Troubleshooting television
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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