The audio circuits in newer televisions are usually contained in one IC. A block diagram outlining the television’s audio system is shown in FIG. 1. In a television, the audio signals go through the video IF processing stages prior to arriving at the audio processing circuits. Therefore, problems with the television’s sound might not occur in the audio processing stages, but earlier in the television’s tuner or IF sections.
Processing Audio Signals
In most televisions, the audio signals are separated from the video signals, color signals, and sync signals at the video detector, which is part of the video IF circuit. Starting at the point where the audio signals are separated from the other signals, the audio signals are processed only as audio. The other signals are handled by other circuits in the chassis. See FIG. 2 for a typical schematic of a mono audio processing IC.
FIG. 1. The shaded areas show the audio stages.
Sound IF Amplifier
The video detector outputs the FM audio signals to the audio IF stage where the 4.5 MHz audio IF signal is created by beating together the sound and video frequency (41.25 mixer frequency and the 45.75 oscillator frequency). The audio signals, at this point, are really FM radio signals. Remember, the video signals are carried on AM frequencies, and the audio signals are carried on FM frequencies.
The sound IF amplifier is a fixed radio frequency amplifier—it amplifies only one frequency, 4.5 MHz. There can be slight variance in the signal frequency, but the signal must be very close to 4.5 MHz for the audio processing stages to produce the correct sound. The 4.5 MHz signal is amplified and output to the audio IF detector.
When you troubleshoot the audio IF stage, measure the IC terminal volt ages and compare the values you get to the values on the schematic.
FIG. 2. A schematic of an audio processing IC.
Audio IF Detector
The audio IF detector receives the 4.5 MHz FM signal and traps the clean audio signal from the FM signal. The audio detector rarely fails. However, if it is a tunable detector that has not been tuned properly, you might hear a buzz in the sound. When the signal is clean, it is output from the detector and sent to the audio amplifiers, volume/tone control circuits, and audio output circuits.
Stereo
When an FM audio signal is transmitted, the signal contains the combined left and right signals (L+R). There also is a modulated signal which is the left signal minus right signal (L-R), which is necessary for the stereo sound. This signal is carried on an AM frequency carrier that is differentiated by the signal strength from the video signals also carried on the AM frequency. This encoding signal is used by the television to demodulate (decode) the stereo signal and produce stereo sound. Another part of the regular audio signal, called a pilot signal, tells the receiver whether or not the signal being received is stereo. Remember, many television stations still do not broadcast in stereo. Sometimes there is also a SAP (second audio program) signal which is part of the regular audio signal. SAP is used to broadcast in a second language. All of these signals are considered to be composite audio signals.
Non-stereo televisions can receive all of these signals, but if the television does not have a stereo demodulator, the television cannot decode the stereo signal. The 15.734 kHz FM modulated signal goes to the audio processing circuits, where it is demodulated, filtered and amplified. It then goes to either the A/V switching circuit or the audio amplifier/control circuits. In addition, some televisions can produce a stereo-like sound that is not true stereo. This is done by splitting the audio signal between two speakers. Likewise, some televisions can receive but not demodulate and reproduce true Dolby Surround Sound.
The pilot signal is used for detecting stereo broadcast signals. After the pilot signal is detected, the audio processing circuits tell the system control IC that stereo is available. The system control IC will determine if the audio signal is a stereo, SAP or mono signal. The audio processing circuit then outputs the proper signal to the audio amplifier/control circuits.
Low bandpass filters are used to separate the composite audio signals. The 15.73 kHz regular monaural audio (L+R) signal is sent to an amplifier where it is amplified and output to the audio matrix.
The 31.468 kHz AM modulated stereo signal (L-R), and the 15.734 kHz pilot signal, are sent to the audio processing circuits. The 31.468 kHz is demodulated into the L-R stereo signal. Then, the demodulated stereo signal is sent to the noise reduction circuit where the noise resulting from the high frequency of the pilot signal is eliminated. The cleaner L-R signal is sent to the audio matrix, where it is combined with the regular audio signal (L+R) in order to produce stereo sound.
The SAP signal, which is filtered out by the 78.670 kHz filter, also passes through a demodulator, then through a noise reduction circuit because it carries noisy high frequencies. The SAP signal then is on standby until the SAP function is activated by user choice.
Surround Sound
Surround Sound creates in a television the effect of movie theater sound. Three common types of surround sound circuits are:
1. Matrix is stereo sound in which one speaker produces the regular stereo sound and the other speaker produces the L-R signal. The result is a richer sound.
2. Hall matrix is the same as matrix except that there is a time delay of 32 msec added to the rear speakers so that the sound has some reverberation and creates the effect of being in a concert hall.
3. Dolby reproduces sound recorded in Dolby mode. There is a left and right channel. Also, there is a center channel, which is the monaural signal (L+R). There is a 20 msec time delay which causes the listener to think that the sound is coming from all over the room. This type of system requires a center speaker. Dolby ProLogic further enhances the total sound effect and provides sound from four speakers—left, right, center and surround.
If the television program or movie you are watching is recorded and trans mitted using true Dolby Surround or Dolby ProLogic you will need Dolby processing in your television or in an external surround sound processing system attached to your television.
Audio Amplifier
The audio amplifier amplifies the clean audio signal. The volume control is in the audio amplifier. When the signals are sufficiently amplified, the signals from the audio amplifier are sent to the speaker(s) which produce the sound. Normally, the audio amplifier is equipped to mute the sound whenever the system control tells it.
Troubleshooting Symptoms
Listen carefully to the sound from the speakers. As you do, vary the volume control. Usually, you can hear problems with a television’s audio system. The sound might be missing, weak, distorted, intermittent, or contain interference. However, problems with the sound system can begin outside of the audio circuit. If the sound problem is accompanied by a picture problem, the problem is probably being caused by a faulty IF processing circuit. For example:
1. Poor sound quality accompanied by an unstable picture suggest a problem in the AGC circuits.
2. Sound and picture do not track properly, suggesting a problem with the detector coil or poor sound alignment.
3. A hum or buzz in the sound accompanied by a running sync suggest a problem in the sync or AGC circuits.
4. Squealing or whistling, distorted or intermittent sound, or no sound, even at maximum volume, accompanied by a normal picture and raster suggest a problem in the audio processing circuits.
To troubleshoot the audio processing circuits, you can use an MIS signal generator, and an oscilloscope to inject a signal beginning at the output of the video detector and test each output to input in the audio circuits until you reach the speakers. Also, you can use an external audio amplifier and trace the signal from the input of the last amplifier back through the audio circuits until you reach the output of the audio processor.
No Stereo
If there is no sound at all, check all of the audio processing circuits. If the television is supposed to receive stereo and the sound is not in stereo, make sure the program you are listening to is broadcast in stereo. If the program is supposed to be in stereo, and the monaural sound is not distorted, the pilot demodulation circuit (L-R) might be faulty. Using an oscilloscope, check that the correct signal is being input into the demodulator. Remember, the signal is really the composite audio signal minus the SAP signal, or in other words, all signals below 45 kHz. Then, test the output from the demodulator circuit. Continue testing circuits, and the components and connections around the circuits in the stereo demodulator path, until you locate the faulty circuit, component or connection. If you inject a test signal, remember that to test the regular audio processing circuit, use a mono signal of 15.734 kHz, FM modulated. To test the stereo (L-R) signal, use a signal of 31.468 kHz, AM modulated.
If there is no monaural sound but there is distorted stereo sound, check the audio IF processing IC.
If there is noise in the stereo signal, check all of the circuits in the stereo processing IC, starting with the low bandpass filters. The filters might be letting in extraneous frequencies that the noise reduction circuits are not eliminating. Or, the noise reduction circuits might not be eliminating the high frequencies correctly, which might indicate a faulty IC.
No SAP
Follow most of the troubleshooting guidelines for troubleshooting stereo. Also, if you inject a test signal when troubleshooting the SAP processing circuits, use a 78.670 kHz FM modulated signal.
Troubleshooting Example
If you suspect that a speaker is faulty or open, inject an audio signal into the input of the speaker and listen. If you hear a hum or tone from the speakers, the speakers are probably operating normally. However, you still have to listen carefully to the audio produced by the speakers. There could be a warped, torn or broken place in the cone. Also, measure the resistance in the speaker’s coil using an ohmmeter. The number of ohms should match the value on the speaker’s specifications. Also, if the speaker is working, you should hear a single click as you attach the ohmmeter.
If you do not hear atone, the speaker may be open:
1. Substitute another speaker for the suspected speaker.
2. Turn the volume control to midrange, because you cannot conduct tone tests with the volume turned down.
3. Check the audio amplifier’s voltages. The values should match the values on the speaker schematic. If they do not, there might be an open connection or a leaky transistor or capacitor in or around the audio amplifier stage. Also, the voltage being received might be too high or too low. Use signal injection to check the inputs and outputs, and locate the faulty component, connection, or IC.
4. Test the components around the audio amplifier stage for opens, shorts or value changes. The values should match the values on the audio schematic. If the values are not correct, replace the faulty component.
No Sound
You can use signal tracing with an audio signal tracer to test the circuits when there is no sound. In audio stages, where there is sufficient gain, you can use a 500uF electrolytic capacitor and a speaker with a clip wire. Clip one side of the speaker to a good ground and the other speaker lead to the capacitor. Input a signal from an audio signal generator, and using the end of the capacitor lead, probe for the signal at the output.
You can use a voltmeter to test the voltages, and an audio signal generator and an oscilloscope to check the waveforms:
1. If the speakers do not produce sound, not even a hissing noise when the volume control is at maximum, there might be an open connection in or around the audio amplifier.
2. If there is a humming sound, the speakers and audio power amplifier are working correctly, the problem is probably with the voltage from the power supply or the audio driver amplifier.
3. If you do not hear hum, the problem is with the speaker(s) or the audio power amplifier. For example, the speaker could have a broken wire in the voice coil or an open coupling capacitor to the speaker.
Weak Sound
Weak sound can be caused by a leaky or open transistor or capacitor, a resistance change or voltage that is too high or too low. Also, if the cone in the speaker is defective or frozen, the sound will be muffled, distorted or weak. If the sound is weak and slightly distorted, some frequency variations may be getting through the detector coil. Therefore, adjust the detector coil to fine tune the audio take off if the circuit is adjustable.
If the detector coil and the speaker cone are working properly, use signal tracing to test the circuits. Supply the signal with a signal generator and trace the signal with an oscilloscope. Also, measure the voltages on all IC pins and for all components around the IC to make sure the voltages sup plied to the IC are correct. When checking components, you may need to lift one side of a component to break the circuit before you test it for an accurate measurement.
Also, a change in the resistance of the collector and emitter resistors can cause the sound to be weak or distorted. Use a DMM to check for open connections or leaky components. Also, make sure to check the capacitors.
Squealing or Whistling Sounds
If you hear a squealing or whistling sound, the source of the problem can be in other circuits of the chassis, but usually there is a faulty transistor or capacitor in one of the audio stages. Check each of the components around the IC, and compare the measurements with those on the schematic.
Humming and Putt-Putting Sounds
If you hear a humming sound, there might be a faulty capacitor or filter regulator in the circuit. Also, there might be a short or a bad solder connection. Listen for the hum with the volume turned up and down. Use signal tracing and test the voltages at all input and output connections. Check the IC and all of the components around the IC. A poor ground connection or broken path in the audio circuits can also produce hum or putt-putting.
Most audio problems are found to be in the audio circuits, but puff-putt sounds, also called “motor boating”, can be caused by wiring problems in the horizontal circuits. If the picture is also erratic, check the other video circuits as well. When you locate a transistor that might be the source of the problem, short the emitter and base terminals of the transistor. If the motor boat sound disappears, replace the faulty transistor.
Crackling or Popping Sound
If you hear a crackling or popping sound when you turn on the television, check the audio amplifier stages and the power supply filters. Faulty transistors will crackle when heating up. Therefore, you can use a cold spray to test the transistor. Take the transistors out of the circuit before you test their resistance. Open filter capacitors in the audio circuits may also cause pop ping sounds.
Distorted or Intermittent Sound
If the sound is distorted, suspect a faulty speaker, transistor, resistor, sound alignment or IC. These components cause most of the sound distortions you hear from the speakers:
1. If the sound is tinny or “mushy” when the volume is turned down, or dull, low-pitched, and vibrates (often called blatting) when the volume is turned up, check for a faulty transistor, resistor, speaker, sound alignment, or IC.
2. If the sound is extremely distorted, check the speaker, the sound alignment, and check for leaky or shorted transistors, capacitors or connectors. Remember to test the bias and base resistors when you remove a transistor from the circuit for testing.
If the sound is intermittent or erratic as if the audio frequency is drifting, or if the sound is muffled, the detector coil might need adjustment, or the audio IF stages may be defective. Use a special adjustment tool and make small adjustments to the coil until the sound is clear. Use an oscilloscope to test the audio signal’s waveform, output from the detector coil, until the signal is adjusted properly.
Loose wiring, and open or shorted connections can also cause intermittent sound problems. With the chassis and speakers on, lightly pull on components to see if they are loose. With the chassis and speakers on, use a cold spray to locate cold solder joints or heat-sensitive components.
Quiz
1. What is the SIF frequency?
2. Describe a key purpose of the pilot signal regarding stereo sound.
3. Is the SAP signal AM or FM modulated?
4. At what stage is the volume control located?
5. At what frequency is the stereo signal modulated?
Key
1. 4.5 MHz.
2. It tells the system control that a stereo s is present.
3. FM.
4. The audio amplifier.
5. 31.468kHZ