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Above have been described the parts of a stereo system in the form of modules and problems that are often encountered. In this section you will be explained how to track damage to a stereo amplifier. In this case, only one of the two identical amplifiers will be discussed. If damage occurs, one of the two amplifiers is damaged. By checking first and turning off one of the amplifiers, then you listen to one part alternately. Figure 6.65 shows a block diagram of a stereo amplifier consisting of two front amplifiers and a final amplifier.
Figure 6.65: Block Diagram of Stereo Amplifier System
The first front amplifier usually consists of an IC1 and is only used to amplify the phono output signal. The second front amplifier usually consists of an IC2 and a driver transistor (Q1), and is used to amplify the output signal from the tuner, tape, VCD/DVD or other inputs. The function selector switch, volume control and tuner control are usually always placed before the power amplifier. Different outputs, at different levels, produce audio signals for speakers, headphones. The center channel output, a network that can be used to produce output for a third speaker. Signal level measurements need to be made to determine whether the amplifier is functioning properly.
Figure 6.66: Audio Level Graph For The Amplifier In Figure 6.59
Figure 6.66 shows a level diagram, to show the relative signal levels, either above or below dBm and in millivolts. If you use the level diagram information for the block diagram of Figure 6.62, you can easily determine the faulty part. For example, if the input signal at point 2 is 140 mV, then at point 4 the signal should be 0.8 volts, by setting the volume control to the maximum position if the signal is less than 0.8 volts, then there is something wrong with the front amplifier. Frequency response, gain linearity, freedom from noise and other interference are also important parameters in stereo amplifier operation.
Recognizing Damage to Stereo Systems
Damage to the stereo system can basically be divided into two groups, namely: mechanical damage and electronic damage. Because most electronic parts consist of ICs, damage or interference often occurs in the mechanical part.
1. Mechanical damage
The drive mechanism on the tape and tuning is a mechanical part that often experiences interference. On the tape, a rubber drive belt is usually used to transfer the rotation of the motor to the tape transport. Because this drive belt has a limited service life, it is often a source of interference. If you feel vibrations in the electric motor, but there is no movement in the tape transport, then most likely the damage is in the drive belt. If this happens, then you must replace the drive belt with a really good one.
The part that is no less important as a source of damage is the motor itself. Lack of lubrication, poor mechanical adjustment, will cause the motor shaft and drive to jam. You can find out when checking for damage to the drive belt. By giving a little lubricant, this problem can usually be solved.
2. Electronic damage
Since the electronic circuits in tuners and audio equipment are subject to failure, failures are most often found in the power amplifier and power supply sections. This is where the components are most stressed and the exciters tend to increase, which shortens the life of capacitors and semiconductors. Most power amplifiers are push-pull. If any of these push-pull circuits fail, it will cause distortion in the audio output and this will be immediately recognizable by ear. The audio transistors that are part of the power amplifier can be checked with an ohmmeter to see if they are open or shorted (See Chapter 4). If you must replace these transistors, make sure that the replacement part is correct, and that it is installed in the correct place in the correct manner.
Audio transformers are also often damaged. Speaker coils can break, caused by current shocks. However, damage to the speaker is often purely mechanical. The voice coil mechanics can bend and this causes friction around its surface. Speaker damage (broken speaker cones, loose connections, etc.) can also occur.
3. Acoustic Damage
People often ignore this damage. They think that if the audio circuit works well, the speakers are good, and if the signal from the source is good, then good sound can be obtained. In fact, the acoustic situation in a particular place is far from ideal. The same hi-fi equipment will produce different sounds when used in different environments.
Wall hangings, floor carpets, room size, and the position of doors and windows will affect the actual sound quality. Figure 6.66 can be used to provide an overview of acoustic problems.
Figure 6.66: Overview of Acoustic Problems
The correct installation of the right and left speakers will produce perfect sound, if the listener sits at point B. Suppose a window is installed on half of the wall behind the listener, and wall hangings cover the remaining half of the wall. If the listener moves to point A, he will still be able to hear directly, both from the left and right, but the strongest reflections will be obtained from the surface of the window. These reflections depend greatly on the dimensions of the room, and serious interference may occur to the sound received directly. If the listener moves to point C, the wall hangings will absorb the sound and will prevent reflections, and he may get a much better sound than at A.
In a very large room there is a dead point, which is a very low volume, because there is an elimination of reflected sound and that which is received directly. In addition, there is also a hard point, which is the sound received feels very sharp. Therefore, first make sure the environment is very good for installing your audio equipment, in order to obtain perfect sound quality.
So it is very complex to find damage to a stereo system, so special skills are needed for that.
Identifying Damage to Modular Stereo Systems
Before you turn on the system, pay attention to the following questions:
- How does the unit work? (don't ask what's broken, but what are the symptoms). How does it sound?
- When do the symptoms appear? Irregular or all the time?
- From every source (AM, FM, phono etc.) At all volumes? Were there any particularly powerful household appliances at the time?
- Did the symptoms come on gradually or suddenly?
- Do these symptoms change as the device gets hotter?
- Has the user moved anything, or changed/added components to the system?
- Are there any strange/unfamiliar sounds or smells? Are the light panels on or dim?
If you hear symptoms, to identify whether the power amplifier is damaged (no sound, distortion, crackling sound, lamp unit is off or flashing), then do not turn on the system. This symptom also appears if there is damage to the power supply.
This problem can cause disasters, such as smoking, sparks and damage. If these symptoms occur every time the device is turned on, it indicates a power surge that is causing damage to the device. Look for the problem in:
- Connections around the power supply,
- Power switch
- Power Processor
- Check the large capacitor on the power supply
- Check the wiring at the power switch.
1. System Check
If you are sure that there is no damage to the supply, then turn on the system, and then observe the symptoms. Remember the following steps to determine module failure.
- Perform a hum check, by removing various blocks from the system. If necessary, remove the switch, turn everything off, and disconnect all cables.
- If the problem appears in several places, check the processor system used in all operating modes. Then check sequentially until the last part (speaker)
- Remember that cables and connectors can also cause noise, or crackling sounds.
- Is there a poor connection (shaky for example)?
- If the module is new, the connection is suspect. Is each module connected correctly?
2. Module inspection
Begin visual inspection of the module
- Hot sign
- The area is clean, while the other part is dirty and full of dust. This indicates that someone has just repaired something around the area.
- Misplaced components because someone previously repaired or modified the circuit.
- Foil that has been damaged by heat.
- Misfit components (different sizes, ages, manufacturers, etc.)
- Check other parts (PCB, corroded plugs, cables showing heat damage, etc.).
Types of Damage and Symptoms of Amplifier Circuits
Below is a table of the types of damage and symptoms that occur if damage occurs to the amplifier circuit, be it the initial amplifier or the power amplifier.
Table 6.3: Damage to Small Signal Amplifiers (Pre-amplifiers)
| Jenis Kerusakan | Gejala |
|--------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------|
| Komponen bias rusak, rangkaian terbuka atau harga resistor terlalu besar | Titik kerja bergeser menuju titik mati transistor cenderung mati, gejalanya terjadi distorsi besar, atau bahkan tidak ada keluaran |
| Kapasitor kopling atau decopling hubung singkat | (gejala sama dengan diatas) |
| Kapasitor kopling terbuka | Bias DC normal: tidak ada keluaran, karena sinyal dari tingkat yang satu tidak dapat diteruskan ke tingkat berikutnya. |
| Kapasitor decopling terbuka | Terjadi umpan balik negatif seri, sehingga penguatan menjadi lebih rendah. |
| Saluran daya kapasitor decopling terbuka | Menaikkan derau pada frekuensi 100Hz, pada keluaran penguat. Masukan penguat depan normal. |
| Saluran rangkain umpan balik terbuka | Penguatan menjadi tidak stabil dan kemungkinan dapat terjadi osilasi. |
| Nilai kapasitor kopling dan decopling berkurang | Respon frekuensi rendah sangat kurang; lebar pita berkurang |Table 6.4: Power Amplifier Faults
| Jenis kerusakan | Gejala |
|-------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------|
| Resistor bias dari rangkaian terbuka atau nilainya membesar | Pada penguat kelas B akan memperbesar distorsi crossover. |
| Kapasitor keluaran hubung singkat | Sekering putus atau transistor menjadi panas. Lakukan pengukuran resistansi untuk mengetahui komponen yang rusak. |
| Potensiometer bias di set pada harga yang kurang tepat | Kenaikan distorsi crossover. Transistor Daya menjadi panas. |