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CONTENTS AT A GLANCE
The Color TV Signal Color TV signal standards
Color TV Receiver Operation The “head end” or tuner section The IF stages and video
amplifier/detectors Video detector Video amplifiers Luma delay line
The chroma processing circuits The chroma and luminance stages Color-killer circuit operation Sandcastle circuit operation Functions of the sync circuits Vertical sweep deflection operation Horizontal sweep deflection operation Sound converter stage operation Sound IF amplifier operation The sound (audio) detector Audio amplifier stage TV power-supply operation Sweep circuits and picture tube
operations
Deflection yoke problems Key voltage readings
Inoperative computer monitor problem
Another monitor problem High-voltage problems
Horizontal oscillator, driver, and output problems
The vertical sweep system Large-screen projection TV
operation
What To Do When Your TV Has Problems
Problems and what actions you can take
Digital/HDTV TV Operation Overview
HDTV picture quality Set-top converter box Digital video formats Digital television signal Notes on compatibility
Introduction to the DTV Delivery Systems
HDTV formats and modes Will NTSC broadcasts dissapear?
Standard-definition and high-definition basics
Digital/HDTV questions and answers
The Color TV Signal
Before looking at the block diagram operation of a typical color TV, see what the color TV signal (which comes to your TV via antenna, cable or DBS dish) energy components contains.
The video signal that comes from the TV transmitter is an electrical form of energy that en- ters the free space in some form of electromagnetic waves. No one at this time has an expla- nation of what makes up this electromagnetic form of energy. It travels at the speed of light.
For this transmitted wave to carry intelligence, it must be varied (modulated) in some way.
Your color TV is thus receiving from the TV transmitter visual images in full color, as well as audio sound. The U.S. color TV system must be compatible with black-and-white television standards. Compatibility is when the system produces programs in color on color TVs and black and white on monochrome TVs. Conversely, color TVs will receive B&W pictures when they are being transmitted.
The color TV signal contains two main components, luminance (black & white or brightness) information, and chrominance (color) information, which is added to the lumi- nance signal within the color receiver circuits to produce full-color pictures.
The transmitted color TV signal contains all of the information required to accurately reproduce a full-color picture. The U.S. standard-frequency TV channel width for a color picture is 6 MHz.
The color signal contains not only picture detail information, but equalizing pulses, hor- izontal sync pulses, vertical sync pulses, 3.58-MHz color-burst pulses, blanking pulses, and VITS and VIRS test pulses. The horizontal blanking pulse is used to turn off the elec- tron scanning beam from the gun in the picture tube, at the end of each scan sweep line.
Vertical blanking pulses are used to blank out the beam at the top and bottom of the pic- ture, so you will not see any of the transmitted pulses used for picture control and testing.
The color (chrome) is phase-and-amplitude encoded relative to the 3.58-MHz color burst and is superimposed on the black-and-white signal level. This video information is used to control the three electron beams (red, green, and blue) that scans from left to right with 525 horizontal lines across the face of the picture tube. How the color picture tube works is explained later in this chapter.
COLOR TV SIGNAL STANDARDS
The signals from the color TV transmitter are reproduced on the screen of the TV to closely match those of the original scene. The black-and-white and color signals have an FM (frequency modulated) sound carrier and an AM (amplitude modulated) video carrier with a channel bandwidth of 6 MHz.
The portion of the black-and-white signal that carries the video picture information is called the carrier amplitude, which is the brightness or darkness of the original picture information modulation. Now, a portion of the color video signal that carries picture information con- sists of a composite of color information and amplitude variations. A unique feature is the 3.58-MHz color syncburst, which is located right back of the horizontal sync pulse. This color pulse is often referred to as “setting on the back porch of the horizontal sync pulse.” Every horizontal scan line of video color information contains the picture information, horizontal blanking, sync pulses, and a color burst of at least eight cycles.
The picture information in a color TV signal is then obtained from the red, green, and blue video signals that the color TV camera generates as it scans the scene to be transmit-
116 HOW COLOR TVSAND PC MONITORS WORK
ted. The luminance (Y) and chrominance (C) signals derived from the basic red, green, and blue video color signals have all of the picture information required for transmission. They are then combined into a single signal by algebraic addition. The final color product contains a chrominance signal, which provides the color variations for the picture, and the luminance signal, which provides the variations in intensity or the brightness of the colors.
The three different color TV broadcast standards in the world are not compatible with each other. In the United States, the NTSC standard was devised by the National Television System Committee and in 1953, was approved by the FCC (Federal Communications Com- mittee). The NTSC system is now used in the USA, Canada, Japan, and other countries.
Most of Europe uses the PAL system, and the SECAM standard is used in Russia. Before the year 2006, all TV video signal transmissions will be changed over gradually to com- pressed digital video and sound format. This system was approved by the FCC in 1997.
In Chap. 5 you will find more information on how flat screen TV/Monitors, high-definition TV (HDTV), and home theater “large screen” receivers operate.
At the conclusion of this chapter you will find an overview of the operation and some changeover notes for the new digital HDTV television system that is now being intro- duced nationwide.
Color TV Receiver Operation
Now, follow a block diagram of a typical modern color TV receiver and see how the var- ious sections work together to produce a color picture.
THE “HEAD END” OR TUNER SECTION
The color TV signal enters the TV receiver at the electronic tuner. This is where you change stations on your TV, via an antenna, cable system, or a DBS satellite dish. Refer- ring to the overall TV block diagram in (Fig. 4-1), you will note the electronic tuner has RF amplifier, oscillator, and mixer stages that convert the RF carrier signal to a 45.75- MHz signal IF that is fed to the video IF stages or strip. Most modern TV tuners are re- motely controlled and have AFC (Automatic frequency control) to lock in the TV. Figure 4-2 shows the electronic tuner mounted in a late-model color TV.
THE IF STAGES AND VIDEO AMPLIFIER/DETECTORS
After amplification and signal processing in the IF stages, the video and audio signals are found at the output of their detectors. The audio has now been detected in the 4.5-MHz au- dio IF section and stereo audio is fed to the right and left audio power amplifiers, then onto the speakers. Some TVs also have MTS/SAP and DBX decoders.
VIDEO DETECTOR
An AM (amplitude modulation) detector, also called a diode detector, converts the picture frequency within the video IF stages down to a video frequency. Right after this detector is the 4.5-MHz sound trap to remove a frequency that would result in a heterodyning of the
COLOR TV RECEIVER OPERATION 117
118
4.5MHZ audio if-mts/sap 4.5MHZ filter
4.5MHZ if amp
Audio det.
mts/sap decoder DBX decoder
Right audio
Audio amps Left audio
R L Ext. audio inputsExt. audio outputs Luma / chroma 1st luma amp 1st
chroma amp
2ndchroma amp
2nd luma amp Burst gateACC Colorkill
Color demod
matrix Shift 90
matrix matrix Subcarrier gen. afpc
3.58 osc
Tint
Y R G B BiasFocus High voltage
CRT
CRT drivers Yokes Vert.Horiz. Flyback/IHVT TriplerHigh voltage Focus HV return power supply CRT filament HV shutdownHoriz. B+ inhibit
Horizontal
OSC.BufferDriverOutput
Vertical HAFCOSC.Driver
Out/damper Common Horiz. flyback pulse
Sandcastle Sync. separator
Vert. integrator Horiz. difference Sync.
Horiz. flyback pulse
V&H sync. Luma in
S. video input video inputvideo output Remote control receiver
AGC Horiz. flyback pulse on/off Main power supply
45.75 MHz video if
1st if amp
If saw filter2nd & 3rd if ampVideo det.
AFC VHF, UHF, CABLE, TUNER RF ampOSC Mixer Video Y/C Filter EXT.
EXT. EXT.
Y/C switch Video buffer amp
Y/C (comb) filter
luma chromadelay B+ Low voltages
Audio switch EXT.
EXT. FIGURE4-1A block diagram of color TV/monitor system.
picture and audio frequencies of the video IF circuits. Without this trap, dark bars would appear across the picture and cause hum in the audio.
VIDEO AMPLIFIERS
The video amplifiers amplify the video from the detector to levels great enough to drive other circuits.
LUMA DELAY LINE
This is a device that electronically delays the Y signal (monochrome video), or luminance signal about 0.8 microseconds to match the delay in the chrome circuits. The reason for this is that the color signal is delayed more than the luminance because more circuits are used to process the color signal and they are tuned to a narrower-frequency bandwidth.
THE CHROMA PROCESSING CIRCUITS
From the video detector, the signal goes to some video-processing stages. You will find a comb filter, used to produce sharper pictures and the delay line. Chroma and video signals along with vertical and horizontal sync signals are also obtained from the video stages.
COLOR TV RECEIVER OPERATION 119
FIGURE 4-2 The electronic tuner “head end” used in a TV.
THE CHROMA AND LUMINANCE STAGES
The composite video signal from the detector is fed to the first chroma amplifier and also to the luminance (B&W) amplifier in the luma/chroma block. The 3.58-MHz oscillator and subcarrier generator is used to extract the chrominance signals. This is accomplished in the color demodulator stage. These signals go to the matrix circuits and then onto the cathodes of the color (CRT) picture tube guns. This block also contains blanking circuits, burst gate, and color killer stages.
COLOR-KILLER CIRCUIT OPERATION
The color-killer circuit has a couple of basic functions. Its performance during black-and- white picture transmission is to keep high-frequency signals or noise from being amplified via the chroma amplifiers. This keeps the color snow or confetti from being seen within the picture. It also keeps you from seeing color rainbows from around fine detail and edges of a black-and-white picture. This stage is also used to kill the color signal during weak or snowy signal conditions. Thus, the killer circuit must know the difference between the color burst signal and interference or noise conditions.
SANDCASTLE CIRCUIT OPERATION
Most modern color TVs have a sandcastle circuit located within an IC. The sandcastle cir- cuit is a special device used by design engineers to inject three mixed signals into one pin of this IC. The IC separates these three signals and uses them for various internal func- tions. The three signals are the horizontal sweep pulses, a delayed horizontal sync pulse, and a vertical sweep pulse.
After separation inside the IC, the horizontal sweep, also called flyback pulses, provides horizontal blanking for the output signals of the chip, and the delayed sync pulses sepa- rates the color burst from the back porch of the horizontal sync pulse, and the vertical pulse provides vertical blanking. If you would look at this pulse on an oscilloscope that is developed from this chip, it appears as a sandcastle, thus the name given this circuit. If one of the input pulses is missing or the IC is defective, the TV screen will be blanked out.
FUNCTIONS OF THE SYNC CIRCUITS
The basic function of the TV sync circuits is to separate the horizontal and vertical sync pulses from the video signal. These separated pulses are then fed to the horizontal and ver- tical sweep stages to control and lock-in the color picture. These circuits need good noise immunity to maintain good, stable vertical and horizontal picture lock in.
Some color sets have the sync and AGC (automatic gain control) circuits combined.
Normally the AGC circuit develops a bias in proportion to the sync pulses peak-to-peak level, which is then used as a dc voltage to control TV receiver gain. Keyed AGC circuits are used because they provide better noise immunity.
VERTICAL SWEEP DEFLECTION OPERATION
The vertical sweep oscillator stage receives a sync pulse from the vertical integrator stage, which forms and develops this pulse. This sync pulse keeps the vertical oscillator running
120 HOW COLOR TVSAND PC MONITORS WORK
at the vertical scanning rate. Some sets have digital countdown and divider circuits to per- form this task. The oscillator feeds the buffer and driver stages. The output current from the vertical power amplifier stage is then applied to the vertical winding of the deflection yoke, which is located around the neck of the picture tube. A pulse from the vertical out- put stage is used for picture tube screen blanking. Some of these pulses can also be used for convergence of the three color beams in the gun of the picture tube.
HORIZONTAL SWEEP DEFLECTION OPERATION
Older color TVs have a horizontal circuit that consists of a sawtooth generator that would drive the horizontal sweep and high-voltage generating transformer. This circuit is con- trolled by an AFC (automatic frequency control) circuit that compares the frequency of the oscillator with the sync pulse coming from the TV transmitter and then produces a correc- tion dc voltage for any oscillator frequency drift that might occur. The deflection current for the horizontal yoke coils, along with picture tube high voltage and focus voltage, plus other pulses, are generated by the horizontal sweep transformer. The horizontal sweep and HV stages need very good voltage regulation to produce a good sharp color picture.
Figure 4-3 shows various adjustment controls found on most TVs. These are horizontal hold (Horiz. Hld.), brightness level, vertical hold (Vert. Hld.), vertical height/vertical lin- earity, and sometimes a color killer and AGC adjustment.
Modern color TVs use a digital countdown divider system to generate and control pulses to drive the horizontal sweep stage. Figure 4-4 shows the horizontal sweep and
COLOR TV RECEIVER OPERATION 121
FIGURE 4-3 Some of the adjustment control locations on TV receivers and some monitors.
high-voltage transformer section. Figure 4-5 shows the high-voltage lead and cup that plugs into the picture tube HV button. This lead will carry a voltage of 25,000 to 32,000 volts. Use caution because this voltage can still be present at this cap—even when the TV is turned off.
The horizontal output stage in these modern TVs are usually of a pulse-width design that not only sweeps the electron beam across the picture tube, but also develops other dc voltages to operate other circuits in the color TV chassis. This eliminates the heavy weight and costly price tag of a power transformer and also improves the efficiency of the ac power and current that the TV uses. A safety high voltage shutdown circuit is also used.
SOUND CONVERTER STAGE OPERATION
This stage converts the audio frequency in the video IF passband circuit (41.25 MHz) down to the sound IF frequency of (4.5 MHz) by heterodyning (beating) the picture and sound frequencies together and tuning to the different frequency. This stage is usually a diode detector and a 4.5-MHz tuned circuit.
SOUND IF AMPLIFIER OPERATION
This stage is sometimes called an audio IF amplifier. This stage is used for amplifying the 4.5-MHz sound IF signal to a level that is high enough to be detected by the sound detector.
122 HOW COLOR TVSAND PC MONITORS WORK
FIGURE 4-4 The high-voltage transformer found in a TV set.
THE SOUND (AUDIO) DETECTOR
The sound detector is also called the audio detector. This part of the circuit converts the FM-modulated 4.5-MHz sound IF frequency to an audible sound frequency that drives an audio amplifier stage(s).
AUDIO AMPLIFIER STAGE
This circuit is used to amplify the audio frequencies from the detector to power levels great enough to drive the speaker(s). If the TV receiver is MTS equipped, there might be two amplifiers (stereo) and the MTS-decoding circuits will precede the audio amplifiers.
TV POWER-SUPPLY OPERATION
As with any electronic device, the power supply is the heart of the device and makes all the other systems operate. If the TV is dead or not working properly, look at and check out the power-supply section first. The problem could be simple, such as an off/on switch that is defective, a blown fuse, or tripped “off” ac breaker for the wall socket. And check the con- dition of the power cord and be sure that it is plugged into the wall socket. Figure 4-6 shows the location of the main TVs power supply fuse located on the circuit board. Replace fuse with the same current rating if it has blown. If it blows again, suspect a short circuit or power-supply problems.
COLOR TV RECEIVER OPERATION 123
FIGURE 4-5 Location of the picture tube’s high-voltage anode connection. Use caution when working around this section of a TV set or monitor.
The transistor power supply regulator heatsink is shown in Fig. 4-7. If the regulator or sweep output transistors are defective (shorted), they might cause the main fuse in the power supply to blow.
This should now give you an overview of how these blocks and circuits in a color TV work together and what could go wrong. Take a closer look at the circuit operation within some of these blocks and see how they work and what to do when they don’t.
SWEEP CIRCUITS AND PICTURE TUBE OPERATIONS
The following section of this chapter shows some advanced troubleshooting of color TV and computer monitor circuits. The horizontal and vertical sweep circuit operations and troubleshooting will be all worked in together.
Vertical sweep circuit operation You can use the following information for color TV and computer monitor troubleshooting to isolate the vertical oscillator, driver, and sweep
124 HOW COLOR TVSAND PC MONITORS WORK
FIGURE 4-6 The main power clip-in fuse used in some TV sets.
output stage problems. The vertical driver and output stages amplify the vertical oscillator signal, which provides the current drive needed for the vertical deflection of the yoke. A defective driver circuit, output stage, or yoke can cause loss of deflection, reduced height, or vertical linearity picture problems. Before you use signal injection to troubleshoot a ver- tical sweep problem, use a dc voltmeter (DVM) to confirm that you have proper bias volt- ages on the output stage components. The vertical stages are usually dc coupled to get good linearity. A wrong dc voltage affects all the components in the oscillator, driver, and output stages. A dc bias problem must be repaired before you can effectively use signal injection in the vertical stages. Use an analyzer, such as the Sencore VA62 or TVA92, to inject vertical and horizontal sweep signals into the circuit (Fig. 4-8).
Collapsed vertical raster This problem will show up as a thin white horizontal line across the screen (Fig. 4-9). To isolate the trouble, inject the analyzer’s vertical drive signal into the output of the vertical driver circuit (Fig. 4-10).
COLOR TV RECEIVER OPERATION 125
FIGURE 4-7 The heatsink for the regulator transistor and fuse that could blow if the transistor is shorted.