Before discussing further about color TV, try to think about how it is possible for us to hear a radio broadcast or for us to see and hear a TV broadcast? This is called telecommunications (long distance communication). This one-way communication can occur because there is a transmitter and receiver and each has requirements that must be met for the communication to occur. The requirements are: the information sent in the form of sound (on the radio) or sound and images (on the TV) is carried by a carrier signal, which we know as modulation (the circuit is called a modulator) at a certain frequency. On the radio there are two ways of modulating, namely AM (amplitude modulation) and FM (Frequency Modulation), while on the TV with the FM system. This modulation frequency is what allows us to capture a radio station or TV station broadcast. When we search for the frequency wave of a broadcast, it means that we are matching the frequency of our receiver with the frequency of the transmitter. So even though there are many radio and TV broadcasts everywhere that are captured by the radio / TV receiver antenna at home, we can only hear or see one transmitter station at a certain frequency. If we want to listen or see another transmitter station, then we have to search by changing our receiver frequency (tuning) which is adjusted to the frequency of the transmitter we are looking for. This is a one-way telecommunication process, one only transmits while the other receives.
Television is a device for capturing image broadcasts. The word television comes from the words tele and vision; which have the meanings of far (tele) and visible (vision). So television means visible or can see from a distance. The discovery of television is aligned with the discovery of the wheel, because this discovery was able to change world civilization. In Indonesia, 'television' is informally called TV, tivi or teve. The beginning of television certainly cannot be separated from the basic discovery, the law of electromagnetic waves discovered by Joseph Henry and Michael Faraday (1831) which was the beginning of the era of electronic communication. Then successively discovered the cathode ray tube (CRT), black and white television system, and color television system. Of course the development of this science will continue to advance, especially with the discovery of LCD, which makes TVs in this era thinner with picture results that are no less good than tube TVs.
So in this era we must know very well about TV because almost all households have TV either black and white or color. Are you ready to learn it?
The television (TV) that we know consists of two types, namely:
- Black and white television (figure 6.67).
- Color television (figure 6.68).
Figure 6.67: Example of a Black and White TV
Figure 6.68: Example of a Color TV
On a black and white television, the image cannot be seen according to its original color. Whatever is seen on the screen only appears in black and white. This is very different from a color television, namely the color of the image that appears on the screen will look like the original.
The images we see on the television screen are the result of a camera's production. The image objects captured by the camera lens (figure 6.69) will be separated based on three basic colors, namely red (R = red), green (G = green), and blue (B = blue). The results will be transmitted by the television transmitter.
Color TV transmitters transmit signals:
- Audio (sound)
- Luminance (image brightness)
- Chrominance (color)
- Vertical / horizontal synchronization
- Burst
Figure 6.69: Image taken by camera and transmitted to TV
In a color television set, all natural colors that have been separated into the basic colors R (red), G (green), and B (blue) will be mixed back in the color matrix circuit to produce a luminance signal Y and two chrominance signals, namely V and U according to the following equation:
Y = +0.30R +0.59G+0.11B\
V = 0.877 ( R - Y )\
U = 0.493 ( B- Y )
In addition to images, television transmitters also carry sound signals that are transmitted together with the image signal in frequency modulation (FM) to avoid noise and interference. The television transmitter systems that we know include: NTSC, PAL, SECAM, and PAL B. NTSC (National Television System Committee) is used in the United States, the PAL (Phases Alternating Line) system is used in England, the SECAM (Sequential Coleur a'Memorie) system is used in France. Meanwhile, Indonesia itself uses the PAL B system. The things that distinguish these systems are: image format, carrier frequency distance, and sound carrier.
Color TV Working Principle
The complete block diagram of a color TV is:
Figure 6.70: Block Diagram of a Color TV Receiver
Figure 6.71: Example of a Color TV Series
Figure 6.72 shows a color TV block diagram, as follows:
Figure 6.72: Block Diagram of a Color TV Receiver
In general, the block has the following functions:
a. Tuner Circuit
An example can be seen in Figure 6.73. The tuner circuit functions to receive incoming signals (TV waves) from the antenna and convert them into IF frequency signals.
Figure 6.73: TV Tuner
The tuner has three main parts as follows:
- RF Amplifier. Functions to amplify the signal received by the antenna.
- Local Oscillator. Functions to generate high frequency signals. The oscillator frequency is always made larger than the RF frequency received by the antenna (as much as the frequency-RF+IF).
- Mixer. By the mixer the RF signal and the oscillator signal are mixed to produce an intermediate frequency or IF. PAL tuners generally have an IF frequency of 38.9MHz, but some have a frequency of 38MHz, while NTSC tuners have an IF frequency of 42.75MHz.
b.. IF (Intermediate Frequency) Amplifier
This circuit (figure 6.74) functions as an amplifier of the output signal produced by the Tuner up to 1,000 times, because the Tuner output is a weak signal and is very dependent on the distance of the transmitter, the position of the receiver, and the landscape. This circuit is also useful for removing other unnecessary waves and reducing interference from the sound carrier wave that interferes with the image.
Figure 6.74: IF amplifier
c. Video Detector Circuit
This circuit functions as a composite video signal detector that comes out of the image IF amplifier. In addition, this circuit also functions as a damper for all disturbing signals because if there are other signals that enter, it will result in poor image quality. One of the signals that is dampened is the sound signal.
d. Video Amplifier Circuit
This circuit functions as a luminance signal amplifier originating from the video detector so that it can run a glass screen or CRT (cathode ray tube). In the video amplifier circuit there is also an ABL (automatic brightness level) circuit or automatic light intensity regulator which functions to protect the voltage circuit. high voltage overload caused by strong light on the glass screen.
e.AGC (Automatic Gain Control) circuit
The AGC circuit (figure 6.75 / 76) functions to automatically regulate input gain. This circuit will stabilize the changing television signal input itself so that the output it produces becomes constant.
Figure 6.75: AGC circuit. The red circles indicate the AGC components that are located inside some of the ICs and some of the Tuners.
Figure 6.76: Other Model AGC. Some TV brands have a Stand-alone AGC as Indicated by the Cross Mark
f. TV Wave Receiver Stabilizer Circuit
The TV receiver stabilizer circuit includes AGC and AFT. AGC (automatic gain control) will amplify the signal if the received signal is too weak. Conversely, if the received signal is too large, AGC will automatically reduce the signal. Meanwhile, AFT (automatic fine tuning) or fine tuner will automatically adjust the image carrier frequency of the IF amplifier automatically.
g. Synchronization Deflection Circuit
This circuit consists of four blocks, namely (figure 6.77):
- Synchronization circuit,
- Vertical deflection series,
- Horizontal deflection series,
- High voltage generator circuit.
Figure 6.77: Synchronization Deflection Circuit shown by Black Line Boundary
h. Sound (Audio) Circuit
The sound we hear is the result of the work of this circuit (figure 6.78), the IF carrier signal of the sound will be detected by the frequency modulator (FM). Previously, this signal was separated from the image carrier signal.
Figure 6.78: Sound Circuit
i. Power Supply Circuit (Power Supply)
This circuit functions to change AC voltage to DC which is then distributed throughout the circuit. In Figure 6.79, the power supply circuit is limited by the white line on the PCB and the area inside the red box. The area inside the white line is the input circuit which is the high voltage area (live area). Meanwhile, the area inside the red box is the power supply output which then distributes DC voltage to the entire TV circuit.
Figure 6.79: TV Power Supply Circuit
j. Horizontal Deflection and High Stress
The horizontal deflection circuit (figure 6.80) functions to provide sawtooth current to be fed to the yoke deflection coil, so that the electron beam on the CRT can scan in the horizontal direction correctly. In addition, the horizontal circuit is also used as a high voltage generator for the CRT anode and to generate several types of medium voltage and other low voltages.
Figure 6.80: Horizontal Deflection Circuit. Partly Located Inside the Flyback Transformer
The parts of the horizontal circuit include:
1. Horizontal Oscillator
As a horizontal frequency pulse generator. In the CCIR system the horizontal frequency is 15,625 Hz, and in the FCC system the horizontal frequency is 16,750 Hz.
2. Horizontal Driver
The horizontal driver is used to amplify the horizontal frequency of the oscillator to provide enough current to drive the horizontal output transistor (HOT), so that the HOT transistor acts as a switch.
3. Horizontal Output (HOT)
The horizontal output section serves to provide sawtooth current power to be fed to the horizontal deflection coil. From the HOT transistor it is then capacitively coupled to the yoke deflection coil. In general, the HOT transistor of a color TV gets a DC voltage of around 110 V.
Plyback transformers (FBT, HVT) are installed in the HOT section, utilizing the sawtooth current during horizontal retracement which can induce very high voltages.
4. Horizontal AFC (Automatic Frequency Control)
The image on the TV set must be synchronized with the image from the TV transmitter, therefore horizontal and vertical synchronization is required. The High Pass Filter (HPF) circuit is used to separate the horizontal synchronization signal, this circuit is easily affected by noise, so the horizontal oscillator is always equipped with an AFC circuit, which functions to keep the frequency and phase of the horizontal scanning signal always stable.
In the AFC section, a phase control VR is sometimes installed which functions to adjust the horizontal position of the center.
From the explanation above, for further clarity, a special block diagram is provided for the color section (figure 6.81) as follows:
Figure 6.81: Block Diagram of the Color Section of a TV
The function of each block in Figure 6.81 is:
1. Color Amp
A chrominance amplifier that amplifies a color tone signal (about 4.43 MHz) with a bandwidth of 2 MHz. It contains attenuated (modulated) color difference signals (V and U) and also a burst signal with a horizontal sync pulse.
2. Color splitter (color splitter)
Separates the V signal from the U signal where the V signal is rotated 180º while the U signal is not rotated. In this block there are NTSC and PAL lines and some resistance.
3. V-Demodulator and U-Demodulator
To detect V signal and U signal. This section receives color carrier wave and signal simultaneously and must be in phase with both V signal and U signal. If the received signal is NTSC then the carrier wave entered into V demodulator must be entered in phase 90°, while for PAL signal the carrier wave entered in phase 270°. If the phases of the signal are correct, then these signals will be amplified through this section and the amplification for both signals is not the same.
4. PAL switch
During the incoming NTSC signal, the PAL switch passes the signal from the crystal oscillator without any phase shift. While when there is a PAL signal, the signal is passed with a 180° phase shift, making it 270°.
5. FF (Flip Flop)
The PAL switch is driven by a Flip-Flop or bistable multivibrator. This Flip-Flop is driven by a clock signal called the identification signal which comes from the phase discriminator which is then amplified by an amplifier. In a burst signal, each change in the signal line one to the next always changes its basis, because the phase discriminator also outputs an alternating voltage. During the NTSC signal the voltage is positive, and during the PAL signal the voltage is negative. By using a positive clock signal, the FF is brought to a condition such that the PAL switch rotates the signal 180° during the PAL signals. When the NTSC signal comes in, the horizontal final amplifier sends a clock that makes the FF to another stable condition. So now the PAL switch is in a condition that does not rotate the signal phase.
6. BURST Amp
The burst signal amplification contains the burst signal, the chrominance signal and the pulse from the horizontal final amplifier. The amplifier can amplify only at the time when the horizontal pulse enters the amplifier. The burst signal is also input while the amplifier is amplifying, thus producing the output voltage to control the BURST Amp via ACC and turn off the color via CK.
7. Color Killer (CK)
To suppress the color amplifier when the color difference signal / chrominance because it is receiving black and white broadcasts (contability principle). This color suppression is necessary, so that when receiving black and white, the color part does not amplify the noise signals that will appear on the image screen. However, if there is a color tone signal sent to the amplifier by the burst, a control voltage will be generated so that the color killer does not work (the color killer will work if there is no BURST signal sent).
8. ACC (Automatic Color Control)
This block works the same as AGC, namely controlling the gain automatically, if the burst signal increases due to the increase in the gain of the color killer, the BURST Amp produces an ACC voltage which is the steering voltage sent to the color amp.
9. Demodulator (V and U)
To separate the color difference from the SPW made in this circuit. Here, an SPW of 4.43 MHz must be made from the demodulator crystal whose phase is the same as that sent during the NTSC line reception, the SPW is shifted 90º while during the PAL line reception the SPW must be shifted 270º. The demodulator results which are still V signals and U signals are re-amplified so that they change again into the color difference RY and BY (which is the reverse process of the transmitter).
10. AFPC (Automatic Frequency and Phase Control)
It functions so that the phase and frequency of the SPW are exactly the same as those sent (even though they are suppressed), so control must be carried out, especially the VCO voltage.
Color TV Damage Tracking
The easiest and most reliable technique to track damage to a color TV is to use the Symptom-Function Technique, because you can clearly see the symptoms of image damage that occurs on the screen / CRT and the symptoms of sound damage on the speaker.
For example: assume that the video (TV picture receiving) drive transistor is broken. This means that there will be no picture on the CRT. Does this also mean that there will be no raster? Of course not, because the raster is produced by the vertical and horizontal deflection circuits and requires a high voltage, which is obtained from the horizontal output of the transformer. So the CRT will light up but will show a blank screen. Does the audio have any effect? Of course not because the audio signal starts coming out before the video drive circuit. To conclude then the truth that there is no picture on the CRT, but there is sound and raster, it is certain to suspect one of them, namely the video drive or video output stage.
Below is a table of various symptoms of damage to a color TV and an estimate of which circuit function caused the damage to occur.
a.TV is completely dead (indicator light is not on)
Power supply circuit. Input to output regulator circuit. Look at figure 6.82 regulator circuit on TV PCB. In general, television power supplies have voltage outputs of 115v, 24v, 12v, and 5v.
Figure 6.82: Arrows Indicate Fragile Components
b. The TV and indicator lights are completely dead and the sound of the switching transformer vibrating is heard.
Horizontal circuit (figure 6.83), usually the ones that are easily damaged are the flyback transformer, horizontal transistor and capacitor.
Figure 6.83: Red Area Lines Show Components That Are Easily Damaged in Horizontal Circuits
c. The indicator light is on but the TV cannot be operated.
- Horizontal series
- Regulator circuit, usually the voltage limiting diode is damaged.
d. No raster but sound is normal (screen remains dark)
Video amplifier circuit, light amplifier circuit, high voltage circuit (figure 6.84) or CRT (figure 6.85).
Figure 6.84: High Voltage Area
Figure 6.85: CRT (Cathode Ray Tube) Filament Breaks Easily
e. Raster one horizontal line.
- The vertical circuit and its oscillator.
- Vertical deflection series.
Figure 6.86: One Line Raster
f. Black stripes on the layer that cannot be removed.
- Horizontal oscillator circuit, usually dry electrolytic capacitors (look dull / cracked).
- It is rarely found on new TVs, usually because the components are old.
Figure 6.87: Black Stripes Cannot Be Removed from Raster Even Though Synchronization Has Been Set.
g.Part of the image is shifted horizontally
Sync circuit, video buffer circuit and AGC circuit. Usually dry electrolytic capacitors or leaking diodes.
Figure 6.88: Horizontal Displacement
h. The image moves continuously up/down
Vertical oscillator circuit. The new TV occurred due to its ceramic capacitor leaking.
Figure 6.89: Rolling Up / Down
i. The black line is slanted and moving up/down continuously.
Sync separator circuit, sync amplifier circuit, AGC circuit and noise cancellation circuit.
Figure 6.90: Black Line Moves Continuously.
j. Narrowed image
Power supply output circuit, horizontal deflection circuit and yoke coil.
Figure 6.91: Narrowing Left / Right
Figure 6.92: Horizontal Region
k. Horizontal Widening
Horizontal width control potentiometer, power supply circuit and CRT anode voltage.
Figure 6.93: Widened Image
l. Shortening the image height
Vsize and Vline potentiometers and vertical deflection circuit (transistors).
Figure 6.94: Shortened Image
m. Vertical elongated image
Vertical deflection circuit, vertical regulator potentiometer or dry elko.
Figure 6.95: Longitudinal Image
n. Low image contrast
The mixer circuit reaches the video amplifier circuit.
Figure 6.96: Difference between light and dark is less clear
o. Slanted lines or mesh patterns appear on the image.
External interference, such as nearby radio transmitters. Keep the antenna away from sources of interfering frequencies.
Figure 6.97: Thin Slanted Line
p. TV picture appears blue/red/green/cyan/yellow only.
RGB circuit (resistor value increased / transistor damaged), try setting Vr on RGB or CRT.
Figure 6.98: Image color is missing.
q. Good picture but no sound.
Audio circuit between audio IF and speakers
r. The image on the screen is not clear but still colorful; the sound is normal.
Video detector circuit is damaged
Figure 6.99: Image is unclear but color is still there
s. The image on the screen scrolls to the center along the horizontal axis; the sound is normal.
Vertical circuit, usually the capacitor.
Figure 6.100: Partial Image Folded in Vertical Direction
t. The image on the screen is unclear; the color is blurry; the sound is normal.
The video amplifier is damaged, usually the transistor.
Figure 6.101: Unclear Image and Color
u. The image on the screen is black and white; the sound is normal.
The color amplifier is damaged, usually the transistor.
Figure 6.102: Colorless Image
v. Image on screen is broken; sound is normal
The video final amplifier is damaged.
Figure 6.103: Missing Image
w. Raster is spotty, image is missing and sound is hissing (missing).
- The tuner circuit is damaged
- It could also be because the AGC circuit is not working.
Figure 6.104: Mottled Raster
Meanwhile, for TV antennas, the things that need to be considered are as follows:
- TV antennas that are outside the house have a certain lifespan because they are exposed to rain and heat all the time. So if it is fragile, it must still be replaced.
- If the TV picture is blurry, try rotating and pointing the antenna while watching the picture on the TV until the picture is clear again.
- If it remains blurry, try checking the connector on the antenna that is connected to the cable to the TV, most of them are corroded and need to be cleaned with sandpaper.
- There is no special handling of the antenna, the most important thing is that the connection of the cable to the antenna connector and the cable to the TV connector must be good, so that the image produced on the TV is good.
As with the amplifier repair section, TV repairs must also produce a repair report that can later be used as input for other repair technicians. One of the fill-in sheets.
TV Receiver Working Principle
The models and types of TV circuit blocks vary, depending on the brand of TV used. In general, the blocks have the following functions:
1. Television Antenna
TV antennas capture RF signals from television transmitters. Antennas are classified based on their construction, there are 3, namely:
- Yagi Antenna
- Logarithmic Period Antenna
- Loupe Antenna
Another classification based on the frequency band of the waves received is:
- Low VHF Channel
- High VHF Channel
- UHF Channel
Figure 5. Yagi Antenna
Figure 6. Logarithmic Period Antenna & Figure 7. Loop Antenna
2. Tuner Circuit
This circuit consists of a high frequency amplifier (HF amplifier), a mixer and a local oscillator. The tuner circuit functions to receive incoming TV signals and convert them into IF frequency signals.
Tuner Circuit
3. IF (Intermediate Frequency) Amplifier Circuit
This circuit functions as a signal amplifier up to 1000 times. The output signal produced by the tuner is a weak signal and is very dependent on the distance of the transmitter, the position of the receiver and the landscape. The red circle shows the IF circuit which is partly inside the tuner.
Figure 9. IF amplifier
4. Video Detector Circuit
Functions as a composite video signal detector that comes out of the image IF amplifier. In addition, it also functions to dampen sound signals that will result in poor image quality.
5. Video Amplifier Circuit
This circuit functions as an amplifier for the luminance signal originating from the video detector so that it can run a picture tube or CRT (Catode Ray Tube).
6. AGC (Automatic Gain Control) circuit
The AGC circuit functions to stabilize the changing television signal input itself so that the output produced is constant. The red circle shows the AGC component located in part of the IC and part of the tuner.
AGC circuit
7. TV Wave Receiver Stabilizer Circuit
The TV receiver stabilizer circuit includes AGC and AFT. Automatic Fine Tuning functions to automatically adjust the image carrier frequency from the IF amplifier.
8. Synchronization Deflection Circuit
This circuit consists of four blocks, namely: synchronization circuit, vertical deflection circuit, horizontal deflection circuit and high voltage generator circuit.
Figure 11. Vertical Deflection Circuit (Note the Gray Block Line)
Figure 12. Horizontal Deflection Circuit (Note the Gray Block Line)
9. Sound Circuit
The sound we hear is the result of the work of this circuit, the IF carrier signal of the sound will be detected by the frequency modulator (FM). Previously, this signal was separated from the image carrier signal.
Figure 13. Sound Circuit
10. Power Supply Circuit
Functions to change AC current to DC which is then distributed throughout the circuit. In the picture, the power supply circuit is limited by a white line and a red box. The area inside the white line is the input circuit which is a high voltage area (Live Area). Meanwhile, the area in the red box is the power supply output which then distributes DC voltage to the entire TV circuit.
Figure 14. Power Supply Circuit & Power Supply Circuit Schematic
11. Chrominance Amplifier
This amplifier amplifies the frequency of 4.43 MHz for the chrominance signal modulated in the V signal (RY signal) and U signal (BY signal). The amplifier bandwidth is 2 MHz.
12. Color Synchronization
In the color synchronization circuit, the color synchronization burst signal is extracted from the composite color video signal.
13. Automatic Color Control (ACC)
If the amplitude of the burst signal increases, the ACC outputs a steering voltage that reduces the gain in the color section.
14. Color Killer (Color Killer)
This circuit is useful for suppressing the color amplifier, when there is no incoming chrominance signal. This occurs when receiving a black-and-white signal.
15. 180 Phase Switching Circuit (Color Splitter)
From the chrominance amplifier, the signal is fed to the colour splitter. This colour splitter separates the signal modulated with the V signal from the signal modulated with the U signal. The colour splitter consists of a PAL switch and several resistors. At the end of each line, as the PAL line is pulled, the V signal is rotated 180 . The U signal does not experience phase rotation.
16. Color Demodulation
By using a color demodulator, the color difference signals are demodulated from the U and V signals. Because in the transmitter, the signals are modulated with a suppressed carrier system and only the two side band sub carriers are present. In order to demodulate it back into the original color carrier signal, a 4.43 MHz sub carrier is required with the exact same phase and frequency as in the transmitter.
Summary
- The Power Supply provides its voltage to all parts of the amplifier.
- The tuner receives the signal from the antenna and amplifies and converts the received frequency into an IF signal (33.4 MHz and 38.9 MHz). The sub carrier signal is still carried by the Video IF signal.
- The IF amplifier and detector respectively amplify the IF signal and detect the video signal. The audio IF signal is also generated in this detector after the 33.4 MHz and 38.9 MHz IF signals are mixed in the video detector.
- The voice IF signal is amplified by the voice IF amplifier and detected by the FM detector.
- The audio amplifier amplifies the audio signal from the FM detector. Then the audio signal is converted into sound by the loudspeaker.
- The AGC circuit regulates the gain of the RF and IF video amplifiers, so that the video signal output remains at a constant amplitude.
- The detected video signal is amplified and fed into the CRT cathode.
- Part of the video signal is separated from its synchronization pulses.
- Horizontal synchronization pulses are given to the horizontal oscillator through the AFC.
- The vertical sync pulse triggers the vertical oscillator to synchronize.
- The vertical and horizontal deflection signals enter the deflection coil as well as the convergence coil.
- The sub carrier signal is taken from the video amplifier via a band pass amplifier.
- After the chroma demodulation process by the chroma circuit, the signals (B -- Y) and (R --Y) are obtained.
- In a matrix circuit, the signal (G -- Y) is generated from the signals B -- Y) and (R -- Y)
- The Y signal at the CRT cathode and the signals (R -- Y), (G -- Y) and (B - Y) produce the influence of the electron beam between the cathode and the grid according to the signals R, G and B.