Basic Car Amplifier Repair Tutorial not to be confused with the full repair tutorial (now available as a downloadable file)
Many people need help repairing their amplifiers but are afraid to ask for help because they don't want to sound like an idiot (asking stupid questions, etc...). The information on this page will help prevent asking questions that may be deemed as stupid. 'Stupid' questions are generally those asked by someone without enough knowledge about the subject to ask meaningful/useful questions. If you take the time to read this page in a manner that will allow you to absorb most (or all) of the information here (expect to spend at least one full hour reading and re-reading -- maybe a bit more if you're struggling to understand it), you will learn the proper terminology and the basic operation of an amplifier. In most instances, most of your questions will be answered here and you will only need a bit of clarification for your specific amplifier.
To help the page load more quickly and to save bandwidth, many of the photos will be loaded from links. When you're done viewing the photo, close the window. If you leave it open and return to this page, the next photo may load in the background window and it may appear as if the link isn't working. The green links are photos. Most of the other links are to web pages on this site. If you're serious about learning to do this type of work, you should follow the links and read the pages from top to bottom.
Amp Going Into Protect Mode?
If your amp is going into protect mode and you need to confirm that the amp is at fault, read through THIS page. If you know that the amp is at fault and you want to try to repair it yourself, continue reading here.
If You Need to Have an Amp Repaired
The information on this page is for those who are interested in trying to repair their own amplifiers. If you have no interest in repairing your own amp, you should look for a local repair shop before shipping the amp to be repaired. For most amps, the round-trip for shipping alone will be approximately $60 if you use a shipper with real tracking. For large amps, the shipping can be well over $100. If you don't know any local repair shop or you don't know which repair shop is reliable, call all of the local car audio shops in the area and ask them who they'd recommend to have an amp repaired. If you find that most shops send their work to one particular repair shop, that shop likely does quality work.
When I'm not overwhelmed with repairs, I will sometimes take in repairs that have to be shipped to me. If you need to have an amplifier repaired, email me. More details on payment, shipping charges and other information regarding amp repair can be found HERE.
If You Need Internal Photos
If you need internal photos of an amp to identify burned or missing components, email me. If I have them I'll email them to you.
Tips on Taking Photos when You Need Help with a Repair
Many people ask for help with their amps when trying to do their own repairs. Many times (most of the time) the photos are absolutely awful. It's generally not because their camera can't perform well enough. It's generally due to a lack of effort to learn what the camera can or cannot do. Near the bottom of this page, there will be a section dedicated to helping you take better photos. Better photos will make it much easier for me to help you get your amp repaired. If you've already read the repair information on this page and only need to see the new tips on photographing amps, click HERE.
Tips for Email or Forum Posts when Asking for Repair Assistance
If you're repairing an amplifier and need help, it's important that you do everything possible to make the transfer of information from you to me as efficient as possible. A few years ago, I had only a few people asking for help. Now I have to respond to hundreds of requests (literally) every week. To get to as many as people as possible, I need the information sent to me to be clear and concise. A new section has been added to help you get the information I will need. If you've never read this page, continue reading below and you'll come to the section I've referred to here. If you've read this page repeatedly and want to go to the new section, click HERE. If you're too lazy or getting the amp repaired isn't important enough to CAREFULLY and COMPLETELY read this section, please don't email me for help. Too many people need help with repairs to help those who won't put forth the effort required to get their amp repaired.
Schematic Diagrams for Amplifiers:
I will buy schematic diagrams. I mainly need diagrams for Korean and Chinese made amplifiers. If you have any that you want to sell, email me and I'll let you know if I'm interested. There are some manufacturers that will provide schematics for free. I will pay for those also (if I don't yet have them) because I don't have time to collect all of them.
This page is here to help answer many of the questions that I get about basic amplifier repair. This is essentially a 'Cliffs-Notes' type page and isn't nearly as detailed as the tutorial I sell. However, it should answer many of the basic questions. Unless otherwise noted, this page will be dealing with class AB amplifiers (most full range amps are class AB or some variant, class A, class B). If you have a question about a repair, feel free to email me.
Be Pro-active if you want to Learn to Repair Amplifiers:
If you're interested in doing repair work and you have to constantly wait for someone to provide every minute detail, you probably won't make it in this business. For example, if someone states that the datasheet for an IRFZ44 provides specific information for that component and you ask the person helping you what a datasheet is, you are not well suited for this type of work. Someone well suited for this type of work would have immediately Googled 'datasheet' and 'IRFZ44' and would have immediately had the answer. This would save the time between the posts or emails from the person helping you with the repair (or whatever you're doing) and shows that you're thinking and willing to put forth a bit of effort. This tells me a lot about a person's willingness/drive to learn. Too many people are lazy and would prefer to wait on an answer because that lets them relax and gives an excuse for doing nothing. If you're the type of person that prefers waiting, you are not likely well suited for any job where you have to think (and this type of work requires you to constantly use logic to find various faults). This may sound harsh and may make you angry (some people are very easily angered) but I know what it takes to make it in the repair business and being lazy won't cut it.
Basic Amplifier Layout:
Most amplifiers have switching power supplies. Generally, the power supply will be on the end of the amplifier near the B+ and ground terminals. The audio section of the amplifier is generally on the other end of the amplifier (most commonly the end where the RCA jacks are located). The power supply produces the various voltages required for the audio section. If the power supply is blown, there will be no audio (in most cases).
The following is a list of things that will help you from making a few of the most common mistakes. In other words... If you don't want your amp to go up in flames, read the following very carefully (don't just skim over it).
If the amplifier is under warranty and you break the warranty seals, the amp will not be covered by the manufacturer. If it's under warranty, you should send it back to the manufacturer for repair.
If you decide to repair the amplifier and you make mistakes, it could cost more than having the amp repaired by a repair shop. If you want to learn more about electronics, it may be worth the extra time and expense. If you simply want your amp to work properly and you have no interest in learning more about electronics, pay someone to repair your amplifier.
When powering up an amplifier that you're repairing, you should do so with a 10 amp ATC/ATO fuse in the B+ power line. A 15 amp ATC/ATO fuse may be required for amps rated over 1000 watts. If there is a serious problem causing excessive current draw, the fuse will help protect the amplifier. This is only used for low power and initial testing/troubleshooting. If you don't use a small fuse a very minor slip of a meter probe could result in hundreds of dollars worth of damage in a tiny fraction of a second.
If the amp blows the fuse and you can't determine why (you can't find any defective parts and there is no direct short across the B+ and ground terminals), you need to power the amp up via a 2 ohm high-power resistor (25-100 watt resistor) or some other current limiter (some people use an automotive headlamp - an H6054 works relatively well). The resistor or headlamp will go in series with the B+ power supply wire. They limit the current going into the amplifier but generally allow enough current so that you can troubleshoot the problem. Please note that the voltage at the amplifier will be significantly less than the supply voltage when the current limiter is inline and the amplifier has a fault that causes excessive current draw.
Whenever possible, you should have the power semiconductors clamped tightly to the heatsink. It takes only seconds for a transistor to overheat and fail. Failure can occur even with the 10 amp fuse or current limiter in line.
A note about PPI amplifiers...
Some of the PPI amplifiers use the bottom cover to clamp the transistors to the heatsink. If you remove the bottom cover from one of these amplifiers, the transistors will not be clamped tightly to the heatsink and may fail within a few seconds if power is applied. The bottom cover must be securely fastened to the amp any time power is applied. If troubleshooting requires that the cover be removed, it may be necessary to build clamps to hold down the transistors so they don't overheat.
Any measurements made with the multimeter set to ohms or diode-check will be done with the amp disconnected from the power source. Voltage readings will be done with the amp powered up. Be sure not to allow the meter probes to slip and short between terminals of the components when measuring voltage. It's very important that you don't come into physical contact with any of the components inside the amp when it's powered up. After removing power from the amp, wait at least one minute before touching any components.
When measuring the voltage, the amplifier's main ground terminal is generally (but not always) a good reference point (for placement of the black meter probe). If you get strange voltage readings, email me with the make and model number of the amp and I'll try to help you find the correct reference point.
Commonly Used Terms
On this page, you will see the terms below many times. The following section is to help prevent confusion and provide additional information. For more detailed information, follow the links provided.
Transistors are used to control the flow of current. They are like valves. There are many different types of transistors. In the power supply, you will almost exclusively find FETs (Field Effect Transistors). In the audio section, the power transistors (those mounted to the heatsink that drive the signal to the speaker terminals) may be either FETs or BJTs.
The power transistors in this amplifier are fully encapsulated. This is somewhat rare for the power transistors (fully encapsulated rectifiers are relatively common). If the amplifier has fully encapsulated transistors, you need to use the same type as replacements. The tab of the transistors here are completely insulated. In normal transistors, the metal tab is connected to the center terminal. If the tab is allowed to contact the heatsink, it will cause the amp to fail. In amplifiers that don't use fully encapsulated components, you will see that they have insulators between the transistor and the heatsink.
Resistors: Resistors are used in every amplifier. They can be virtually any size. Generally, larger resistors are used where higher power dissipation causes the resistor to run hot. In THIS photo, you can see the large 'emitter resistors' and the smaller resistors. The small resistors are in a part of the circuit that causes very little power dissipation. The larger resistors are capable of dissipating more power and would fail (from overheating) if they were as small as the other resistors. All else being equal, larger resistors have more surface area which allows them to dissipate more heat than a smaller resistor.
The transformer is used to step up the voltage from 12v to whatever is needed for the audio section of the amplifier. At 12v, the maximum power you can get is ~20 watts into 4 ohms. With higher voltage, you can produce more power. The main 'rail voltage' generated by power supplies in class AB car audio amplifiers is generally between ±30v and ±50v. In large class D amplifiers, the voltage can be greater than ±150v (I've measured more than 300v across the rails in some amps).
In most amplifiers, the rail voltage can be measured on the center leg of the 3-legged, dual rectifiers. In most class AB and class B amps (most full range amps fall into one of these categories) you will also find rail voltage on the center legs of the output transistors.
Capacitors: Capacitors are used to pass AC signals or to store energy. When used to pass AC signals (blocking DC), they are being used as coupling capacitors. When used to store energy, they are being used as filters. The large 'rail' capacitors are used to store energy. They provide energy between the pulses of the switching power supply. In THIS image, the rail capacitors are the ones on the left (blue). The capacitors on the right are the B+ filter capacitors. They are generally rated for higher operating temperatures. They also generally have lower ESR. This is important because high ripple current can cause capacitors with higher ESR to run hot and fail.
If you work on an amp that has a problem with the power supply, you're almost certainly going to see/hear the term 'gate resistor'. Here, the term indicates the way the resistor is being used in the circuit. The resistor passes the drive signal from the drive circuit (IC or transistors) to the gate leg of the power supply FET. For most FETs, this is the left-most leg of the transistor. Click HERE for the datasheet of a common FET.
Understand What Your Multimeter is Telling You
When checking semiconductors, you will use a multimeter. To be able to do any troubleshooting, you need to know what your meter is telling you. Try the following in ohms AND diode test modes:
Meter probes separated and not in contact with anything:
This is an open circuit. If your meter's probes are across two points in a circuit and the meter reads the same as when the probes are not in contact with anything, the circuit is open (no connection). More accurately, this tells you that the circuit is open to DC current (the circuit is not capable of passing the DC current that the meter is applying to the circuit). If you were using a meter that applied an AC voltage to the circuit (multimeters apply DC to the circuit), the reading may be different. Meters from various manufacturers will have various displays for an open circuit. 'OL' is very common. A Radio Shack meter of mine displays 'OF'. Another meter I have displays '1. '.
Meter probes touching:
When you touch your meter's probes together, you should read all zeros (in diode test mode) or something very near 0 ohms (when set to ohms/resistance mode). If you don't have similar readings, you meter may be defective, it may have a low battery or the meter leads may be defective.
Before you do any testing, you should set your meter to ohms and touch your probes together to make sure the meter is working properly and the leads are not defective. This is especially important when you're using your meter to confirm that there is no voltage in a circuit. Of course, you must remember to set the meter to voltage mode before touching the probes to a circuit that could possibly have voltage on it. Some meters will be badly damaged if connected to a live circuit when they're set to ohms or diode check.
There are many different ways that an amp can fail but the two most common failures are shorted output transistors and blown power supply transistors (< those are not blown). There are several types of protection circuits in amplifiers. The most common are over-current and thermal. The over-current protection is supposed to protect the output transistors. Sometimes it doesn't work well enough to prevent the failure of the output transistors but it will work well enough to shut the supply down before the power supply FETs are destroyed. If the amp remains in protect mode, goes into protect mode or blows the fuse as soon as the remote voltage is applied, shorted output transistors are almost certainly the cause.
If the fuse protecting the amp is too large, if the protection circuit doesn't respond quickly enough or if the power supply is poorly designed, the power supply transistors may fail. If you see a lot of black soot on the power supply transistors (near the power transformer), the power supply transistors have failed. Soot on the board doesn't necessarily mean the transistors have failed. Sometimes, technicians don't clean up the mess from a previous failure.
Transistor Failure/Checking Transistors:
In general, when a transistor fails, it will either short (common for output AND power supply transistors) or open (common for power supply transistors). Transistors act like valves. They control the current flowing through a circuit. A shorted transistor acts like a valve that's stuck open (passing too much current). In the case of an output transistor, the shorted transistors tries to deliver the full rail voltage to the speaker output terminal. If you've ever seen a damaged amp that pushed or pulled the speaker cone to its limits when the amp powered up (common on some Rockford amplifiers), that was almost certainly due to a shorted output transistor. When checking transistors, you most commonly look for shorted connections inside the transistor. You do this by using a multimeter to look for low resistance connections between the transistor's terminals.
I used the terms short and open on the previous paragraph. A short (short circuit) is a path through which current flows that should not be there. An open (open circuit) is a break in the circuit.
Right-click to zoom in.
When checking power transistors in the power supply or the audio section of an amplifier, you look for shorts between the legs. If you get a reading of near zero ohms with any combination of the meter probes across the legs of any individual transistor, the transistor is likely shorted (or is in parallel with a shorted component).
If you check 3-legged rectifiers in the board, you'll find a short between the outer legs (for the most common 3- legged rectifiers). The short you're reading is actually the windings of the power transformer. If there is no short between the center and outer legs of the rectifier, it's likely OK. THIS page tells you how to check the most common transistors. Scroll down to the section labeled "Checking Field Effect Transistors". The tests for BJTs are farther down the page. THIS page shows a slightly different way to check transistors and has applets to allow you to send me or post screen caps of the readings from transistors (click the black bar at the right of the applet for instructions).
When power supply transistors fail dramatically (smoke, flames, soot blown out onto the board...), they generally short internally (all terminals shorted together) but the high current flow causes the third leg to fuse open. Sometimes, there is no visible damage but the third terminal opens internally. In these instances, you can see a direct short between the first and second terminals.
This page is a VERY basic introduction into car audio amplifier repair. If you're interested in learning more about troubleshooting, the commonly used components in car amplifiers and the way they work, you may be interested in the full version of the repair tutorial. The full version also contains many examples of common problems with many of the most common amplifiers. This includes tips for common problems that may not be apparent when replacing blown components which can result in premature failure of the amplifier. If you're interested in learning more, click HERE. As of now, the 'downloadable' version of the tutorial is $20 less than the DVD version.
In most amplifiers, you'll find groups of parallel components. The components are used in groups because a single component can't handle the stress. When in parallel groups, the components MUST have virtually identical characteristics. If one or more are just slightly different, the load will not be shared equally and it can cause the amp to fail prematurely. When one component in a parallel group fails, all in the group MUST be replaced for optimum reliability. Even if the replacement part has the identical part number, it will not be exactly the same as the original parts and won't share the load properly. The date or production code is a good way to identify parts that are very similar. Parts with the same date code will have identical markings printed/etched on their face. If there is any difference in the markings, they may not be well matched. If the parts have the same date code, they are going to be closely matched. Generally, when you order parts, you get the same date code but that's not always the case. If you buy 6 parts and the distributor has them prepackaged in packs of 1s, 5s, 10s, you will get one pack of 5 and one single pack. These are unlikely to have the same date code. If you are not ordering a full stick of parts (50pcs/stick for TO-220 parts), order more than you need so you can be relatively sure you will get enough matched parts.
The following image shows the parallel components for an audio amplifier. The configuration will vary from amp to amp but the basic layout of the parallel components will be similar to what you see here. For switching power supply components, there are no emitter resistors. This makes the matching of the components that much more critical.
Datasheets are documents that tell you virtually everything about a particular component (including pin configuration, the pin numbering scheme and the names of the individual terminals -- particularly important for transistors). For a single transistor, they can be 10+ pages long. For ICs (Integrated Circuits -- chips), they can be 20+ pages long. Datasheets can be used to find the specifications like current capacity and maximum operating voltage. This is handy when you need to substitute a part that's no longer available. If you need a datasheet, use Google and search for 'datasheet' and enter the part number of the component. If you use Octopart, you don't need to enter the term 'datasheet'. A link to the datasheet will be provided when the results for the search are returned. If you use Mouser or Digi-Key for parts, when you look up a part, a link will be provided for the datasheet on the information page for the part.
The following are 3 datasheets for commonly used components.
These are FETs that you'll find in some amplifier's power supplies. They are rugged and relatively inexpensive.
This part is packaged in several different ways. The different packages are used for various types of layouts (surface mount or through-hole). The specifications are very similar except for the power dissipation rating. The larger components can dissipate more heat so they are rated higher.
This is the most commonly used PWM control IC. It's used to drive the power supply transistors. Most commonly, the IC drives a buffer circuit and the buffer drives the FETs. If you learn this IC, it will make it MUCH easier to troubleshoot amplifiers. After you've learned this IC, other driver ICs will be easier to learn. They all do the same basic thing but have slightly different features.
In general, it's best if you use replacement parts that are the same exact part number as the original. In some cases, it's OK to make a substitution if the replacement has better specifications than the originals. Until you know an amplifier very well, use the original parts. In class D amplifiers, you should not make any substitutions. In some cases, the new replacements will vary enough from the originals that the same part number component will not work properly.
Surface Mount Components
Surface mount components make a repair slightly more difficult. They are typically smaller and the pads can be damaged easily if you're not careful. Using good equipment and good technique, you will be able to replace them easily. In THIS photo, you can see several surface mount resistors and a surface mount semiconductor (a diode). These commonly burn when the power supply fails. To remove them, you apply new solder to both sides, heat one end for 2-3 seconds and then move the iron to the other side. When the solder melts, the resistor will slide off of the pads (if it's done quickly enough that the other side hasn't had time to cool).
You read the value off of the surface mount resistors just as you would for resistors that use color codes (except you don't have to remember the colors). The resistors marked 680 are 68 ohms. The resistor marked 101 is 100 ohms. Refer to the Resistors page for more information on reading resistor values.
Good Quality Equipment/Tools
As with any type of work, it's easier to do a job with good quality tools. The following are some of my suggestions.
When someone asks me to recommend a soldering iron, I always recommend Weller soldering irons. My favorite is the WES51. They can be purchased for ~$100. If that's more than you want to spend on an iron, the WP-35 is another good iron. It's ~$45. If you've never used anything other than a budget/RS brand iron, you won't believe how much easier it is to make good solder joints with a professional quality iron.
To replace defective components, you'll need to remove them from the board. To remove to old solder, I generally use a desoldering pump. They are efficient and relatively inexpensive. I recommend the Edsyn DS017. I've tried others but none have been as reliable. You can purchase these directly from Edsyn.
Good quality desoldering braid is very good at cleaning up the pads for surface mount components. Don't buy the braid from Radio Shack. I've tried it several times and in every case, it was old and too oxidized to do any good. I generally buy Chem-Wik 10-100L. That's the 100ft roll. You'd likely buy the 10-5L or the 10-25L. This braid has a rosin flux that helps draw the solder into the braid. You can purchase Chem-Wik braid from Digi-Key.
I've used a lot of different brands of solder but now I almost exclusively use Kester 44. I've been using it for more than 10 years and the quality of the solder and the flux has always been top-notch. When you select a solder, you want rosin core solder, not acid core solder. Acid core solder is for soldering copper pipes or radiators, not components on circuit boards.
A good quality, reliable digital multimeter is an essential tool. The DMM is used check virtually all of the components in a piece of electronic equipment. As long as the meter is reliable and accurate, it doesn't need to have a lot of features. THIS meter is just about as basic as you can get but it can perform virtually any test you need for basic repair work. I have lots of other meters (some much more expensive) but this is the meter I use on a daily basis (if you have a Fluke 10, 11 or 12 that you want to sell, email me). In addition to the basic volt/ohm functions, your meter should also have a diode-check function. If you're going to buy a meter, I strongly suggest buying a Fluke brand meter.
An oscilloscope isn't required to do all repairs but it allows you to confirm that the output is clean and that the power supply waveforms (particularly the gate drive waveforms) are as they should be. You don't need a 200MHz scope for audio repair. I've used a 2MHz scope (Tek 5110) and it worked perfectly well. If you want an inexpensive scope that will do virtually anything you'll never need for car audio amplifiers, you may want to try to find an old Tektronix 465 or 465B.
If you don't understand precisely what a scope does and how they function, read the Oscilloscope page of this site (page #73). There are a lot of people who have been using a scope for years and still don't know how to use them. For those people, there is a section labeled 'Best Initial Settings'. That section tells you precisely how to set the scope for the most common tests performed in car audio amplifiers.
At the END of this page, I've posted a couple of photos and more information that may help you in your search for a usable scope.
When working on car audio amplifiers, it's not convenient to have to reinstall it in the vehicle for testing. You'll want to use an AC to DC converter (regulated 12v power supply). These can vary in price. Surprisingly, Pyramid/Tenna supplies are pretty good. You'll need a supply rated to supply 35 amps continuously. If you can afford it get a larger one. I'd suggest buying a supply that has a meter to show current draw. Without the ammeter, you don't know if an amp is pulling excessive current.
If you're on a tight budget, you can get by with a battery and battery charger but as soon as you can afford to buy something better, you should do so. It's not safe to charge a battery in a confined space and most chargers produce a lot of electrical noise/hum (not something you want when troubleshooting an amplifier).
For those who will do a lot of repairs, you will need a sturdy set of shelves. The following shelving unit is one that I built to for my shop. If you're interested on building it, you can find the 3D model and information for the software to view it on one of my OTHER sites. Go to page 10a in the directory of the site.
Back to Repair Information...
Gate Drive Signal:
I previously mentioned power supply drive circuits. Here, I'll go into a bit more detail. The TL594 (or it's close relatives, the TL494 or the KA7500) produces the square wave drive signal. In some amplifiers, the power supply voltage is regulated. For regulated power supplies, the square wave output has a variable duty cycle. It modulates the pulse width to maintain the target rail voltage. This means the the 'on' time won't always be 100% (50% per output). For more information on PWM supplies, follow THIS link. For unregulated supplies, the pulse width is constant. For the TL594, the output is typically taken from pins 9 and 10 (MTX, Sony and a few other amps use pins 8 and 11 as the outputs). This is driven into driver transistors. In THIS photo, the drivers are the small rectangular components with the circuit board designations Q903, Q909, Q904 and Q910. They act as buffers and produce higher drive current than the TL595 can produce.
The driver transistors drive the square wave signal into the gate resistors (the resistors connected to the gate terminal of the power supply FETs). In the photo, resistors R924, R928 and R934 are the gate resistors for 1 of the 2 banks of power supply FETs (3 FETs per bank in this amp). In most amplifiers, the value of the gate resistors is between 27 and 100 ohms. These are a bit higher at 120 ohms. It's common for the gate resistors to fail when the power supply transistors fail. As I mentioned before, the gate often shorts to the drain (terminals 1 and 2 of the FET -- look at the IRF1010EZ datasheet, page 9). When this happens, the drivers have to work against the internal short of the FET to try to pull the gate voltage down. It's essentially impossible because the terminals are fused-together internally and terminal 2 is essentially connected directly to the B+ terminal of the amp. If the drivers are tough, the gate resistors will fail. if the drivers can't handle the current (when trying to pull the gate voltage down), they will fail (unless the gate resistors fail first).
Rectification and Filtering:
The output of the power transformer is sent to two dual rectifiers. All of the positive pulses pass through one rectifier and to the positive rail capacitors. The other other rectifier passes all of the negative pulses to the negative rail capacitor. This rail voltage is the source of power for the outputs transistors in the audio section of the amplifier.
When troubleshooting an amplifier, you must confirm that the power supply is producing both positive AND negative rail voltage. There are a few amps that don't have negative rail voltage. The class D amps based on the HIP4080 driver IC will only have positive rail voltage. Lower voltage (±15v) will be produced for the preamp section of the amp but the power amplifier section doesn't use a negative power supply. 'Chip amps' which operate off of the B+ voltage (like the internal amplifier of a head unit) do not have a switching power supply and will not produce a negative power supply voltage.
Linear Voltage Regulators:
In most amplifiers, the rail voltage is much too high for the op-amps. To lower the voltage, voltage regulators are used. There are many types of regulators. In this amplifier, Zener diodes are used to set the regulated voltage. Series-pass transistors (BJTs) are used to increase the output current capacity of the regulator. The regulators for this amp can be seen HERE.
You must confirm that the pre-amp op-amps have both positive and negative supply voltage. You will find regulated voltage from ±10v to ±18v in amplifiers. ±15 is the most common voltage for the op-amps.
Getting a Clean Input Signal:
In most amplifiers, the input circuit is either an active noise cancelling circuit or the amplifier has an isolated secondary. You can find more information about these on THIS page. This amplifier uses an active noise cancelling circuit. Most budget amplifiers use an isolated secondary. The input op-amp is generally very near the RCA jacks like THIS. It takes 2 op-amps for each input. Each of the 8 pin op-amp packages in the photo contains 2 op-amps.
Not all op-amps are in flat packages. THIS dual op-amp is in an SIP (Single In-line Pin) package (vs the DIP, Dual In-line Pin package). The op-amps in this package are being used in the filter (low pass crossover) circuit. The low pass/bypass switch on this amp selects between the crossover or the full range signal.
Power Amplifier Section:
After the switch, the signal is fed into the power amplifier section. In virtually all commercially available, solid state amplifiers, there is an error correction circuit. It greatly reduces the distortion at the output of the amplifier circuit. The 'brains' of this error correction circuit is the 'differential amplifier'. The input signal is fed into the input of the differential amplifier. A fraction of the output circuit is fed back into the other side of the differential amplifier. If there is any difference in the two inputs of the differential amplifier, the error will be corrected. In this amplifier, the two transistors of the differential amplifier are in one package. This insures that the transistors are closely matched and maintain the same temperature (which helps insure that they remain matched).
For most of the transistors using the Japanese numbering system, you add a 2S prefix to the part number. The dual transistor above is a 2SC5168. For transistors manufactured by KEC, the prefix is KT instead of 2S.
In most amplifiers, the differential amplifier is driven into a voltage amplifier. A differential amplifier can't swing its output fully from rail to rail but the voltage amplifier can (or at least get very close to the rails). The voltage amplifier drives the driver transistors and the driver transistors (2SC3421 and 2SA1358) drive the output transistors. The output transistors drive the speakers.
Sometimes, when the output transistors fail, they will cause the drivers to fail. If you're repairing an amplifier that has blown output transistors, you should check the driver transistors also.
In the driver transistor photo, you can see a small transistor between the output transistors on the heatsink. This is a bias compensating transistor. Previously, I mentioned that the dual transistor package insured that the two transistors maintained the same temperature. If the transistors are not at the same temperature, their electrical characteristics will not match (even if they are otherwise identical). The same is true for the output transistors. If you set up an output section to where there is always a tiny current flowing through the transistors (class AB operation), this current will change as the temperature of the transistors changes. To compensate for the changes in the electrical characteristics of the output transistors, a bias compensation circuit is used. The small bias compensation transistor prevents the bias current from changing significantly. Without it, the output current would increase significantly as the temperature of the output transistors increased.
Many times, there is a bias adjustment potentiometer. It allows you to set the bias/idle current of the output transistors. If the bias is set too high, the outputs will run too hot and be more likely to fail. If set too low, the output could be distorted. There have been many instances where someone has gone into an amplifier and turned the bias pot up (thinking that they were increasing the power output of the amp). This caused the amp to fail. Unless a potentiometer is accessible from the outside of the amplifier, you should not turn it unless you know what it is. Having the bias current set too high is like having the idle setting on your car's engine set to 3000 RPMs instead of 750 RPMs.
Troubleshooting a Dead Amplifier
Previously, we looked a scenario where the amp would try to power up but would go into protect or blow a fuse. Troubleshooting a dead amplifier is different.
When an amplifier will not power up, you have to confirm that you have ~12v across the B+ and ground terminals. You also must check the fuses (all must be intact). If those check out, you need to open the amp and look for signs of damage. If there is no visible damage, you need to begin checking voltage on various points. Connect your black meter probe to the chassis ground terminal of the amplifier. Measure the voltage on the center leg of the power supply FETs. It should be at or near 12v. If that's as it should be, check the voltage on each pin of the TL594 (or whatever driver IC the amp uses). Using the datasheet, determine if the IC is operating properly. Each IC has various input pins (to error amps or other internal control circuits). Determine if the IC is operating but being shut down or if the IC isn't operating properly. If the IC has the correct voltage on the 5v reg and the correct voltage on the sawtooth waveform pin, the IC is likely OK. The following is a list of basic checks:
Make sure the IC has B+ voltage on its control and B+ inputs. In general, the TL594 will have 12v on pins 8, 11 and 12. Without voltage on pin 12, the IC won't power up. With no voltage on pins 8 and 11, it can't have output (most MTX, some Sony and some Xtant amps are different).
All of the PWM control ICs have 5v regulators. Measure the voltage and make sure it's within a few percent of the rated voltage.
On the TL594, pins 3 and 4 must be below ~3v for the IC to produce output. If it's above 4v, something is shutting the IC down.
Pin 7 on the TL594 is ground. It must be directly connected to chassis ground.
For an unregulated power supply, the output pulses on pins 9 and 10 will read ~1/2 of the B+ voltage when using a multimeter set to DC volts. If the power supply transistors have failed, after you remove them, make sure that you read ~1/2 B+ voltage all of the way to the gate of EACH power supply FET. If it's too low or too high on any single FET or either bank of FETs, there is a problem. If it's a problem on a single FET, the gate resistors is likely open for that FET. If the problem is the same for a bank of FETs, the problem is in the IC or in the buffer/driver circuit.
Pin 5 should have ~1.5v DC on it. This is actually a sawtooth waveform but it typically reads ~1.5v on a multimeter. Previously, I stated that the voltage on pins 3 and 4 had to be below ~3v. Pin 3 has to be below the peak voltage of this waveform. Pin 4 must be ~0.15v below the peak voltage.
Pins 1, 2, 15, and 16 are the error amps and drive pin 3 internally (see datasheet for internal schematic). The error amps are used for various purposes. It's common for pins 1 and 2 to be used for voltage regulation. Pins 15 and 16 are often unused. When unused, pin 15 is tied to 5v (pin 14) and pin 16 is tied to ground. These are comparators and you can read about those HERE (demo ~1/2 of the way down the page).
Protection circuits are beyond the scope of this basic troubleshooting guide but I'll provide a basic introduction to them. In general, protection circuits are designed to prevent damage to the amplifier and, in some cases, they prevent damage to the speakers.
The most common protection circuit is the thermal shutdown circuit. It is designed to shut the amp down if it gets too hot.
Most amplifiers also have an over-current protection circuit. It's designed to shut the amp down if too much current is drawn from the speaker outputs or when there is an internal fault that causes excessive current draw.
Some amplifiers have DC offset protection. This prevents damage to the speakers in case the amp fails in a way that causes rail voltage to be driven to the speaker output terminals.
Over and under-voltage protection is employed in some amplifiers. If the B+ supply voltage is too high or too low, the amp will shut down.
Any of these can shut the amp down. If there are no dedicated indicators to tell you which fault has caused the shutdown, you must determine which fault is causing the shutdown and then find the defective components. With a schematic, it's generally not too difficult. Without a schematic (many manufacturers won't provide schematics), it can be much tougher.
Troubleshooting an Amp that's Not Completely Dead (shows some signs of life)
If you're asking for help with a repair and you include answers to the following questions in the initial email/post, it will generally greatly speed up the process.
Do you have both positive and negative rail voltage on the output transistors?
Do you have both positive and negative regulated voltage on the power supply pins of the audio op-amps?
Does operating any of the switches or potentiometers through their entire range make a difference in the fault (channel distorted, intermittent, low...)?
In most amplifiers, you'll find both positive and negative rail voltage. If you have one but not the other and the amp isn't a class D amp (some class D amps only use positive rail voltage), you need to determine why you don't have both positive and negative rail voltage. If you don't have both positive and negative regulated voltage on the power supply pins of the audio op-amps, you need to trace back the circuit to the regulators to see why you don't have the proper regulated voltage (generally ±15v). Many times, a distorted or intermittent channel is due to nothing more than a dirty contact in a potentiometer or a switch. Sometimes it's simply caused by having the switch between positions.
The power LED is not a good indicator of the condition of an amplifier. Just because the LED is lit, that doesn't mean the amp is operating. In many cases, it's simply an indication that the amp has B+ and remote voltage. In some amplifiers, the power LED will be lit with only the remote voltage applied (B+ isn't required). Amps like those made by Sony may have all green lights lit even if an amplifier has a blown power supply. Amps like those made by Rockford, must have a working power supply for the power LED to be lit.
In many amplifiers, the protection LED will only light if the power supply is working and the audio circuit trips one of the protection circuits (over-current/DC offset). If the power supply is blown, it's generally not possible for the audio circuit to trip the protection circuit and therefore the protection LED will not light.
Checking Bipolar Transistors
In virtually all amplifiers, you will find 'bipolar transistors' (BJTs). To check them, you need to understand their basic construction (as it applies here). In most BJTs, you'll have nothing more than two diodes. As you know, a standard diode passes current in only one direction. When checking them with a multimeter set to 'diode check', you should find that the meter reads ~0.6v in one direction and reads as an open circuit when the probes are reversed. You should find the same thing when checking bipolar transistors. Looking at the two images below, you can see how the diodes are orientated in the package. Note the locations of the base, collector and emitter. The placement of the probes on the B, C and E are important. The pin-out of the transistors can have the B, C and E in any of the 3 positions. You need to download the datasheet for the transistor to determine the pin configuration (B, C, E). For most power transistors in the TO-220 case/package (below) or the larger cases, the pin configuration is virtually always BCE.
In these two images, you see that the forward voltage drop is 0.6v. In all other combinations of probe placement, the meter should read the same as when the probes are open (not in contact with anything). Any other reading means that the transistor is defective. Understand that the 0.6v is simply a common value. Anything close to 0.6v with no reverse leakage (current flowing through the internal diodes backwards) means the transistor is likely OK.
If you don't fully understand how diodes function, visit the DIODES page of the site. The CHECKING SEMICONDUCTORS page (bottom half of page) has more test procedures for other types of transistors.
Power should not be applied to the amp when checking transistors that have not been removed from the board.
If you need help checking transistors, fill in
THIS form, produce a screen capture image and email it to me. The case style of your transistor isn't important when using this form. Simply have the text/part number facing you and the legs down. Don't forget to enter the transistor's part number in the form. You need to pull the transistors from the board for definitive testing/results. BEFORE entering all of the values, make sure that you can produce the screen capture.
If you're sending me multiple screen caps, please zip them into one file before sending them. In the name of the zipped file, include the amp model and your email address. This helps me find the files if I need to refer to them later.
When saving the files, do NOT save the screen caps as BMP files. The file size will be too large. Save as GIF, PNG or JPG files. Set the JPG quality to 85% or higher.
Removing the transistors from the board will provide more definitive results.
Links for Those Who Want More Detailed Information
If you've made it this far, and you're still interested in doing this type of work, read the first 12 chapters of the site until you understand them completely. Then read the following chapters (some of these are links previously seen in the text above):
For Those Who Want to Earn Money Repairing Amplifiers
The tutorial I sell (click banner below) has MUCH more detailed information and covers a much wider range of amplifiers and problems. If you're interested in doing this as a side-line job, the tutorial will be a great help. If you're an engineering student, you'll learn a lot about electronic components but you won't get much information about the way those components are used in real world products. The tutorial will help fill in large gaps that won't be filled by college courses.
Car Audio Forum
For those who want to visit a nice respectable forum, click the link below. There have been many repair discussions.
Taking Good Photos of Your Amplifier's Circuit Boards
It may seem a bit strange that there is photographic information on an amplifier repair page but there are thousands of amplifiers on the market that I've never seen and will never have in my shop. Photos, especially good quality photos, allow me to see the amp in question and will often make it possible to help the owner get it repaired.
It's not necessary to have an expensive camera or accessories to take decent photos. If the first few (or 10, 20, 50...) photos you take aren't very good, keep trying. If you don't have the patience required to get a good photo, you certainly don't have the patience needed to repair your amplifier. Please don't submit cell phone images. As of this time, most are useless for this purpose. I'm sure that in a few years they will get better but they're not there yet (mid 2011). If you want to compare your cell phone images to a 7 year old, lowly 2MP camera (Nikon Coolpix 2200), try to match the quality of the following image. The reason I mention cell phone images (and provide a sample to which they can be compared) is because of the rude responses that I've gotten from people who are offended when I tell them that their iPhone (or whatever phone) images aren't good enough. If you don't have a digital camera, you probably know someone who will let you borrow their camera.
If you're going to buy a camera, it's generally best to buy one that uses commonly available, inexpensive batteries (AA preferred). Also make sure that, if the camera requires the use of proprietary cables, the cables are included or are available relatively inexpensively. If you're going to buy a camera, go to a site like the Imaging-Resource and look at the sample images (particularly the macro images) to see what the camera can do. Viewing the sample images can also be helpful if you own a camera and cannot get acceptable images because it allows you to see what the camera is capable of.
It's often good to crop a photo. If you only need to post a section of the image you captured or you want to remove something from the image, you can use a program like Irfan-View. It's a free download and very easy to use. After downloading the program, you open the image in Irfan-View, drag a rectangle around the area you want to keep and hit ctrl-y. This removes everything that's not in the selected rectangle. Then you simply save the image. To reduce the file size with virtually no reduction in quality, set the JPEG 'save quality' to about 85. If you'd like more information on the capabilities of Irfan-View, you can find it on my computer site.
Virtually all cameras allow you to select the overall resolution of the image. For most cameras, a resolution that produces an image that's 2000 to 3000 pixels wide is a good choice (common resolutions in that range are 2048x1536, 2816x2112 and 3264x2448). Unless the photos is very good and only includes a few closely spaced objects, the minimum resolution (often 640x480) is useless.
Macro or Normal Mode:
You'll have to try your camera in both normal and macro modes. Some cameras (like the Olympus C-740 that you'll see here) work better in normal mode. The Nikon 2200 (also used here) works better in macro mode. To switch to macro mode, look for the button or setting that looks like a tulip/flower. That's the universal symbol for the macro mode.
Angle at which Photo is Taken:
Most people try to take photos with the camera body (film plane) parallel with the circuit board. With proper (advanced) lighting techniques or natural/ambient light, that may be ideal. When using an on-board flash for lighting, it's generally not a good idea. Take the photo at a slight angle. Try several angles to see which works best. You want the least angle possible without having the flash reflect off of the board and back into the lens.
Flash or No-Flash:
In general, it's necessary to use the flash to get good photos. Without the flash, the camera will slow down the shutter to produce acceptable exposure and the photo will be blurred unless you use the timer and a tripod. If you can't get good photos with the flash, you can use sunlight to get enough light to allow the camera to set a higher shutter speed. Don't use direct sunlight. If there is a cloudy overcast, you can take the photos without any cover. If there is no overcast, you will need to be under-cover (like a carport).
Most cameras have a zoom function. The image quality is generally better near the center of the zoom range than it as at either extreme. When framing the image, set the zoom to the middle of its range and move the camera back/forth to photograph the area of the board that you need help with.
Auto or Manual:
For those not familiar with their cameras, the full auto mode should be tried first. Some cameras (like the 2200 here) only have auto or scene modes (no fully manual mode) so there is little choice. If you're familiar with the manual modes and know how to use them, you're likely just bored if you're reading this. In manual mode, you can select the shutter speed, the aperture, the ISO and the flash power. If you can use manual mode, try to use the highest F-stop/number you can. This will provide a greater depth of field (more components in sharp focus). Using a shutter speed of at least 1/60 of a second it important to prevent motion blur (even if your camera has image stabilization).
Nikon Coolpix 2200:
The following camera and diffuser was used to take the next circuit board image. The simple wire frame and tracing paper are a diffuser to help make the lighting a bit more even. The photos without the diffuser were pretty good but this simple device makes them better. For those who are not familiar with cameras, the tripod socket on the bottom of the camera is a standard 1/4"-20. The screw was tightened (by hand) until it bottomed out in the socket and then the nut was drawn down on the ring terminal. The tripod sockets are typically relatively fragile plastic so you have to be very careful to avoid breaking them.
As you can see, a 2MP camera can take relatively good quality photos. Click on the image to view it in a new window. If you're using Firefox or Chrome, click on the image after it's loaded to view it at full resolution. This camera cost only about $20 (plus shipping) on eBay. If you buy a camera like this or a similar one, be aware that most have broken battery door latches. Read the description and look at the auction photos carefully before buying one.
The following photo was taken outdoors with a heavy overcast.
This is an old 3MP camera that I keep around to take photos of waveforms on an oscilloscope. They are available used on eBay for about $50. You should be aware that they have a problem with keeping the memory (internal backup battery is dead in most of them) but they'll work OK on full auto. They work in other modes but you have to reset all settings every time you power it up. In the first two photos, you can see the diffuser I used on this camera. The camera has an accessory lens holder (available for about $10 on eBay - buy a black one instead of a silver one) if you buy this camera or one similar (C-720, 730, 750, 760). Again, I used a diffuser. This is another simple and cheap device that makes the photos much better.
The photo below was taken with the camera body parallel to the board. As you can see it's not nearly as good as the one above. Basically, the only difference was the angle at which the photo was taken. The components are all in focus in the image below but the bright reflection is distracting and fatiguing, especially if you have to look at multiple photos or if you have to study the photo for more than a few seconds.
This is a good camera but not really well suited for this type of work unless you have accessory lenses and the required adapter. The macro mode is useless (I can't find any difference from the normal mode) and the super-macro is only good for photos taken within a couple of inches. To take photos with this camera, the amp has to be about 3 feet away from the lens. This means that the amp has to either be propped up so you can step back and photograph it or you have to place the amp on the floor and photograph it. Here, I used another soup-bowl diffuser. I couldn't use the full auto mode with this camera. I had to use the manual mode setting the aperture and shutter speed to get the right exposure.
Many of the visitors of this site claim that they want to learn to do repair work for profit. If you have no desire to work towards taking the very best photos that your camera can produce, it likely means that you're not a quality oriented person and should not be doing work for others.
Tips for Email or Forum Posts when Asking for Repair Assistance
For those who read this and see something that they've done and think I'm singling them out, rest assured that I'm not. Nothing is here because it happened one time. If it's here I've seen it so many times that it's gotten annoying and this is an attempt to prevent seeing it again. When asking for help, take time to think about the problem and include all information that you think is relevant. Putting forth a bit of extra effort could mean the difference between getting the desired answer in a few minutes or having it take a few days (or longer).
Proper Grammar and Spelling:
One of the most important things you need to do when asking for help is to use proper grammar. This includes punctuation, capitalization and proper spelling. If you're using Firefox, there is a spell-check add-on for it. Download it and use it.
It may seem like I'm being unreasonable asking for this but this is technical and the language needs to be precise so I don't have to ask for clarification of everything you send me. As a general rule, you should attempt to write at approximately the same level as the person with whom you're corresponding. If you're in a forum where there is nothing but slang, then it's appropriate for you to reply that way. If the person corresponding with you is taking the time to use proper grammar, punctuation, capitalization and paragraph breaks, you should do the same. I'm willing to help just about anyone (free of charge) but you have to do your best to help me to help you. If this is too much to ask, please look for help elsewhere. If you're from somewhere other than the US and/or English isn't your native language, do your best and I'll try to help.
If you ask me for help and use chatspeak, LEET or truncated/abbreviated words (ur for you are, str8 for straight, etc...), don't expect me to reply. If I do reply, it will likely be to ask you to re-submit the question, written properly. If you're not intelligent enough to ask properly, you're unlikely to be intelligent enough to complete the repair. Many of the replies require 10 minutes to an hour to find the answer. If you can't take the few seconds that it takes to spell out words, you shouldn't expect anyone to spend time trying to help you.
If you're someone with a legitimate learning disability and cannot use proper grammar for that reason, you can still use capital letters and periods. The spell-checker in your browser will take of spelling errors. It will at least show me that you're trying.
Circuit Board Designations:
When asking for help on the forum, please use the circuit board designations when referring to components on the board. If you say 'the transistors on this side of the board do this' and 'the other ones do this' it gets very confusing to those who are trying to understand, via this very vague description, what you're trying to say. The components will generally have circuit board designations printed next to them on the board. Use those designations. Instead of the previous description, you'd have something like this 'transistors Q101, Q102, Q103 and Q104 do this' and 'Q105, Q106, Q107 and Q108 do this'. I know that this takes extra effort but it makes it much easier for others to help you.
In the following image, you can see that virtually all of the components have a circuit board designation. The prefix tells you what type of component it is (R - resistor, Q - transistor, D - diode, C - capacitor...). R147 is the pink resistor. R52 is the large gray resistor. Q21 and Q9 are transistors. C37 is a capacitor.
While I have a lot of photos and schematic diagrams (been collecting them for quite a long time), I don't have information on every amp. In some instances, you'll need to supply a reasonably good quality photo so that I can read the circuit board designations and make sense of what you're asking. The photo section on this page is provided to help you get the best possible photos. For those who don't have a camera that can take good quality photos (only have cell phone or web cam), you can use a lower quality image and mark up the photo in Irfanview, Microsoft Paint (free with Windows) or your favorite photo editing software.
Many times, a schematic diagram or photos from an amp 'similar' to the one you have will be used for a reference. If you believe that something doesn't seem quite the same on your amp, ask questions or ask for clarification.
Reference Point when Measuring Voltage:
For some reason, people think that they should place the red probe on the positive terminal of the amp when measuring voltage. I would never have imagined that anyone would do this and I still can't understand why they do it but it's happened multiple times so it must be a widely held belief. The only time (that I can think of) that you would place the red probe on the B+ terminal is when you were measuring the voltage on the B+ terminal. You would have the black probe on the amplifier's main ground terminal for that measurement.
When measuring most voltages, you will place the black meter probe on the amplifier's primary ground (the amplifier's main ground terminal). You'll then place the red meter probe on the point where you need to measure the voltage. Use the amplifier's primary ground for the reference point (black probe) unless otherwise instructed (for those asking for help on either the car audio forum at DIYAudio.com or via email). In some instances, you'll need to use the secondary ground as the reference. If that's the case, I'll generally tell you what point to use. The non-bridging speaker terminals (the speaker terminals that are NOT used when the amp is bridged) are often the reference point (black probe placed there) for the secondary ground.
As an example, if you're measuring the voltage on the power supply driver IC (TL494, TL594, SG3525...), you'd place the black probe on the amplifier's ground terminal and touch the probe alternately to each of the pins of the driver IC. If you're asking for help and I need these voltages, I'll generally give a list for you to fill in the blanks. If it's via email, simply fill in the voltages after each pin number. If it's on the forum, copy and past the list. The list is precisely as it is for a reason. There is a blank space after each pin number to make it easier to read. If the space doesn't get copied, it's simple enough to add a space before you type the value. There is a colon after each pin number so that the person filling in the blanks doesn't use hyphens (which can be mistaken for a negative sign). There is a reminder to list the IC number (circuit board designation) if there is a chance that it will be confused with another IC. This is generally only an issue when you're listing the voltages from multiple similar ICs.
THIS is a plain text file (open with Windows Notepad) that has no formatting and retains the spaces as they should be. Right-click and save to your computer if your browser opens it and adds formatting, Copy and paste the number of pins for the IC you're measuring the voltage on.
Voltage 'Across' or Voltage 'On' a Component Terminal:
When you're asked to measure the voltage on a terminal of a component, you'll almost always have the black probe on a ground reference point. If you're asked to measure the voltage across a component (typically 2 terminal components), you place one probe on each end of that component. Don't give the voltage on each end of the component (with the black probe on ground) unless asked to do so because reading from ground to the two points is less accurate. If you're asked to measure the voltage with the black probe on the source leg of an FET and the red probe on the gate leg of the FET and you don't know which leg is the gate and which is the source, look up the datasheet for the FET and search the datasheet for the pin configuration. Sometimes it's near the top, sometimes it's buried in the body of the datasheet. It may take 10 or 15 minutes to do this but if you have to post a question asking which is which and no one replies for a day or the person helping you can only get to the forum once a day, it will save an entire day because you won't have to wait for an answer you can find on your own.
Don't Forget the Units!:
When measuring voltage, resistance, whatever... the units are VERY important. For example if you post the resistance between two legs of a semiconductor and only post '200', in most instances, that's absolutely meaningless. In most instances, 200 ohms and 200k ohms makes a huge difference. In the following images, you can see that one meter reading is approximately 3.3k ohms. The other is 3.6 ohms. If you just say that it's 3.3 it means nothing.
If you look closely at the first value, you can see that there is a 'k' in front of the symbol for ohms. In the second one there is no 'k' in front of the symbol for ohms.
If you're trying to make measurements and you're coming up with readings that don't appear (to yourself or to someone helping you) to be correct, post a link to the owner's manual to your meter so that the person helping you can try to determine why you are getting unusual readings. If no online manual is available, post a link to a site that has a good quality photo of the front panel of the meter. The first photo you find may not be the best available. Don't be lazy (unless getting the amp repaired is not really important). Take the time to look at several pages to find the one that shows the front panel the best (all text legible). If you can take a good photo (again, take several and send the best) that's better than the ones you find on the internet, post that photo.
Don't Try to Seem like You Know what You're Talking About if You Don't:
When talking with friends/acquaintances on the net, you may be able to BS your way into making them think you know more than you do. When you're dealing with people who are MUCH more knowledgeable than you are about a subject and you do the same thing, you just seem like an idiot. This applies to everyone. If I went to an advanced math forum and started using terms that I didn't understand, they would think I was an idiot and if I continued to do it, I would confirm their suspicions. Don't use terms that you don't understand. If you're new to repair work and you're attempting to repair an amplifier, you're not expected to know everything. If you did know everything, you wouldn't need the help. Using the wrong term/name for a component is worse than asking what a component is, in my opinion. Intelligent, ignorant people ask questions to learn. People who want to appear to know more than they do just waste the time of those from which they're asking for help.
Those who don't understand that intelligent people can be ignorant, don't know the definition of ignorant. Ignorant means that you don't know. We're all ignorant of something.
Not Everything is a MOSFET:
Not all of the components clamped to the heatsink are going to be the same thing. Some people (a lot of people) refer to all of the 3-legged components on the heatsink as MOSFETs. Some are rectifiers, some are transistors some are regulators. The transistors can even be used as regulators. The components that are transistors can be FETs (MOSFETs) or BJTs. If you call something a MOSFET and you're actually referring to a different component or a different type of component, the person helping you may believe that you know what you're referring to. If you're actually calling a BJT (often used for output transistors) a MOSFET and the amp has only FETs in the power supply, it will be confusing and will waste everyone's time. Don't try to be a know-it-all. No one knows everything and when you're beginning a new hobby/trade, you're not expected to know anything.
Datasheets for Replacement Parts:
In most cases, it's best to use the exact replacement part when repairing an amplifier. Sometimes very slight differences in parts can mean the difference between an amp working and not working properly. When the original parts are no longer readily available, you may need to use a substitute part. If you have a part that you'd like to use as a substitute, post the datasheet (or a link to the datasheet) for both the original and the substitute part.
Many times, when you copy a link from the address bar (especially for something found via the search function on a site), the link will only work from the browser that was used for the search. There are a couple of ways to confirm that the link works universally. The best is to open a different browser. If you're using Firefox, open IE or Chrome. If you're using IE, open Firefox or Chrome (you get the idea) and open the link in the other browser. If the link doesn't work in the other browser, it won't work for others. Opening a second tab in your current browser or opening a second instance of the same browser won't tell you if the link is good.
Transistor Type vs Function:
When referring to components like transistors, there are (at least) two ways to do it. You can refer to the transistor by its type (FET, BJT, JFET...) or by its function in the circuit. For example, an FET could be used in the power supply or in the output stage of the audio power amplifier. If they're used in the power supply, you'd likely refer to them as power supply transistors or power supply FETs. If they're in the output stage of the audio amplifier, you likely refer to them as output transistors or output FETs. If a smaller transistor is driving a larger transistor (or a group of larger transistors), you generally refer to them as driver transistors. These are almost always BJTs so it's not necessary to refer to them as driver BJTs.
BJTs are often referred to simply as bipolar transistors.
State ALL Work Previously Done to the Amp:
When asking for help, be sure to list ALL work that you've done to the amp. List EVERYTHING, no matter how insignificant it seems. Also list all work that appears to have been done to the amp previously. Many times, the (only) fault is due to a mistake made during the previous repair attempt.
Don't Reinstall/Remove Parts Without Letting Others Know:
If you're asking for help and are too impatient to wait for further instructions, don't remove or reinstall parts unless you let those trying to help you know what you've done. If you say that you've removed x-parts and reinstall them without letting others know, the next instructions could be with the assumption that those parts are out of the circuit. This could make a huge difference in the readings and may take several postings/emails to discover. Depending on how busy the person helping you is, that could mean a day or even a week wasted. If you don't know what you did last (with regards to the repair on the forum or via email), re-read the thread.
Don't Break an Email Thread/Chain:
If you're emailing me (or anyone who can help you), maintain an unbroken chain of emails. If you start a new email every time you ask a question, it's very difficult for me (and likely others) to remember what was previously done or even what amp it is. For you, it may be the only repair that you've dealt with and therefore relatively fresh in your mind. For someone that helps a lot of people with repairs, it won't be so fresh. I may have more than 100 emails and forum posts in the matter of a few days. In a day's time, I probably won't remember much of the repair and after a week, it's unlikely that I'll remember any of it without skimming over the unbroken chain of messages. If you continue to break the chain of emails, I'll stop replying. I try to help as many people as possible and if you won't cooperate, I can't help you. If helping one uncooperative person prevents me from helping 3 others due to time wasted, it's not fair to the others.
If you have a thread or email chain that you've started for one amp, don't ask questions about another amp in that chain/thread. Start a new chain/thread and include the make/model of the amp in the subject line of the message. Each time you start a different repair, start a new chain/thread.
On the forums, do not ask questions about your amp in a repair thread started by someone else with a similar amp. Start your own thread. It gets very confusing, especially if the original poster has additional questions about their amp. If you're not intelligent enough to start a new thread, there's a pretty good chance that you're not intelligent enough to repair your own amp.
Problems Concerning Noise or Distortion:
If you ask for help with a repair, you need to be very clear about the problem, especially when it concerns noise or distortion. Noise and distortion should be treated differently when asking for help with a repair.
Distortion makes the output less clear, harder to understand. If your amp only distorts as the output increases (but well before you think it should), you need to confirm that the DC voltage across the B+ and ground terminals of the amp (multimeter probes placed directly on the metal part of the B+ and ground terminals on the amp) is remaining at or above 12v. If it's dropping lower, that could be part of the problem and you need to determine why it's dropping.
Is it distorted at all output levels or only at higher output levels?
Is only one channel distorted or are multiple channels distorted?
Does changing the position of any of the controls make a difference in the quality of the output? When checking this, you need to have a speaker connected to the channel that's defective, listening carefully while operating all controls through their entire range. If the audio becomes clear in one position (even intermittently), you need to include that bit of information when you ask for help.
When it's suggested that you operate all controls through their entire range, that is in reference to the controls accessible from the outside of the amp. There are controls inside some amps that, if turned, can result in the failure of the amplifier or severe damage to the speakers. Do not adjust any internal controls unless instructed to do so. If you have made adjustments to the internal controls, you need to say so when you ask for help.
Many people use the terms hiss, hum, buzz. A hiss would tend to be a higher frequency noise. A hum would tend to be a lower frequency noise. A buzz would tend to be a mid to high frequency noise.
You also need to be specific about it's relative loudness of the noise. Is it barely audible (ear needs to be almost on the speaker to hear it)? Is it moderately loud to where it would be difficult to hold a conversation over it? Is it so loud that it absolutely overwhelms any audio, no matter the volume level?
Is the noise being produced only by the internal components of the amplifier or is the noise audible through the speaker as well? Many amplifier produce significant noise from the various transformers and inductors and it is often not a problem that needs to be or can be resolved. If the noise is not entering the audio signal and would not be audible when the amp was being used normally (all covers on, amp in trunk or in amp rack...), it's probably not something that you need to be concerned with. When asking for help, be sure to note whether the noise is audible or not in the speakers.
Does the noise vary with a change in the position of any of the controls on the amp?
Does the noise go away when no signal source is connected?
Does the noise vary with the level on the signal source?
Are you using a battery-powered signal source (ipod, etc...)?
Are you using a test speaker appropriate for the amp that you're repairing? If you're repairing a multi-thousand watt class D amp and are using a full range speaker with efficient tweeters and hear a hiss, that doesn't necessarily mean that there is a problem. Some class D amps are noisy. If you were to connect a subwoofer, would the installed amp produce enough noise to be audible?
As a side note...
When using a speaker to test an amplifier, don't use an expensive or fragile speaker. To offer a bit of protection for the speaker, you should also insert a non-polarized capacitor in series with the speaker. For full range amps, a 100uf cap is typically a good choice. It will work for subwoofer amps as well but the 100uF cap will block most bass. For testing subwoofer amps, you should use a robust subwoofer and have it where you can see the cone. This allows you to quickly disconnect it if you see the cone pull in or push out due to excessive DC offset.
List of Test Equipment:
When you make your original post or request for help, list the test equipment that you have. Include the make/model for each piece of equipment. This will help those helping you to ask for tests that you can perform with the tools that you have. If you don't, at the very least, have a multimeter, there's really no way that anyone can help you. If you don't have a meter , don't run out and buy one blindly. Start a post asking if the meters that you're considering are a good choice. There are plenty of meters that are all but useless and there's no point in having to buy multiple meters because you made a poor choice with the first one you purchased.
An oscilloscope isn't absolutely required for all repairs but it makes it easier to help with repairs that involve distorted output. You will be asked to post or email a photo of the distorted waveform. When you take the photo, you should set the camera so that it doesn't use the flash. It's not important that the face of the scope be well lit when all that's needed is a photo of the displayed waveform. You'll need to use a sine-wave for the audio source. You can download the following test tones. You'll need to either burn them to a CD or copy them to your digital audio source (MP3player, ipod...). The 100Hz is most commonly used.
If you're not completely familiar with the use of an oscilloscope, you need to read the oscilloscope page of this site (#73 in the directory). You need to read EVERYTHING on that page but pay special attention to the section labeled Best Initial Settings. Read that section at least twice (once before setting the controls and once after setting the controls to confirm that they're as they should be).
Final Word for this Section:
I know that it may seem like I'm a bit of a jerk (others would use stronger language to describe me) but I'm quite frustrated with the younger generation. All they seem to be able to do well is to play video games. Except for a very tiny fraction of the population, that's getting them nowhere. I try to push and prod them to get motivated but it's difficult (I'm referring to those who 'claim' they want to learn). The information and suggestions on this page alone could help a lot of people make a lot of money if they take the time to read it (a good tech can make between $60 and $90/hour) but time after time, they state that they've read all of it and then ask questions that tend to indicate that they have barely skimmed over it -- at best. I've put a lot of work into this page and want people to benefit from it. The language/terms used in parts of it (the work 'lazy' keeps creeping in) may not be pleasant but it seems that too many people require stronger language to motivate them.
There have been a lot of questions about the type of scope necessary to do this type of work. Many people are under the impression that they need an expensive 100MHz scope to troubleshoot audio circuits. That's simply not the case. Below, you can see two different scopes. The first one is a Tektronix 2230 digital storage scope rated for 100MHz. The second photo is the same signal displayed on a Tektronix 5110 oscilloscope rated for 2MHz (not a misprint, 2MHz). Although the 2230 has slightly better resolution, the 5110 has good enough resolution for virtually any problem that you'll encounter. The signal is a 160kHz square wave. This is typically the highest frequency you'll find in the audio section of most class D amps. Some full range class D (T) amps operate at higher frequencies but those amps are not common. Even those amps that operate at higher frequencies operate well below 1MHz.
If you look closely, you can see ripple on the top and bottom of the waveform. This is at a much higher frequency than that of the square wave but it can still be seen in the 5110 display.
Some people will argue that a 2MHz scope will not be able to resolve tiny high frequency glitches. That's true to some degree but that's not really a concern with most audio problems. I don't want you to think that I'm telling everyone to go out and buy a 2MHz scope. This is simply to show that it doesn't take a 100MHz scope to display the types of signals found in car audio amplifiers. If you can get a good deal on a 15MHz or 20MHz scope, don't pass it up because you don't think it's up to the task. Virtually any scope with a triggered sweep and in good working order will suffice.
If you're looking to buy a scope on eBay, email the auction numbers to me and I'll try to help you get one that is in good working order.
When buying semiconductors, you should only buy from reputable distributors. There are quite a few counterfeit/fake components on the market. Sometimes, the counterfeiter will buy cheap components and re-mark them. Others could simply be manufactured with very low standards. If you have an amp that fails repeatedly after repairing it and you suspect that the transistors may be fakes, break open the package and compare the size of the silicone die to those in the following photos. If your transistors look like the die has been glued to the tab or the die is significantly smaller than the ones in the photos below (for the same part/manufacturer), you may have counterfeit parts. You can send the parts to the manufacturer and they will be able to determine if the components are genuine or counterfeit. I believe that the transistors below are genuine components unless otherwise noted.