buffer, predriver, and/or driver. The purpose of the buffer amplifier stage is to prevent loading of the oscillator, which could cause frequency instability or waveshape changes. The predriver and/or driver stages shape and amplify the signal to provide sufficient base drive current to the output amplifier stage. Feedback maintains the proper dc bias and waveshape to ensure that the current drive to the yoke remains constant as components, temperature, and power-supply voltages drift. These stages are dc coupled and use ac and dc feedback, similar to audio amplifier stages. Notice that ac feedback in most vertical circuits is obtained by a voltage waveform derived from a resistor placed in series with the yoke. The small resistor is typically placed from one side of the yoke to ground. A sawtooth waveform is developed across the resistor as the yoke current alternates through it. This resistor provides feedback to widen the fre- quency response, reduce distortion, and stabilize the output current drive to the yoke. This vertical stage feedback is often adjusted with gain or shaping controls, referred to as the vertical height or size and vertical linearity controls. The dc feedback is used to stabilize the dc voltages in the vertical output amplifiers. The dc voltage from the output amplifier stage is used as feedback to an earlier amplifier stage. Any slight increase or decrease in the balance of the output amplifiers is offset by slightly COLOR TV RECEIVER OPERATION 135 changing the bias. Because the amplifier’s waveforms are slightly distorted, the bias change will shift the bias on the output transistors, somewhat, thus bringing the stage back into compliance. Much of the difficulty in troubleshooting vertical stages is caused by the feedback and dc coupling between stages. A problem in any amplifier stage, yoke, or its series components alters all of the waveforms and/or dc voltages, making it difficult to trace the problem. Vertical picture-tube scanning The vertical output stage produces yoke current that then pulls the electron beam up and down the face of the picture tube. The vertical yoke might require up to 500 mA of alternating current to produce full picture tube deflection. A power output stage is now required to produce this level of current. A current output stage commonly consists of a complementary symmetry circuit with two matched power transistors (Fig. 4-18). The transistors conduct alternately in a push- pull arrangement. The top transistor conducts to produce current in one direction to scan the top half of the picture. The bottom transistor conducts to produce current in the oppo- site direction to scan the bottom part of the picture. Most vertical output stages are now part of an IC package and are powered with a single positive power supply voltage. The voltage is applied to the collector on the top transistor. 136 HOW COLOR TVS AND PC MONITORS WORK Cs Rs Qb Qt B+ Cs Rs Qb Qt B+ Cs Rs Qb Qt B+ Cs Rs Qb Qt B+ Yoke current Output voltage Time A Time B Time C Time D FIGURE 4-18 The deflection currents and waveforms during four time periods of the vertical cycle. In this balanced arrangement, the emitter junction of the transistor should measure about one half of the supply voltage on this stage. In series with the vertical yoke coils is a large- value electrolytic capacitor. This capacitor passes the ac current to the yoke, but blocks dc current to maintain a balanced dc bias on the output amplifier transistors. To better understand how a typical vertical output stage works, let’s walk through the current paths at four points in time, during the vertical cycle illustrated in (Fig. 4-18). Starting with time A, the top transistor, Qt is turned on by the drive signal to its base. The transistor is biased on, resulting in a low conduction resistance from collector to emitter, which provides a high level of collector current. This puts a high plus (+) voltage potential at the top of the deflection yoke, resulting in a fast rising current in the yoke. During time A, capacitor Cs charges toward a positive (+) voltage and current flows through the yoke and the top transistor, Qt. This pulls the picture tube’s electron beam from the center of the picture tube up quickly to the top. During time A, an oscilloscope connected at the emitter junction displays a voltage peak, shown as the voltage output waveform in Fig. 4-18. The inductive voltage from the fast-changing current in the yoke and the retrace “speed-up” components cause the voltage peak to be higher than the posi- tive (+) power supply voltage. The current flowing in the deflection yoke during time A produces a waveform, as viewed from the bottom of the yoke to ground. This is the voltage drop across Rs, which is a reflection of the current flowing through the yoke. During time B, the drive signal to Qt slowly increases the transistor’s emitter-to-collector resistance. Current in the yoke steadily decreases as the emitter-to-collector (E-C) resistance increases and thus reduces the collector current. The voltage at the emitter junction falls during this time and capacitor Cs discharges. A decreasing current through the yoke causes the picture tube’s electron beam to move from the top to the center of the screen. To produce a linear fall in current through the yoke during time B demands a crucially shaped drive waveform to the base of Qt to meet its linear operating characteristics. The drive waveform must decrease the transistor’s base current at a constant rate. Thus, the transistor must operate with linear base-to-collector current characteristics. These reduc- tions in base current must result in proportional changes in collector current. At the end of time B, transistor Qt’s emitter-to-collector resistance is high and the tran- sistor is approaching the same emitter-to-collector resistance as the bottom transistor, Qb. Capacitor Cs has been slowly discharging to the falling voltage at the emitter junction of the output transistors. Just as the voltage at the emitter junction is near one half of the pos- itive (+) supply voltage, the bottom transistor begins to be biased ON to begin time C. This transition requires that the conduction of Qt and Qb at this point be balanced to eliminate any distortion at the center of the picture-tube screen. During time “C”, the resistance from the collector to emitter of transistor Qb is slowly decreasing because of the base drive signal and the increase of collector. The signal passes from capacitor Cs through the yoke and Qb. As Qb’s resistance decreases and its collector current increases, the voltage at the emitter junction decreases. This can be seen on the voltage output waveform as it goes from one half positive (+) supply voltage toward ground during time C. The current increases at a linear rate through the yoke, as shown in the yoke current or voltage across Rs waveform (Fig. 4-18). The resistance decrease of Qb must be the mirror opposite of transistor Qt’s during time B. If not, the yoke current would be different in amplitude and/or rate, causing a difference COLOR TV RECEIVER OPERATION 137 in picture-tube beam deflection between the top trace and bottom trace times. At the end of time C, the emitter-to-collector resistance of Qb is low and Qb is slowly decreasing by the base increase of collector begins to discharge, producing current as the deflection yoke approaches a maximum level. At the start of time D, the emitter-to-collector resistance of Qb is increased rapidly and collector current will decrease. This quickly slows the discharging current from capacitor Cs through the yoke and transistor. As the current is reduced, the trace is pulled quickly from the bottom of the screen back to its center. Time A begins again and the cycle is repeated again. This should now give you an overall view of how the hori- zontal and vertical sweep and scanning system produces a picture on your TV or com- puter monitor. The basic inner workings of the color TV and PC monitor have now been covered. Another very important part of the color TV is the portion that you look at, the picture tube (cathode- ray tube or CRT). The working of the color picture tube The CRT works by producing (emitting) steady flow of electrons from the electron gun at the base (neck) of the CRT. These electrons are attracted to and strike the phosphor-coated screen of the CRT, causing the phosphors to emit light. Deflection circuits and a yoke outside the CRT produce a changing magnetic field that extends inside the CRT and deflects the beam of electrons to regularly scan across the entire face of the CRT, lighting the entire screen. The CRT can be divided into three functional parts (Fig. 4-19): 1 The electron gun cathode assembly. 2 The electron gun grids. 3 The phosphor screen and front plate. The color picture tube is the last component in the video chain that lets you actually view a color picture on your TV or monitor. The major sections of a color set have previously been explained in this chapter, so now see how the CRT develops a color picture. 138 HOW COLOR TVS AND PC MONITORS WORK Red gun Green gun Blue gun Deflection yoke Shadow mask Beam convergence at shadow mask Phosphor dots on glass faceplate FIGURE 4-19 An inside view drawing of a picture tube that has an in-line gun assembly, metal dot mask, and phosphor dot triads on a glass faceplate. [...]... delta gun arrangement with a dot shadow mask As shown in Fig 4-20A and 4-20B, the metal mask has evenly spaced holes with RGB phosphors clustered on the glass faceplate in groups of three However, this triad arrangement had convergence problems because the three beams could not be made to meet at the shadow mask holes for certain areas of the faceplate How the electron CRT gun works The electron gun... shows the relationship between shadow mask, electron guns, and the phosphors on the tube’s faceplate As you refer to Fig 4-20A, notice that each beam converges through a hole in the shadow mask, while approaching the hole at a slightly different angle Because of these different angles, the red beam hits the red phosphor, the blue beam the blue phosphor, and the green beam hits the green phosphor However,... typical projection color TV receiver and DBS dish input connections A front view of a typical large-screen projection set is shown in Fig 4-25 This type of TV projects the picture image onto the back of a translucent (Fresnel) screen that can then be viewed from the front As shown in Fig 4-26, the inside view these sets have three separate red, green, and blue (RGB) projection tubes to produce a bright... handle with wing lock-nut provisions Rotation of the focus handle changes the longitudinal position of the lens’ B element 144 HOW COLOR TVS AND PC MONITORS WORK Speaker Speaker RGB crt gun assy FIGURE 4-26 A front view with viewing screen removed of a rear projection color TV set, showing component locations FIGURE 4-27 Drawing of a front screen projection TV set This unit can set on a table or be hung... units are used in home theater installations Projection set lightpath profile A side view of the TV lightpath is shown in Fig 4-28 Note the tight tuck of the lightpath provided by the Delta 7 compact optics For comparison purposes, the lightpath profile of an earlier model projection set is shown in Fig 4-29 Liquid-cooled projection tubes The rear-screen projection TVs use three projection tubes (R, G,... FIGURE 4-34 The picture pulled down from top and bottom This is usually a vertical sweep circuit problem 150 HOW COLOR TVS AND PC MONITORS WORK What to do: I Check the vertical sweep output stage components I It could also be a shorted winding in the vertical coils of the deflection yoke This might show up as keystone raster shape I Check and adjust the vertical hold control I Check and adjust vertical... and problems just covered, you will need a pro- fessional TV technician to solve or correct them You can take a small TV into the service shop However, the large-screen or projection TVs will need to be repaired by a professional servicer in your home 154 HOW COLOR TVS AND PC MONITORS WORK FIGURE 4-39 The picture is unstable, moves up and down, and tends to slip sideways Picture cannot be locked in... majority units 156 HOW COLOR TVS AND PC MONITORS WORK Most sets now marketed use terms such as HDTV-ready, digital-ready, or HD-compatible This term does not indicate the TV set can produce a digital signal, only that it has a jack available to plug in a set-top decoder Most of these types of sets do have enhanced screen resolution DIGITAL VIDEO FORMATS There are several video formats; however, the most... flowchart illustrating these events of the data stream is shown in Fig 4-40 Satellite systems already are transmitting digital HDTV signals Direct TV has two HDTV channels now and plans more in the near future Digital satellite systems will have a head start in sending out high-definition TV over the conventional TV stations NOTES ON COMPATIBILITY Electronics service technicians and TV reviewers are concerned... more “crisp” and detailed picture than the conventional TV Q: How are these HDTV signals received? A: In most locations you should be able to receive HDTV with any standard UHF antenna The exact style of antenna that is required for optimal reception may vary depending on your geographic location and distance from the TV tower Consult your electronic distributor for advice for selecting the optimal antenna . the picture tube faceplate. The waveforms shown in Fig. 4- 16A and 4- 16B represent a current increasing and decreas- ing in level, with respect to time. Figure 4- 16A shows the current increasing quickly. pack 9-5 24- 03 Video input 9-253-03 Stereo decoder Red crt Green crt Blue crt A- 140 37 Secondary control A-12752-05 AFC switch 9 -41 7-01 Stereo interface 85-1735 Membrane keyboard 9 -44 2 Tuner control FIGURE. images on computer displays consist of lines with sharp transi- 140 HOW COLOR TVS AND PC MONITORS WORK FIGURE 4- 21 This photo shows the GE in-gun assembly and the adjustments used for convergence. tions