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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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am fundamentals
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.
am fundamentals1
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
am fundamentals2
AM fundamentals
  • Amplitude modulation (AM): low frequency audio wave varies the amplitude of a high frequency carrier.
am fundamentals3
AM fundamentals
  • Most AM receivers are called superheterodyne because of the mixer stage
  • Antenna receives many radio frequencies (RF) within its frequency band.
am fundamentals4
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.
am fundamentals5
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
fm fundamentals
FM fundamentals
  • Frequency of RF carrier varies with the amplitude of the modulated signal.

FM receiver

fm fundamentals1
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
fm fundamentals2
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.
fm fundamentals3
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
fm multiplex fundamentals
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.
fm multiplex fundamentals1
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.
fm multiplex fundamentals2
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.
fm multiplex fundamentals4
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.
fm multiplex fundamentals5
FM multiplex fundamentals
  • Other methods of FM stereo demodulation:
    • Using bandpass and matrix method
    • Electronic switching method
tv transmitter receiver fundamentals
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.
tv transmitter receiver fundamentals1
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
tv transmitter receiver fundamentals2
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
tv transmitter receiver fundamentals3
TV transmitter & receiver fundamentals
  • Block diagram of a black and white television receiver
tv transmitter receiver fundamentals4
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
tv transmitter receiver fundamentals5
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.
tv transmitter receiver fundamentals6
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.
color tv troubleshooting
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).
color tv troubleshooting1
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.
color tv troubleshooting2
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.
color tv troubleshooting4
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.
color tv troubleshooting5
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.
color tv troubleshooting6
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
color tv troubleshooting7
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
color tv troubleshooting8
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
color tv troubleshooting9
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)
color tv troubleshooting10
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
color tv troubleshooting11
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
color tv troubleshooting12
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
color tv troubleshooting13
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
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