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Troubleshooting Television

Troubleshooting Television. AM fundamentals. Radio wave – electromagnetic energy vibration. Travels at 300 million m/s The lower the frequency, the longer the wave. Audio wave is approximately between 20Hz and 20kHz : can be heard by most people

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Troubleshooting Television

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  1. Troubleshooting Television

  2. AM fundamentals • Radio wave – electromagnetic energy vibration. Travels at 300 million m/s • The lower the frequency, the longer the wave. • Audio wave is approximately between 20Hz and 20kHz : can be heard by most people • Radio wave frequency: waves higher than audio wave frequency, cannot be heard.

  3. AM fundamentals • Physical size of an antenna is proportional to the length of the wave, to transmit and receive audio waves – impractical • Modulated wave used for transmission: a high frequency continuous wave produced by an oscillator is mixed with a low frequency audio wave

  4. AM fundamentals • Amplitude modulation (AM): low frequency audio wave varies the amplitude of a high frequency carrier.

  5. AM fundamentals • Most AM receivers are called superheterodyne because of the mixer stage • Antenna receives many radio frequencies (RF) within its frequency band.

  6. AM fundamentals • The tuner (variable capacitor and a coil) selects a desired band of frequencies and passes them on to the mixer stage • At mixer stage, incoming RF signal is combined with a constant-amplitude continuous wave, which oscillates at an intermediate frequency (IF) usually 455kHz above the incoming RF signal. • Output of the mixer passes through a tank circuit tuned to IF (455kHz). • Local oscillator will be set at RF+IF frequency. example, if RF= 1000kHz and IF=455kHz, then local oscillator set at 1455kHz.

  7. AM fundamentals • Next stage is IF amplification: tuned to the receiver’s specific intermediate frequency for improving selectivity • Detector stage: separate the audio wave from the IF carrier. • Rectify the composite signal, leaves the upper envelope of the composite AM signal • Filter the IF carrier signal through a capacitor to ground, passing only the low-frequency audio signal. • Automatic Gain Control (AGC) feeds back a portion of the signal to provide control for constant volume • Audio frequency (AF) amplifier : amplify the audio signal to drive the speaker

  8. FM fundamentals • Frequency of RF carrier varies with the amplitude of the modulated signal. FM receiver

  9. FM fundamentals • Antenna receives FM signals within its band • Tuner selects a specific band of frequencies. • RF amplifier strengthens the FM signal • Local oscillator generates a constant-amplitude RF signal • RF signal is mixed with FM signal and form an intermediate frequency (IF) • The IF is amplified

  10. FM fundamentals • FM detector convert the frequency variations to audio representation. • Detected audio signal is then fed through a deemphasis network • Deemphasis network restores the relative amplitudes of the signal’s frequency components. At transmitter, the high frequencies are further amplified (preemphasis) to improve the signal-to-noise ratio (SNR) for transmission. Therefore, a reverse process must be done at the receiver.

  11. FM fundamentals • After deemphasis process, the audio signal is amplified to drive the speaker (audio amplifier). • Automatic frequency control (AFC): keeps the receiver oscillator properly tuned

  12. FM multiplex fundamentals • FM stereo transmission • 2 separate microphones picking up audio signals (designated L-left and R-right). • These signals are sent to a matrix network and produce 2 new outputs. (L+R and L-R signal) • The FM transmitter is frequency-modulated by the L+R output, L-R output, combined with a 19kHz pilot signal carrier.

  13. FM multiplex fundamentals • FM stereo frequency spectrum • The low end of the frequency spectrum contains the L+R signal for monophonic receivers (30Hz to 15kHz). • The high end frequency spectrum contains the L-R sidebands with suppressed carrier (23 kHz to 53kHz). A 19kHz pilot carrier signal is also transmitted to be used by the receiver for synchronization.

  14. FM multiplex fundamentals • FM receiver works backward from the transmitter. It receives L+R signal, the sidebands of the L-R signal, and the 19kHz pilot carrier. • If the receiver is not equipped for FM stereo, it responds only to the L+R signal and process it as a monophonic signal • If the receiver has FM stereo, the L-R signal is recovered by mixing the L-R modulation sidebands with a 38kHz carrier (generated in the receiver and uses 19kHz pilot carrier for synchronization) then extracting the original L-R signal.

  15. FM multiplex fundamentals FM receiver

  16. FM multiplex fundamentals • The L-R and L+R signals are processed in a matrix network circuit similar to the one used at the transmitter • In the matrix network: • The L+R and L-R signals are added which produces the original L signal. • The L+R and L-R signals are subtracted to produce only the original R signal. • The original L and R signals are then amplified and sent to their respective speakers.

  17. FM multiplex fundamentals • Other methods of FM stereo demodulation: • Using bandpass and matrix method • Electronic switching method

  18. TV transmitter & receiver fundamentals • Video signal is amplitude-modulated (AM) and sound signal is frequency-modulated (FM). • Composite transmitted signal (from TV transmitter) is a combination of both AM and FM principles.

  19. TV transmitter & receiver fundamentals • TV camera (transducer): convert light energy to electric energy. • Electron beam creates a video line (trace) 525 times per second. • Cathode-ray tube, CRT picture tube (transducer): convert electric energy back to light • Microphone and speaker – transducers for sound system

  20. TV transmitter & receiver fundamentals • Transmitted signal carries synchronization pulses – to synchronize tv receiver with the traces made at the camera. • TV receiver: • Electron gun produces a stream of electrons • Magnetically scan left to right and from top to bottom • Specific phosphors on the screen luminesce when struck by electrons

  21. TV transmitter & receiver fundamentals • Block diagram of a black and white television receiver

  22. TV transmitter & receiver fundamentals • Signal from antenna is amplified, mixed with a continuous wave of a predetermined frequency from oscillator, then sent to intermediate-frequency (IF) stage. • Video detector: demodulates the signal and send the audio part to the audio stages and the video part to the video stages

  23. TV transmitter & receiver fundamentals • Audio signal (FM) is amplified in the IF amplifier, demodulated in the FM detector, again amplified in the audio-frequency (af) amplifier, and reproduced as sound by the speaker. • Video signal (AM) is amplified by the video amplifier, and sent to the grid of the picture tube (CRT). • Automatic gain control (AGC) maintains the signal at a constant level.

  24. TV transmitter & receiver fundamentals • Sync separator removes the vertical and horizontal pulses and applies them to integrating and differentiating circuits. • Modern receivers use microprocessor to interface operator controls and video processor. • Video processor contains audio and video intermediate frequency, source switch, sync separator, mixers and drivers for color units. • Digital high definition televisions (DHTV) contains sophisticated system control functionality which provides higher picture quality.

  25. Color TV troubleshooting • At tv station, 3 cameras scan a scene in unison. Each camera is sensitive to only 1 primary colors (red, blue, and green). • The primary colors are fed into a matrix at the transmitter and creates a luminance (Y), signal and chrominance/color signals (I and Q).

  26. Color TV troubleshooting • Y signal: proper proportions of red, blue, and green, used to modulate the carrier. • I and Q signals: modulate a 3.58MHz color subcarrier (which is suppressed by the modulation process). • The composite signal has a carrier, the Y, I and Q signals, as well as FM audio.

  27. Color TV troubleshooting • If the receiver is black-and-white set, only Y signal is detected and processed. • A color receiver requires a 3.58MHz oscillator to enable the detection of the I and Q signals.

  28. Color TV troubleshooting

  29. Color TV troubleshooting • Positive and negative chroma signals: • Green = -I-Q+Y • Blue = -I+Q+Y • Red = I+Q+Y • 3-color adder circuits sums up the Y, I and Q signals. Resistor values provide the proper proportion of each signal. • Each color signal is sent to appropriate CRT grid to control the beam’s intensity.

  30. Color TV troubleshooting • Color burst amplifier receives a portion of the color signal (sent by the transmitter), and sends it to the phase detector. • Phase detector: • compares the 3.58MHz signal to the color burst; if the 2 signals are not equal, a dc signal is sent to the reactance modulator to pull the signals into precise synchronization • Sends a dc signal to the color killer when the color burst is absent, as it is during monochrome broadcasting.

  31. Color TV troubleshooting • Color killer: prevent any signals from the chroma circuits during a monochrome broadcast. As long as the color burst is present, the color killer is off. • A general localization of color troubles can be made in terms of the raster and monochrome picture quality. • A good raster indicates normal dc voltages • A good monochrome picture indicates normal Y signal and correct dc voltages for the picture tube

  32. Color TV troubleshooting • Absence of color • Defective IC or module in I and Q signal processing stages. • Color killer control may be improperly set or defective. • Bandpass amplifier may be inoperative. • Check presence of 3.58MHz signal and color burst signal at the oscillator and subcarrier

  33. Color TV troubleshooting • Weak or faded color • Improperly adjusted bias control on the picture tube • Improperly adjusted screen and drive controls • Weak transistor or IC in I and Q signals processing stages • Defect in bandpass amplifier • Tuner and IF stages slightly out of alignment • Presence of 3.58MHz signal and color burst signal

  34. Color TV troubleshooting • Screen dominant color • Improperly adjusted color drive • Misadjusted screen controls • Defective picture tube • Color killer • Defective color killer results in color noise (confetti)

  35. Color TV troubleshooting • color bars • Reactance transistor, automatic frequency phase control, defective color burst • Defective chroma processor IC or module • Other color problems • Purity is out of adjustment • Picture tinted in black and white • Flesh tones vary with image position on the screen • Picture tube needs degaussing

  36. Color TV troubleshooting • Total convergence setup • Mostly modern tv sets have a preset convergence circuit • Adjust proper picture size, focus and linearity, and brightness. • Generally use a dot, bar, or crosshatch generator • Purity adjustment • First, bias the blue and green guns to cutoff • Then slide the deflection yoke forward • Adjust purity magnet until the red focuses exactly in the middle of the picture tube • Finally, push the deflection yoke backward until the raster is completely red

  37. Color TV troubleshooting • Static convergence • First, turn on the green gun. • Adjust the red and green static convergence magnets to merge 2 colors in the center of the picture tube until 1 yellow dot appears. • Then turn on the blue gun and merge all 3 colors until the color white is formed

  38. Color TV troubleshooting • Dynamic convergence • Adjust each dynamic control to converge the top, bottom and sides of the picture tube. • Adjust the picture for the best gray tracking • While tv is on and the color turned down, adjust the red, blue and green drive controls until the best overall gray raster is obtained • Adjust screen controls • Set the service switch in the service position • A horizontal line will be displayed across the center of the screen • Then turn all 3 color screen controls counterclockwise, and then slowly adjust each one until the color is just visible

  39. Thank you

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