Troubleshooting Television AM fundamentals l l Radio wave

Troubleshooting Television

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

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

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

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

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

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

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

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

FM fundamentals l l l 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-tonoise ratio (SNR) for transmission. Therefore, a reverse process must be done at the receiver.

FM fundamentals l l 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 l l 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 19 k. Hz pilot signal carrier.

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

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

FM multiplex fundamentals FM receiver

FM multiplex fundamentals l The L-R and L+R signals are processed in a matrix network circuit similar to the one used at the transmitter l In the matrix network: l l l 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 fundamentals l Other methods of FM stereo demodulation: l l Using bandpass and matrix method Electronic switching method

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

TV transmitter & receiver fundamentals l Transmitted signal carries synchronization pulses – to synchronize tv receiver with the traces made at the camera. l TV receiver: l l l 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 fundamentals l Block diagram of a black and white television receiver

TV transmitter & receiver fundamentals l Signal from antenna is amplified, mixed with a continuous wave of a predetermined frequency from oscillator, then sent to intermediate-frequency (IF) stage. l 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 fundamentals l 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. l Video signal (AM) is amplified by the video amplifier, and sent to the grid of the picture tube (CRT). l Automatic gain control (AGC) maintains the signal at a constant level.

TV transmitter & receiver fundamentals l l 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 l At tv station, 3 cameras scan a scene in unison. Each camera is sensitive to only 1 primary colors (red, blue, and green). l 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 troubleshooting l Y signal: proper proportions of red, blue, and green, used to modulate the carrier. l I and Q signals: modulate a 3. 58 MHz color subcarrier (which is suppressed by the modulation process). l The composite signal has a carrier, the Y, I and Q signals, as well as FM audio.

Color TV troubleshooting l If the receiver is black-and-white set, only Y signal is detected and processed. l A color receiver requires a 3. 58 MHz oscillator to enable the detection of the I and Q signals.

Color TV troubleshooting

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

Color TV troubleshooting l Color burst amplifier receives a portion of the color signal (sent by the transmitter), and sends it to the phase detector. l Phase detector: l l compares the 3. 58 MHz 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 troubleshooting l 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. l A general localization of color troubles can be made in terms of the raster and monochrome picture quality. l l 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 troubleshooting l Absence of color l l 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. 58 MHz signal and color burst signal at the oscillator and subcarrier

Color TV troubleshooting l Weak or faded color l l l 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. 58 MHz signal and color burst signal

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

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

Color TV troubleshooting l Total convergence setup l l 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 l l 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 troubleshooting l Static convergence l l l 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 troubleshooting l Dynamic convergence l l l Adjust each dynamic control to converge the top, bottom and sides of the picture tube. Adjust the picture for the best gray tracking l 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 l Set the service switch in the service position l A horizontal line will be displayed across the center of the screen l Then turn all 3 color screen controls counterclockwise, and then slowly adjust each one until the color is just visible

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