Foreword:
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.
Overview:
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 (virtually all 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.
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).
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The following is a list of things that will help you from making a few of the most common mistakes.
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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:
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 this 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 pass much more current 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. - Transformers:
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 measures more than 300v across the rails in some amps). - Capacitors:
Capacitors are used to pass AC signals or to store energy. Whey 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.
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 leads 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.
- Safety:
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.
Amp Failure:
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 is too large 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.
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 leads 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.
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.
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 code is a good way to identify parts that are very similar. 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.
Datasheets are documents that tell you virtually everything about a particular component. 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. The following are 3 datasheets for commonly used components.
- IRF1010EZ
These are FETs that you'll find in some amplifier's power supplies. They are rugged and relatively inexpensive. - MPSA06
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. - TL594
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 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.
As with any type of work, it's easier to do a job with good quality tools. The following are some of my suggestions.
- Soldering Iron:
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. - Desoldering Pump:
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. - Desoldering Braid:
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. - Solder:
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. - Multimeter:
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 7 or 8 other meters (some much more expensive) but this is the meter I use on a daily basis (if you have one 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. - Oscilloscopes:
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.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.
- Power Supplies:
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).
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.
Crossovers/Filters:
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).
Note:
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.
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 lead to the chassis ground terminal of the amplifier. Measure the voltage on the center lead 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 regs. 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.
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.
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 leads 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 leads 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 lead placement, the meter should read the same as when the leads 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. Fill in ALL of the blanks (white fields).
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.
Please Read:
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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.
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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.
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.
Other sites that have information about counterfeit semiconductors:
Reputable, Reliable Distributors:
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):
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.
http://www.diyaudio.com - Car Audio Forum
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.
