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Transcript
ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING AIM: Observation of various test patterns by using a TV pattern generater. APPARATUS REQUIRED : T.V. kit, Pattern Generator ,connecting cord THEORY: Pattern Generator The unique features of this instrument is that it has 12 patterns . Seven in color and five in black and white . The 3 primary colors Red, Blue and Green and added to check the purity of the colors. Guns can be aligned and repaired of the color TV sets with the help of these 3 patterns. RED GREEN BLUE Contrast of the colors can be seen in Horizontal and Vertical position in different modes of patterns provided in the color position, pattern one, color pattern two, color pattern three and color pattern four give different sequence on the screen. Chess pattern can be seen to check the contrast in Black & White. RASTER (CHESS PATTERN) (SIDE CHESS PATTERN) (CROSS BAR PATTERN) Side chess pattern is provided to adjust the yoke position, Crossbar pattern is provided to check the Linearity of the TV sets. Besides that video signals and sound signals (1KHz) provide extra feature to this unique instrument. Audio sound lead is provided along with the P.G. and can be easily connected in the socket provided at the back. This lead can be connected with Tape Recorder to have direct sound on the T.V. set This will enable the T.V. Engineers to see the clarity of the sound . Channel 2,3 and 4 can be had on band 1 and Channels 5 to 12 can be had on Band 3 with the help of push pull switch provided on the front panel. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING PROCEDURE: Switch On the Power ON/OFF switch . Connect the Generator output to the T.V. Receiver and the following operations are to be carried out. OBSERVATIONS: 1. Press horizontal bar switch/or rotate switch for horizontal bar (a) Observe the Horizontal Bar pattern on the T.V. screen, should the Bars move in the Vertical direction. It is possible to hold them with the Vertical Hold Control. This check is for frame time base frequency. (b) Check that the White Bars are of the same height. ( i.e. check of frame time base linearity). (c) Check that the White Bars are of the uniform Brightness over the whole width (i.e. check of low Frequency reproduction). 2. Press vertical bar switch/or rotate switch for horizontal bar (a) Observe the vertical bar pattern on the T.V. screen should the bars move in the horizontal direction, it should be possible to hold them by means of “Horizontal” stability control of the T.V. receiver (i.e. check the line time base frequency). (b) Check that white bars are of equal width (i.e. check of line time base linearity ). (c) Check that the black and White contrast is sharp enough (check of high frequency reproduction). 3. Press cross bar switch/or rotate switch for horizontal bar (a) The white blocks serve for a general check i.e. rapid linearity check and check of unit function response. (b) Observe that white blocks are of the same size check of horizontal and vertical linearity. 4. AGC This can be checked by switching from one pattern to another while observing for any streaks in the pattern. Should the AGC be correctly adjusted no streaks would be seen. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING 5. FOCUS ADJUSTMENT Use of dot pattern Normally dots are slightly oval in horizontal direction if blurred or not clear, Adjustment is required in contrast, brightness or focus circuits. When the dots are tall at the right hand side or dim, adjust fine-tuning in case the condition does not improve, adjust pictures amplifier circuit. Different types of color patterns are available while rotating the switch at color position different types of colors are available at different position, primary colors Red, Blue and Green are the main features of P.G. Model 1000 and P.G. Model 14. Specifications: System : PAL Standard : CCIR Standard Signal : Patterns available * Chess Pattern * Side Chess * Vertical Bars * Horizontal Bar * Cross Bars * Dots * Raster White Colour Signal Frequency : 5.5 MHz with respect to video carrier frequency modulated by 1 KHz internally generated 50 KHz sweep. Color Signal : 4.4 33618 MHz Crystal controlled on all channel. Picture Carrier Frequency Band –I : 2,3,4 Band – III Channel 5 to 12 channels UHF Channel : : 36 Stability : Fairly good Output : Fixed Modulation : AM Negative ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING Sync Pulses Line Frequency : 15625 Hz (Fixed) Frame Frequency : 50Hz (Fixed ) Interlacing : Random Stability : +1% Output Impedance : 300 Ohms balanced Output Voltage : Approx. 10 mV Sound : Frequency Modulation 1 KHz Approx. Accessories : Leather case 300 Ohm RF. Lead Power Supply Requirements : 230V 50Hz Single Phase A (CROSS BAR PATTERN) (DOT PATTERN) RASTER WHITE RASTER (CHESS PATTERN) RED (SIDE CHESS PATTERN) GREEN BLUE BLUE ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING R G B (COLOUR CHESS) RESULT : All the Patterns are study successfully by using pattern Generator. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING AIM: Operation of Monochrome T.V. Kit. a) Study of Monochrome T.V kit b) Study of TV Power supply c) Study of Video Amplifier and EHT voltage Generator APPRATUS REQUIRED : T.V. kit. THEORY: a) STUDY OF MONOCHROME T.V KIT A Television receiver commonly called TV or Television set receives the telecast signals and reproduces the televised scene and sound. The following functions are performed by various stages of a television receiver. 1. To intercept the telecast signals, to select the signals of the desired channel and to amplify these. 2. To separate the video (vision or picture) signals from the combined signals, to amplify these and to reproduce these in to form of picture on the picture-tube screen. This is done with the help of horizontal and vertical sweeps. 3. To separate the FM sound signals, to amplify to detect and to reproduce the sound. 4. To separate the synchronizing signals (in brief sync signal ) from the composite video-signals. 5. To produce horizontal (line ) and vertical (frame ) sweep signals, synchronized with the help of sync signals and to give these to the respective deflection coils producing the two sweeps. 6. To provide power at suitable D.C. voltages for operating various stages, providing supply to various electrodes of the picture tube and for heating its cathode. The working of the various stages/sections of a solid-stage TV receiver has been briefly described here with the help of the block schematic diagram in which various stages have been depicted in the form of boxes. While the number of ICs used in different models of TV receivers may differ, this block schematic which is based on four ICs , represents a fairly balanced circuit optimising the use of ICs. Aerial : The aerial picks up the telecast signals and gives these to the input of the tuner through the feeder wire and an impedance matching transformer know as “ balun”. Tuner: The signals picked up by the aerial are given to the tuner which consists of two stages of RF amplifier and frequency changer (converter). The RF amplifier partly ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING selects the signals of the desired channel and amplifies these. The amplified signals are given to the frequency changer. The frequency changer (converter) performs two functions. Firstly it produces oscillations at a frequency which is higher than the frequency of the incoming signals by the intermediate frequency (IF) ,secondly it mixes the incoming signals and the oscillations. This mixing results in the production of intermediate frequency (IF) signals. Since there are two independent signals video and audio, two frequencies are produced. The usual intermediate frequencies in the system used in our country are 38.9 MHz for picture and 33.4 MHz for the sound respectively. A single channel or multichannel tuner may be used depending on whether a receiver is meant for single channel or multichannel reception. Bipolar tuners having turret tuning or incremental tuning are common in multichannel TV receivers. Electronic tuners are also gaining popularity. Video If Section The picture (vision ) and sound IF signals obtained from the tuner are given to the video IF section. Two or three wave-traps are provided at the input of this section to suppress the signals from the adjacent channels and also to reduce the amplitude of the sound IF signals (to eliminate interference from these signals with picture signals). The main functions performed by this section are amplification of the signals, detection of the picture (video) IF signals to obtain the video signals, pre-amplification of video signals mixing of picture and sound IF signals, resulting in the production of intercarrier sound IF signal (5.5 MHz) keyed AGC for the IF amplifier and producing keyed and delayed AGC voltage for the RF amplifier in the tuner. As the band width of the picture signals is rather large, this section is designed to provide adequate band width. Staggered tuned circuits (circuits tuned to different frequencies ) are used for interstage coupling to obtain the desired band width. An IC such as CA 3068 is normally used in this section. Video Output: The video output stage amplifiers the video signals obtained from the video (picture) IF section and given these signals to the cathode of picture tube. As the bandwidth of the video signals is rather large (from D.C. to abut 5.0 MHz) the video output section has to be designed to provide sufficient gain over the entire band. Three transistors are normally used in the video output section one as the driver the second as output and the third for the horizontal and vertical retrace lines on the picture tube. picture tube : ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING The picture tube is a specially designed cathode ray tube which displays the video signals in the form of picture on its screen with the help of horizontal (line) and vertical (frame) sweeps. Electromagnetic deflection is used in the television picture tubes. A set of two windings one for producing the horizontal sweep and the other for vertical sweep, is placed on the neck of picture tube. These are known as deflection coils or yoke. The sweep signals are given to the respective windings of this yoke. An IC CA 3065 (or similar) is commonly used in this section. The functions given above, this IC also includes an electronic attenuator for volume control and a zener diode regulated power supply for supplying its various stages. Sound Output : The sound (audio) signals obtained from the sound IF section are not of sufficient amplitude to drive the loudspeaker. The output section amplifies the sound signals and gives sufficient output power. Two stages a driver and output are used in this section. The driver transistor (BC 148 or similar) amplifies the sound signals obtained from the sound IF subsystem the output stage, which normally is based on two transistors (matched pair of versa tile transistors 2N 5296 and 2N 6110 or similar types) in a complementary , symmetry single- ended push pull circuit, further amplifies the sound signals and gives them to the loudspeaker. Positive and negative flyback pulses are also obtained from this stage for keyed AGC, line frequency control and suppressing the retrace lines on the picture tube. The line output section utilises two transistor – one as driver and the other as output. A high voltage power transistors such as BU 205/508A is used in the output stage. Vertical Sweep-Section: The vertical (frame) sweep-section produces vertical sweep signals and provides sawtoothed frame-sweep current to the frame (vertical) sweep coil. This section consists of a controlled oscillator the frequency and phase of which are kept in synchronization by the sync signals. This is followed by an amplifier cum output stage which amplifiers these oscillations and drives quiescent current through the frame deflection coil. An IC like IC 2130 is commonly used in this sweep current to the frame (vertical) sweep coil. This section consists of a controlled oscillator the frequency and phase of which are kept in synchronization by the sync signals. This is followed by an amplifier ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING cum output stage which amplifiers these oscillations and drives sufficient current through the frame defection coil. An IC like IC 2130 is commonly used in this section. In some earlier models transistorized circuits have also been used in the sections. b) STUDY OF TV POWER SUPPLY Power Supply: In solid state television receiver power is required at different D.C. voltage for operating various stages. These are obtained from the main power supply and the auxiliary power supply. Main Power Supply : The main power supply provides D.C. voltages of about 110 to 130 volts. This is obtained by rectifying the mains voltage and smoothening it . This power from the main supply is usually given to the video amplifier line output section and the grid of the picture tube. A 12 volt supply is also obtained from it by dropping and stabilizing this voltage, for feeding the line oscillator sub-section. Auxiliary Power Supply : The auxiliary power supply is a part of the line output stage. DC voltage ranging from 12 to14 volts are provided by the auxiliary power supply. These are used for supplying to various sections operating at low voltage. In some television receivers an IC like CA 810 may be used in sound section in place of the transistors. ICs are now available which are capable of performing the complete functions of sound IF and sound output sections. CA 1190 (BEL) is an example of these ICs. The auxiliary supply gives supply at three D.C. voltages –12 volts, 24 volts and 40 volts A voltage of about 6.3 volts from the line output transformer is given to the heater of the picture tube (pins 1 and 8). line oscillator sub – section The main function of the line oscillator is to produce oscillations at the line sweep (15625 Hz). The frequency and phase of these oscillations is controlled by the sync signals. A preamplifier for amplifying the oscillations and sync separator are also included in this section. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING An IC like CA 920 (BEL) is commonly used in this sub-section. This IC combines the following function sync- separator, noise-gating for reducing the effect noise pulses on synchronizing, line oscillator, phase detector for comparing the phase of the line oscillator with reference to sync signals (for correcting the oscillator phase and frequency) and preamplifier. line output : The line output section is one of the most important section in a television receiver. It provides saw-toothed line sweep current to the line (horizontal) deflection coil. It also performs the following functions. 1. Producing a high voltage D.C. known as EHT (around 17,000 volts in black and white television receivers ). This is given to the final anode of the picture-tube. 2. Producing a high voltage D.C. of about 1000 volts giving to the accelerating and focusing anodes of the picture tube. 3. Providing low voltage A.C. for heating the filament of the picture tube. 4. Providing low voltage D.C. at suitable voltages for feeding some of the ICs and transistors. c) STUDY OF VIDEO AMPLIFIER AND EHT VOLTAGE GENERATOR Generation Of EHT As explained under generation of sweep, the collector current of the output transistor builds up a magnetic field around the deflection coil during trace. When the deflection current reaches its peak value the voltage at the base of this transistor suddenly falls to zero and cuts- off this transistor. The magnetic field around the deflection coil will therefore collapse. As the rate of collapse of the magnetic field is very fast a high voltage is induced in the primary winding of the line output transformer. This voltage is further stepped up by the secondary winding which consists of a large number of turns. The voltage obtained at the secondary winding is rectified by a diode and is smoothed by the capacity formed between the inner and outer coating in the picture tube. This provides a D.C. voltage of about 17000 volts. A high voltage is also induced during retrace on the primary winding of the line output transformer. Flyback Pulses During the retrace, pulses are also induced in the windings of the line output transformer. The pulses so obtained are given to the line oscillator IC CA 920 to serve as reference of oscillator for correction of its phase. These pulses are also given to the video amplifier for suppressing the horizontal retrace lines. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING AIM: Switching Mode Power Supply APPARATUS REQUIRED : T.V. kit THEORY: Different types of power supply are used in electronics equipments. Earlier high voltage capacity power supply was made directly from 220/230 A.C. supply. For generating low voltage supply we use resistance in high voltage supply according to necessity. Capacitor and coil are used for filtering A.C. pulse and ripples. After few years, transformers are used for making lower supplies. In both types of power supplies output voltage changes according to input A.C. volts, it means when input A.C. voltage increases output D.C. voltage also increases & if input A.C. voltage output DC volts also decreases. This type of power supply is called unregulated power supply. But when we use ICs for making electronic equipments, it is necessary to use regulated supply. Use of transistorized series regulator does not solve the problem because of working efficiency being up to 25% to 40%. SCRs also losses energy ,because of which we can’t use SCR power supply for high rating. So there is a necessity to use power supply having high working efficiency about 80% with low cost. SMPS power supply The use of Switching Mode power supply started from 1980. In SMPS switching transistors are used for controlling output supply which will ONOFF at particular frequency. Principal Of Switch Mode Power Supply: The output value of any power supply section depends on given input supply. If we stabilized input A.C. voltage by any method then output supply will also stable, SMPS works on the same principle. At first main input A.C. is converted in to high D.C. voltage and by using filter capacitor we get smooth D.C. which is given to switching transistor. By control circuit we can ON-OFF transistor very rapidly. This control circuit generates high frequency square wave pulse (15KHz to 20KHz ) which is given at base of transistor .ON-OFF of transistor depends on this pulse these pulses are given at primary of small size transformer and appeared voltages at secondary are rectified and filtered figure 1 shows block diagram of simple SMPS. In switch mode power supply we get low voltage D.C. supply whose value varies from 12 volt to 20 volt, along with main D.C. supply of +110 volt. For regulating this output, one part of output is given to sense amplifier. This ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING amplifier compares output with reference voltage. If there is a difference in both voltages then error voltage will develop, which is given to control circuit used for controlling ON time (duty cycle ) of switching transistor according to error voltage and develop output regulated voltage if there is increment in output voltage then developed error voltage reduces ON time of switching transistor, so in both stages we get regulated output supply. SMPS also has some characteristic, for example starting of switching process at low current, protecting circuit from high current and voltage and separate output supply with main supply. Most of SMPS use one or two transistors. These days we get necessary frequency for switching and regulating output voltage from I.C. we clarify you that when ON-OFF process of switching transistor is done at a fixed transmitting frequency then we get fixed supply at primary of transformer, because of that value of secondary voltage is also fixed after rectifying and filtering we get D.C. voltage at output. For regulation output D.C. voltage switching process of switching transistor is controlled by one control circuit. We have to keep in mind that supply at secondary of SM transformer is output supply, which is separated from D.C. supply made by input A.C. main characteristic SMPS is that there is separate input and output supplies, which protect main circuit after starting of switching transistor. PROCEDURE : 1. Connect 34 pin connector cable from Dynamic Demonstrator to left side of the CRT cabinet. 2. Connect the Antenna cable to the Dynamic Demonstrator. 3. While switching on the Demonstrator (before plugging into AC-mains ) ensure that SMPS switch is in OFF condition. 4. Do not adjust or align vision if Board preset or coil. 5. Ensure that EHT cable is connected to picture-tube before system is switched ON. 6. Do not keep” horizontal control “ to maximum position as it may Damage horizontal IC, or output transits BU205/508A. 7. Do not interchange horizontal & vertical yoke connections. 8. After checking the above points switch ON the trainer. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING AFTER SWITCHIG ON THE TRAINER THE FOLLOWING ALIGNMENTS HAVE TO BE MADE 1. To get clear picture select the appropriate channel in the tuner. 2. Align “vertical hold “ and horizontal hold” controls for correct picture so that you will not get horizontal and vertical rolling. 3. Adjust the volume control to a suitable audible level. FAULT INDUCING SWITCHES OBSERVED PROBLEM/DEFECT F1 F2 Switch OFF dot ON the screen, sound ok Audio ok but no picture, only snow raster ON the screen Sync signal loss Horizontal line No Sound ,picture ok No picture, Sound ok, Darkness appears on the screen F3 F4 F5 F6 ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING AIM: Operation of Color T.V. Demonstrator. a) b) c) d) Observing the effect of various fault induced by various fault inducing switches. Measurement of signal in RF Tuner section Measurement of signal in Video IF and AGC section. Observation of waveforms Generated by Horizontal Sync Oscillator. APPARATUS REQUIRED : T.V. kit, CRO, Multimeter ,connecting chords THEORY: Lab Color TV Dynamic Demonstrator is a comprehensive trainer. In this Demonstrator all importance has been given to practical study of various parameters involved in a color television. 1. 2. The Block Diagram of the Lab Color television Demonstrator (using ET & T kit ) is printed on the front panel of the Demonstrator. The picture tube is housed in the cabinet and connected to the mother board by means of polarized connector. Fault inducing switches (F1to F2) are provided at the right hand side of the top panel in the Demonstrator to study the relationship between cause and the effect. The circuit of power supply, tuner, Vision IF Sound IF, vertical sweep section, Horizontal sweep section, Horizontal output & EHT generator, Chroma section are assembled in the mother board and explained by means of block Diagram in the trainer. Various test point TP1 to TP24 are provided to check the voltages in different sections using a multimeter and also to observe to the waveforms on an oscilloscope. Antenna connector (impedance matching Balun transformer) is provided in our Demonstrator. Channel selector is mounted in the picture tube cabinet control elements like volume control Brightness control, contrast & color adjust controls are mounted in the picture tube cabinet. 34 pin FRC connector 9pin D connector is provided to connect picture tube and mother board. 3. 4. 5. 6. 7. 8. PROCEDURE: 1. Connect Antenna cable to the Demonstrator. 2. Connect 34 pin connector and 9 pin “ D “ connector from the mother board to the picture tube. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING 3. Connector 24 KV (HT probe Red) from the mother board to the picture tube properly you will have to be extremely careful when you connect this cable. 4. Focus cable from the Demonstrator (white cable) should be soldered at the base of the picture. 5. Check whether all the connections are connected properly. 6. Plug – in power cord to 230V50Hz main. 7. switch ON the trainer. 8. Now ,you can see the picture as relayed by the transmitter depending upon the channel you have selected. 9. Alternatively a patter generator can be connected instead of antenna cable and Different patterns can be observed on the TV monitor. 10. As soon as the picture comes adjust the appropriate brightness, contrast & Color controls for your viewing comfort. Use the fault inducing switches from F1 to F10 and see the following effects: a. F1-To observe retrace lines. If you put F1 switch to UP position you will observe no picture and sound in your Demonstrator you will observe retrace line in your monitor. b. F1- you will notice phase shift in the picture. c. F3- cutoff the GREEN color in RGB spectrum. d. F4– To cutoff the BLUE color in RGB spectrum. e. F5– To cutoff the RED color in RGB spectrum. f. F6- To cutoff the vertical sweep section,you will observe horizontal line on your monitor. g. F7- To introduce the vertical linear variation in the picture. You will observe that the picture is vertically non-linear, sound normal. H F8- To cutoff brightness control. h. F9-To cutoff Audio signal i. F10 To cutoff vision IF section To cutoff 115v from vision IF CAUTION DO NOT KEEP THE FAULT INDUCING SWITCHES F1 – F10 IN THE ON POSITION FOR NOT MORE THAN 45 SECONDS. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING Various test points are provided to test and measure input and output signals j. SMPS section TP1 - +150DC TP2 - + 100DC TP3 - + 15VDC k. Audio section TP4 - +3.6V DC TP5 - +4V DC TP6 - + Audio output should be observed in oscilloscope l. Vertical sweep section TP8 _ TP9 - SEE PAGE NO. 4 m. Vision IF section TP16 - +6V TP17 - +1.25V TP18 - +12 .10V n. Horizontal Driver output f. TP19 TP20 TP21 TP26 TP22 TP23 TP24 - 1.2V +90V + 85V +86V +14.8V +1.2V +85V(Approx) Driver TESTING POINTS OF OUTPUT PIN DETAILS TP4 TP5 TP6 TP7 TP8 TP9 - TP10 TP11 - TP12 TP16 TP17 TP18 - OUTPUT IC 1190 PIN NO:12 OUPUT OF 5.5 MHz COIL OUTPUT OF SPEAKER OUTPUT OF IC 2653 PIN NO: 2 OUTPUT OF IC 2653 PIN NO :1 OUTPUT OF IC 2653 PIN NO: 2 AFTER DIODE IN 4148 OUTPUT OF IC 2653 PIN NO: 6 OUTPUT OF IC 2653 PIN NO:2 (OUTPUT OF IC 2653 PIN NO: 2) OUTPUT OF IC 2653 PIN NO: 4 OUTPUT OF IC 1940 PIN NO:5 OUTPUT OF IC 1940 PIN NO: 3 OUTPUT OF IC 1940 PIN NO:11 ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING TP19 - OUTPUT OF HORIZONTAL DRIVER TRANSISTOR BD 115 BASE TP20 - OUTPUT OF HORIZONTAL OUTPUT TRANSISTOR BU208D COLLECTOR TP21 - OUTPUT OF HORIZONTAL DRIVER BD 115 COLLECTOR BLOCK DIAGRAM OF ET&T COLOR TELEVISION KIT The block diagram of color Television kit given in figure. The function of each block are explained below. ANTENNA The main function of antenna is to accept the electromagnetic wave coming from television transmitter. The antenna receives the electromagnetic waves and converts into RF signals which is given to the television receiver. For better reception of RF signal YAGI antenna is most commonly used in all television receivers in VHF/UHF range for its simple construction and low air resistance. BALUN It is used for matching the impedance of balanced 300 ohm line to unbalanced 75 ohm tuner input impedance. RF signal from antenna is given to the RF tuner through the balun transformer. b) MEASUREMENT OF SIGNAL IN RF TUNER SECTION : RF Tuner It is used for better picture and sound reception the main functions of the tuner are 1. It selects station and rejects others 2. it matches antenna with Television receiver because of this ghost image can be removed 3. It converts RF signal into intermediate frequency by heterodyning it with local oscillator frequency 4. It isolates the local oscillator signals from the antenna for preventing radiation of it through the antenna. 5. it rejects the image frequency which causes the ghost image along with picture The RF tuner selects RF signals of desired (selected) channel, amplifies them and converts them into IF signals the tuner consists of an RF amplifier an oscillator and a mixer stage. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING Local oscillator generates a constant frequency for desired channel RF amplifier the RF signal achieved from antenna and mixer stage converts them into IF signal by heterodyning RF signal with local oscillator frequency The IF carrier frequency present in IF signals for picture and sound are 38.9MHz and 33.4MHz respectively, thus IF signal achieved from tuner is fed to the IF amplifier. C) MEASUREMENT OF SIGNAL IN VIDEO IF AND AGC SECTION. IF PRE – AMPLIFIER It amplifies the IF signal. This stage of amplification is necessary because by the use of saw filter the gain of receiver becomes less. SAW – FILTER On this kit saw filter is used in place of wave trap circuit. It passes only required frequencies and grounds unwanted adjacent channel frequencies. VIDEO IF STAGE In ET & T kit IC1 (TDA3541) is used in video If stage. This stage contains the function of video IF amplifier, video detector video pre-amplifier AFC and AGC circuits etc. VIDEO IF AMPLIFIER : It amplifies the IF signal to provide sufficient gain AGC voltage is applied to all the separate IF amplifiers, except the last IF amplifier from video IF amplifier the signal is applied to the video detector two or three stages are used in this amplifier stage. VIDEO DETECTOR : Signal obtained from video IF amplifier is injected to the video detector. In video detector the signal is demodulated giving back the y-signal (luminance) and the color side-band along with various synchronizing pulses and the color burst signal AFC signal is also given to tuner section for automatic frequency control. Another main function of video detector section is to mix the picture IF (38.9MHz) and sound IF (38.9MHz) frequency to produce a new sound IF at 5.5MHz.from here new sound section. L-C filter or 5.5MHz tank circuit is also used with video detector to remove the 5.5MHz inter-carrier sound signal from video signal. The video signal obtained from video detector is then applied to the video amplifier. This amplifier is coupled to video pre-amplifier as well as AGC sections. SOUND SECTION SOUND IF AMPLIFIER : THE 5.5MHz inter carrier signal from video detector stage is fed to the sound IF amplifier for proper amplification In this kit sound signal is preceded by a crystal (ceramic filter) tuned at 5.5MHz. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING F.M. DETECTOR : 5.5 MHz sound IF signal is amplified by one or two sound IF amplifier stages and then applied to detector stages and then applied to detector stage. In this stage original sound signal is detected from the carrier. AUDEO AMPLIFIER : In this stage voltage amplification and power amplification is given to the audio signal finally fed to the speaker. VIDEO PRE-AMPLIFIER From the output of video amplifier in the video IF stage the video signal is fed to the video pre-amplifier .the signal consists of 1 The luminance or Y-signal 2 The color sub-carrier containing red and blue color difference signals, 3 The horizontal & vertical sync pulses, 4 The color burst signal The video pre-amplifier section amplifies the signal strength from 2V to 6V so that it is able to drive video output stage. Here the division of the luminance & chrominance separation takes place. From video pre-amplifier, video signal is coupled to chroma band pass amplifiers through chroma filter circuit, sync separator and delay line circuit. DELAY LINE From video pre-amplifier, Y-signal (luminance signal) passes through a delay line to amplifier stage. The delay line delays the y-signal (luminance signal) by approximately 0.8 microsecond. The delay line is a special coil with very high value of inductance and distributed capacitances so that the delay speed of the signal through the delay line is greatly reduced. It this delay line is not introduced, y-signal (luminance signal) will reach to picture tube earlier than chrominance signal. The main reasons for required delay in Y-signal (luminance signal) are as under, 1 Chrominance signal has to pass through relatively complex circuit of the decoder and by this reason, it is fractionally delayed as compared to y-signal (luminance signal). 2. the band with of Y-signal (luminance signal) is more than that of chrominance signal the narrower band with signal takes longer time to travel a particular distance. The Luminance Amplifier After the delay line ,the Y-signal (luminance signal) is fed to main Y (luminance) amplifier. The circuit used is also called emitter follower which acts as buffer amplifier to prevent any mutual interference between contrast control and black level clamp circuits. ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING The output signal voltage from Y (luminance) amplifier is fed to the matrix where it mixes with color difference signals to produce original red, green and blue colors. The horizontal and vertical blanking pulses derived from horizontal output and vertical output stages are also fed to Y (luminance) amplifier stage. These pulses ensure that the Y-signal (luminance signal) fed to the matrix is held at black level during retrace periods. The average value of luminance signal fed to the matrix unit determines the mean brightness of the picture appearing on the screen the contrast control is used for adjusting the amplitude of the Y-signal (luminance signal) from the amplifier. Chroma Section IC TDA360/3561 is used in this section. Chroma Band Pass Amplifier From the output of video pre-amplifier the composite color video signal is coupled to chroma band pass amplifier it consists of chroma filter 4.43MHz and two stage of chroma amplifiers. The chroma filter circuit separates modulated chrom sub-carrier signal and the color burst from incoming composite video signals. These separated chroma signals are amplified by the first chroma amplifier which is gain controlled by the voltage developed by the automatic chroma or color control (ACC) amplifier. The output from the first chroma amplifier goes to both the second chroma amplifier and burst pre-amplifier. The second chroma amplifier incorporates color saturation control circuit. The output of color killer also feeds into it because of this the 2nd chroma amplifier is cut off during black and white reception. Color Burst Circuit The color burst circuit consist of the burst pre-amplifier pulse and shaper and the gated burst amplifier. After Demodulation detected U(i. E. B-Y) and V(i.e. R-Y) color difference signals are fed to the matrix from which separate red green and blue color signals are available. Luminance (Y) signal is also fed to the matrix. R-G-B Video Output ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING The color video signals red green and blue are then fed to respective cathodes of picture tube after one stage of amplification video output circuits for each color signal are identical c) OBSERVATION OF WAVEFORMS GENERATED BY HORIZONTAL SYNC OSCILLATOR. HORIZONTAL SECTION IC3 (tda 1940F) is used in this section SYNC SEPARATOR FROM THE EMITTER of video pre -amplifier, composite color video signal is fed the sync separator made inside the IC TDA1940F. here horizontal and vertical sync signals are separated by the use of suitable integrated and differentiator circuit this stage also amplifies circuit. This stage also amplifies both horizontal and vertical sync signals. AUTOMATIC FREQUENCY CONTROL OR AFC CIRCUIT: This stage is included in IC TDA1940F this section compares the horizontal flyback pulse and incoming horizontal sync signal the difference between these two appears as control voltage for controlling horizontal oscillator frequency. HORIZONTAL OSCILLATOR: This oscillator generates 15625Hz saw-tooth horizontal (line ) frequency for horizontal deflection of electron beam inside the picture tube its frequency is controlled by the control voltage obtained from AFC circuit. HORIZONTAL DRIVER : The signal obtained from horizontal oscillator is very weak and so it is necessary to give voltage and power amplification to this signal in this stage voltage amplification is provided and sent to the horizontal output stage through the horizontal driver transformer transistor T9 (BDS 115) is used in this section as horizontal driver transistor. HORIZONTAL DRIVER TRANSFORMER : it is used to match the high impedance of driver stage with the low impedance of out put stage. HORIZONTAL OUTPUT STAGE : this stage consists of a transistor and EHT transformer power amplification is provided to the horizontal scanning frequency (15625Hz) and then this frequency is fed to horizontal deflection coil and EHT (horizontal output transformer) ORIENTAL INSTITUTE OF SCIENCE & TECHNOLOGY ELECTRONICS & TELECOMMUNICATION DEPARTMENT TELEVISION ENGINEERING Some auxiliary power supplies are obtained from this stage such as 25v, 12.6v,13v 150v and supply for heater high voltage (about 20KV to 25KV) is also produced for final anode of picture tube horizontal fly back pulse is achieved from EHT (horizontal output transformer). VERTICAL STAGE In this kit IC TDA2653A is used in vertical section. This section consist of vertical trigger cum vertical oscillator vertical driver and output amplifier. VERTICAL OSCILLATOR : it is triggered by the frequency of vertical sync pulse achieved by the integrator circuit thus it provides the 50Hz saw-tooth frequency necessary for the vertical deflection of electron beam inside the picture tube. VERTICAL DRIVER : It provides voltage amplification to the vertical signal, obtained from the oscillator. VERTICAL OUTPUT: It provides the sufficient power amplification to the vertical scanning frequency (50Hz) and then it is Frequently Asked Question’s: 1. 2. 3. 4. 5. 6. What do you understanding by interlaced scanning. How interlaced scanning is reduces flicker and conserve bandwidth Why is the picture signal AM and sound signal FM modulated Define aspect ratio Why is Electro magnetic preferred over Electrostatic deflection Why front porch and back porch provide along with the horizontal sync. Pulse. 7. what are the functions of brightness and contrast control.