Replacing the transistor with a more powerful one in the amplifier. Repair of automobile amplifiers. Repair of car amplifiers CALCELL

Hi all .

Recently I dug up this topic, I think that the coverage of this topic will not be superfluous, well, you never know what.

"Tarantino" Dec 25, 2012, 04:26 AM
So, after my feeder to the antenna suddenly died (the central conductor fell off) - given the active phase of work on the air - the "loss compensator" KL-203P proved to be a caustic smoke in a matter of seconds. I really wanted to make wishes - I thought a genie from a transistor, but, alas. Hasty attempts to avoid the "fire" by turning off the device were unsuccessful. The amplifier is gone. No response to transmission. The diagnosis was confirmed by autopsy. The search began. A lot has been written here and on other resources about the circuitry, simplicity and vulnerability of this device, but the discussion of the "cut down" (erased manufacturer's marking) used output stage transistors, codenamed MOS RM3, remained a mystery to me. As a result, having collected valuable information from the forums, my own experience, (as well as some amount of money), I compiled a list of the most suitable transistors and analogues in my opinion.
I bought a bunch of transistors (which were available for sale) not as an experiment, but to save the pocket of members of the forum, like-minded people, and in general for the sake of communication lovers.
Tried almost everything I bought.
Outcome:
1. MOS RM3 - original ~ $30 to order. did not even try to buy. will work, according to a reliable source.
2. 2SC1307 (supposedly even more original original from Toshiba) - not found. didn't try.
3. MS1307 (allegedly original) - not found. didn't try.
4. IRF1310N (supposedly analogue) - tried. didn't fit. The power meter showed a decrease in power. Those. with incoming 15W - the output was 7-8W. (I didn’t mix up the connectors. I even mixed them up on purpose - even worse.) At the same time, the SWR of the tuned antenna (1.1) went off scale for 5. Perhaps you need to change the gate offset, then they will work.
5. IRF510 - tried. They work. 8W input - 60W output. With an increase in input power - an increase in output remained practically unchanged. Maximum 70W output with 20W input. At the same time, the radiator could safely iron clothes.
6. IRF520 - tried. Did not earn. The SWR went up. station in defense. But the amplifier turned on the transmission. Power could not be measured. May also work with gate voltage bias
7. IRF530 - tried. slightly better than the IRF510. The output power is the same. The temperature regime, other things being equal, is more moderate. a couple of times I observed "sticking" (self-excitation) during transmission.
8. IRFZ24N - delivered. My hopes exceeded all expectations! It's him! I thought. Didn't improve anything. I put everything. Input/output power: 1W/35W, 5W/100W!, 10W/130!!!W, 15W/135W.

It is worth noting that I have already installed IRFZ24N and not hoping for success. It turned out to be the cheapest of all given - 38r. http://www.quartz1.com/price/find_ru.php?text=irfz24n . The rest are 80-130 rubles.

The temperature regime is more than moderate. At 8-10W at the input, it heats up predictably under continuous loads and does not even require additional cooling.

When dismantling native transistors, I encountered a partial detachment of the conductive tracks from overheating with a soldering iron. Not much, but be careful. The last 2 transistors were soldered only by the legs, having previously bitten them off from the field workers themselves. Don't forget the mica pad and thermal paste for final assembly.

Hooray! Everything works without any complaints!
By the way, for the Voronezh KP101/12 amplifiers, I believe, a similar replacement of the output transistors will not have negative consequences. But I haven't tried it.)

Unfortunately, obsessed with the goal of fixing the UM, I did not take a photo of the process and measurements. Although, in general, what is there to take a picture - and so, everything seems to be clear. If anyone needs - write. I'll try to do it."

It's no secret that to get high-quality sound and powerful bass in a car, you definitely need a power amplifier. Today, fortunately, you can find car amplifiers for every taste on the market, it all depends on your specific needs. A 200-400 watt amplifier is enough to power standard car acoustics, but among us there are true connoisseurs of sound pressure, audiophiles and music lovers who will not surprise you with a couple of hundred watts of sound power.

It was for such people that class D amplifiers were invented - digital audio frequency amplifiers that have high efficiency, compact size and many other advantages.

Unfortunately, a car amplifier sometimes breaks down, in some cases the repair is more expensive than the initial cost of the amplifier itself, so it is highly advisable to consider or try to repair it yourself, because sometimes a blown fuse can be the cause of the breakdown. Having at hand a simple and cheap multimeter with a diode continuity mode, you can find most of the defects that are very often observed in many car amplifiers.

Any car amplifier consists of three main parts - a voltage converter, a block with power amplifiers and a filter block (crossover).

The voltage converter or inverter is the most vulnerable part in any amplifier - 90% of the problems are associated with this particular node. The converter, in fact, feeds the entire amplifier, including the filter unit.

Exceptionally, all voltage converters are made according to a standard push-pull circuit using a PWM controller, most often on TL494. Then everything is standard - a driver, power transistors, a transformer, a rectifier and a filter unit. In some (cheap) amplifiers, non-stabilized inverter circuits are implemented - in a word, there is no output voltage control, of course, this is quite bad, but not at all a necessary process if the amplifier is not sensitive to the supply voltage and is a cheap model.

Converter transistors - they are the ones that fail most often. In cheap Chinese amplifiers, transistors are strangely marked, even if you cannot find similar transistors, then you just need to know one thing - the keys can always be replaced with IRFZ40 / IRFZ44 / IRFZ46 / IRFZ48 or with more powerful IRF3205, the choice of keys is actually quite large, I just listed the most available options. In general, N-channel high-power field-effect transistors are used exclusively in all automotive ULF inverters - up to the brutal IRF1404.

Initially, we check the board by eye - sometimes visible defects can be observed (burnt resistor, broken tracks on the back of the board, etc.)

Before replacing transistors, you must first check the power fuse, the diode on the plus and minus buses (when the power is reversed, it also burns out), and only after you are convinced that everything is ok with these parts, we replace the keys.

For a more professional repair, an oscilloscope is indispensable. Initially, you need to check for the presence of rectangular pulses on the 9th and 10th output of the generator microcircuit, if they are, then the microcircuit is working. Next, we check the presence of the same pulses after the driver - on the gates of the field keys. If there are no pulses, then most likely the problem is in the driver, if there are, then without hesitation we replace the field-effect transistors.

It is extremely rare that there is a problem with a power amplifier, the converter burns out first while preserving the amplifiers. Other failures are possible in the converter, although very rarely. There is a problem with the input and output capacitors or the diode rectifier, which rectifies the high frequency alternating voltage from the transformer.

Here I will share my modest experience in the field of repairing car amplifiers. I hope the information will be useful to novice radio mechanics in their difficult task of restoring audio equipment, as well as to motorists who are familiar with electronics and who want to fix their amplifier on their own.

To begin with, I would like to talk about how to turn on a car amplifier without a car radio and at home. Read more about this. This will be needed when repairing the car amplifier.

If you do not have a sufficiently powerful power supply at hand, then any one for a voltage of 12V and a current of 1 - 3 amperes will do. But here it is worth understanding that we need it only in order to turn on and adjust the amplifier. We will not operate it at full power, so the current consumption will be minimal.

I also strongly recommend reading or taking note of the material on car amplifier converter device. This information is very important.

Well, now, examples of repairs from real practice. Basically, they relate to one of the main blocks of any car amplifier - a voltage converter, or in other words - an inverter.

Repair of car amplifiers CALCELL.

1. Fault: auto amplifier goes into protection. The red LED PRT (Protect) is lit on the front panel. After a couple of turns on, the amplifier stopped showing signs of life at all - the PRT LED stopped glowing.

The cause of the malfunction was the 2N4403 transistor in the circuit of the TL494CN microcircuit (converter). One of his crossings was broken. In addition, the 10Ω resistor (Ohm) burned out. In the photo, R7 is him. While the resistor "endured" - the amplifier turned on, but went into protection. As it burned out, the amplifier stopped turning on at all.

The pinout of the bipolar P-N-P transistor 2N4403.

Why did the amplifier go into protection? The fact is that this transistor is part of the on / off circuit. Due to the breakdown of the P-N junction of the transistor, the amplifier did not turn on and went into protection.

There was no suitable replacement for the PNP transistor 2N4403 at hand. Therefore, a risky attempt was made to take the same transistor from the preliminary stage of one of the amplifier channels. Thankfully they were there. Yes, think about it, I decided, well, I’ll pull out the transistor from there, solder it to replace the faulty one, check the amplifier. Oh yes, he did. But after a few seconds after switching on, I smelled a burning smell. It turned out that due to the lack of 1 small transistor, the powerful complementary transistors of the UMZCH output stage began to heat up terribly. Fortunately, the transistors survived. Therefore, I do not advise so "cunning".

The replacement of the transistor was complicated by the fact that it was smeared with some kind of rubber glue, with which kegs of electrolytes were glued to the board.

2. The CALCELL POP 80.4 amplifier does not turn on. Protective fuses blown.

The device came "dead", apparently after an incorrect connection. After a quick inspection of the parts without soldering, it turned out that the 11V zener diode was broken in the "strapping" of the TL494CN PWM controller chip. A breakdown of the TL494CN chip itself was also found. When measuring the resistance between pin 12 (+ power, Vcc) and 7 (- food, GND) the multimeter showed - "0". Apparently, the power supply voltage of the amplifier was greatly overestimated.

After replacing the TL494CN chip and the zener diode with 11V, an attempt was made to turn on the amplifier. But, after turning on, the red PRT LED lit up for a few seconds (as it should), and then complete silence .... The power supply from which the amplifier was powered went into protection due to overcurrent.

It turned out that one of the two groups of MOSFET transistors on the converter board is very hot. Transistors of the other group are cold. After checking 3 STP75NF75 transistors that were warming up, it turned out that they were broken (Source - Drain). The 2N4403 transistor was also broken, which is a buffer for this converter arm. You can familiarize yourself with the diagram of a typical converter (inverter) of an auto amplifier in more detail.

After replacing the 2N4403 buffer transistor and three STP75NF75 MOSFETs (marked as P75NF75), the car amplifier began to work properly.

3. Amplifier CALCELL POP 80.4. When the amplifier is turned on, the red LED lights up. PROTECT and after a few seconds it goes out. The amplifier does not turn on - there is no indication.

This happens when the converter goes into protection due to high current consumption or a short circuit in the load. The load in this case is all four amplifiers, filter block and preamplifiers.

The most likely reason for the protection trip is the failure of the output transistors. The CALCELL POP 80.4 amplifier uses powerful bipolar transistors as output transistors. You can evaluate their correctness here this methodology, and soldering transistors is not necessary at all. As a rule, the breakdown of the transistor junction is easily determined, the multimeter starts beeping disgustingly with a buzzer - a signal that there is zero resistance between the transistor terminals.

It is worth considering that with such a quick test, the parts associated with the tested transistor (low-power transistors, etc.) can affect the readings. Therefore, if there is any doubt, we solder and check the transistor separately. It is not uncommon that it is precisely the elements associated with our transistor that are pierced, and not the transistor itself. Some amplifiers, such as the SUPRA SBD-A4240, use MOSFETs as output transistors. MOSFETs can be tested universal tester, since a conventional multimeter is not always suitable for such purposes.

Let's go back to our amplifier. For greater clarity, I will refer to the circuit diagram of this amplifier - car amplifier circuit CALCELL POP 80.4. When checking the output transistors of one of them, the Base - Collector (B-C) transition "rang" as broken. On the diagram, it is designated as Q312 ( 2SA1694). To check the performance of the amplifier, I soldered the faulty transistor and its complementary pair - the transistor 2SC4467 (Q311). I turned on the amplifier, but it again went into protection. It means there is something burning somewhere. In addition, low-power transistors Q309 were very hot ( MPSA06) and Q310 ( MPSA56). The check showed that both junctions of the Q309 (MPSA06) transistor were broken.

Since there was no complementary pair 2SC4467 / 2SA1694 on sale, I decided to replace it with more powerful analogues - a pair 2SA1943/2SC5200 manufactured by TOSHIBA. Here they are. They are heavy to the touch and inspire confidence.

After installing the new 2SA1943/2SC5200 transistors, it turned out that they are too large and because of this, the board does not fit into the case.

I had to bite out a small part of the printed circuit board so that they retracted into the case and fit snugly to the surface.

After replacing the amplifier began to work properly.

During the electrical run, I noticed that even without a load, low-power transistors in the preamps heat up quite noticeably. When playing music with rich bass, the heating increases. The amplifier worked for two subwoofers (one per bridge).

It is possible that prolonged operation at maximum power led to overheating and failure of the low-power transistor MPSA06 (Q309), and this, in turn, to the breakdown of the B-K transition of the powerful transistor 2SA1694 (Q312) in the output stage of the amplifier.

4. Non-standard case. The CALCELL amplifier just bought in the store was brought in for repair. According to the owner, after connecting the power, smoke came out of the vents of the amplifier.

After opening and examining the printed circuit board, it turned out that the conclusions of one of MOSFET transistors converter has traces of solder paste, solder balls. Here is a photo.

Apparently, a current went through the remnants of the solder paste when turned on. Because of this, the rosin in the paste heated up and began to evaporate in the form of a white haze. After that, the amplifier did not turn on due to the solder bridge formed during the solder paste reflow. It's no secret that cheap electronics made in China don't pass presale checks. Hence such "blunders".

Repair of car amplifier Lanzar VIBE 221.

Diagnosis: amplifier won't turn on. No LED indication. Judging by the appearance of the printed circuit board, they tried to repair the amplifier, and even the key MOSFET transistors in one of the converter arms were replaced. Instead of native IRFZ44N, STP55N06 were installed. But the amplifier ordered a long life. Also in the gate circuit of the MOSFETs were "burnt", but serviceable 100 ohm resistors. When checking the buffer transistors 2SA1023, which "rock" the IRFZ44N mosfets, it turned out that they were in good order.

After replacing the TL494CN SHI controller chip, the amplifier started working. Just in case, buffer transistors 2SA1023 and diodes 1N4148 were replaced in the base-emitter circuit of these transistors.

Car Amplifier Repair Mystery.

Problem: Amplifier turns on but no sound. car amplifier Mystery 1.300 a typical representative of the so-called monoblocks. That is, it is a monophonic amplifier. The sound power declared by the manufacturer is 300W. Such amplifiers are usually used to drive a powerful low-frequency speaker, that is, a subwoofer or subwoofer.

After opening and inspecting the printed circuit board, it turned out that several transistors (2SB1367 and 2SD2058) were poorly soldered, there was soldering degradation and excessive heating of the soldering points. The transistors appear to be part of the 15V regulators in the secondary power circuits. These stabilizers serve to power operational amplifiers and amplifier filters. In another way, this node can be called preamplifier. It is to him that we connect the very "tulips" through which an audio signal is sent from the car radio. Naturally, if there is no power to the preamplifier, then there will be no sound.

Why did it happen? The fact is that overheated transistors do not have a heatsink, their body is plastic. They stick to their own conclusions. There is no additional attachment. Due to overheating and constant shaking (after all, it was installed in the car), the soldering collapsed and the contact was broken. Therefore, the stabilizers stopped working. A little more and the transistors would simply fall out of the mounting holes!

After restoring the soldering of the transistors, the amplifier was fully operational, but the noticeable heating of the transistors suggested that after a while there would be a repetition.

It was decided to install heating transistors on a homemade radiator to reduce heat. Also update the pin soldering and make it more reliable. Here's what came out of it.

At the same time, adjacent transistors were planted on the radiator, which heated up less - to stiffen the structure. Since the transistors are in a plastic case and do not have a metal flange, I also applied heat-conducting paste KPT-19 to the place of thermal contact with the radiator.

Among other things, on the printed circuit board of the monoblock there was a clearly "inflated" electrolytic capacitor at 3300 µF * 63V in the secondary rectifier. In the power supply - inverter, 2 electrolytic capacitors are usually placed, because food amplifying stages bipolar, in the region of ± 28 - 37 volts. The adjacent electrolyte looked better and was not "bloated".

It was decided, just in case, to replace the electrolyte that was swollen with a new one at 4700 µF * 63V (this was in stock). During the electrical run of the car amplifier, it turned out that the replaced electrolytic capacitor warms up slightly. It turned out that it is heated by powerful resistors located nearby. For reference, the neighboring electrolyte does not have such resistors nearby. This is an obvious oversight. As you know, heating has a bad effect on electrolytic capacitors, since the electrolyte dries out faster and their capacity decreases.

Fusion FP-804 car amplifier repair.

Fault: amplifier does not turn on. There is no indication. After opening the cause, it was not necessary to look for a long time. Everything in the converter burned out MOSFETs HFP50N06 (original - STP50N06), as well as some 47 ohm resistors in the gate circuit of some of these transistors. Also knocked out buffer transistors 2SA1266.

Instead of the burned out HFP50N06 transistors, IRFZ48N were installed, replaced with new 2SA1266 buffer transistors, burned-out 47 Ohm resistors, and also, just in case, the TL494CN SHI controller chip.

The device turned on and began to work properly. But my joy was short-lived. Three days later, the owner of the amplifier called me and said that there was a weak monotonous whistle in the rear speakers. The whistling was only heard when the engine was running.

The first thought that came to mind was noise from the generator that enters the audio path of the amplifier. This happens with hastily made wiring and close proximity of the supply and signal (interconnect) circuits. But the wiring and interconnect cables were of high quality, which I was convinced of. A day later, they brought me an already "dead" Fusion FP-804 amplifier with a familiar diagnosis: it does not turn on.

The most interesting thing was that the power indicator "Power" barely noticeable. But I did not pay attention to this. After the autopsy, it turned out that all the same MOSFETs were knocked out again. So this amplifier turned out to be in my pile of scrap - they gave it away for parts.

After some time, I decided to restore this amplifier, and I wanted to figure out what is the reason for the general burnout of rather expensive mosfets in converter. I bought new transistors to replace the faulty ones, installed and ...

At the first start, I witnessed an enchanting show. Immediately after switching on, a growing whistle was heard - a slow start of the converter, and then I saw sparks jumping from the center of the toroidal transformer.

Here it is - a malfunction! Breakdown of the windings in the transformer. If I hesitated and didn’t turn it off, I would completely burn out this batch of MOSFETs.

After that, it became clear why the green LED was dimly lit. "Power" with 12V power connected. The current entered the secondary circuit through a breakdown between the transformer windings and slightly "highlighted" the power indication LED. This is the first time I've encountered such a problem. The only way out is to rewind the toroidal transformer.

Schematic diagram of the auto amplifier Fusion FP-804 (aka Blaupunkt GTA-480).

SUPRA car amplifier repair.

Car amplifier SUPRA SBD-A4240.

Fault: Turns on normally - " green LED". But when a signal is applied to the inputs, there is no sound in any channel. The amplifier is silent.

This error is not typical. For a better explanation of the troubleshooting technique, I will refer to the circuit of this amplifier. Diagram of car amplifier Supra SBD-A4240(will open in a new window).

Measurements of the supply voltage in the secondary circuits did not give anything - everything is normal. After a cursory check, a broken 7.5V zener diode was found (indicated as ZD4 in the diagram).

A broken zener diode led to the disconnection of the signal circuits of all amplifiers, since it is installed in the input signal blocking circuit (Q3, Q101, Q201, Q301, Q401, ZD3, ZD4).

This circuit blocks the passage of the audio signal to the inputs of the preamplifiers. "Blocking" of the signal occurs for a short time, immediately after turning on the amplifier. This is done in order to avoid a "click" in the speakers.

Since there was no 7.5V zener diode available, a 5.6V zener diode was installed instead of the broken one (this led to slight signal distortion, later I installed a 7.5V zener diode). After that, 3 channels began to work with slight distortions, and 1 channel gave out strong distortions with signs of self-excitation of the amplifier. When the tweezers touched the input of the sound signal ("tulips"), a periodic "gurgling" was heard in the speaker.

Suspicion fell on the input filter unit, the one that is implemented on operational amplifiers - KIA4558 microcircuits (in the diagram U1-A and U2-A). Therefore, in order to determine where the malfunction lies, the signal circuit was broken, going from the output of the input filter block to the input of the preamplifier. This is done simply - one terminal of the electrolytic capacitor is soldered (in the diagram it is C108).

Next, touch with tweezers the output of the resistor R115 or the output of the base of the transistor Q103. Thus, we apply a "signal-to-noise" to the input of the preamplifier. Moreover, if the amplifier is working, then in the speakers we will hear a characteristic hum. But in this case, along with the hum in the dynamics, I again heard the nasty "gurgling". It became clear that the problem should be looked for in the preamplifier, and not in the input filter block.

The search for a faulty element in the preamplifier was complicated by the fact that it was made on low-power transistors (in the Q102 - Q116 circuit), of which there are quite a few. Checking these transistors without desoldering from the board (for breakdowns of transitions) did not give a result. Therefore, it was decided to solder all the preamplifier transistors and check them more carefully.

This also did not give a result, although it was possible to detect two 2N5551 transistors, which caused distrust. Checked them out universal tester, and they were determined every other time as broken. I had to replace them with new ones. All other transistors turned out to be serviceable, as were other elements of the circuit: diodes (D3 - D5) and capacitors. BUT! I didn't check the resistors.

During an external examination, I noticed that on the case of one of the resistors (in the R124 diagram - 47 Ohms) there is a barely noticeable burnout. When checking, it turned out that the resistor was open.

Since the resistor R124 is installed in the emitter circuit of the transistor Q106 (2N5551), its breakage led to incorrect operation of the amplifier and the same “gurgling”. After replacing the faulty resistor, the amplifier began to work properly. It was also replaced by a new transistor Q106. As already mentioned, when checking a pair of transistors 2N5551 came under suspicion. Perhaps one of them is the transistor Q106, in the circuit of which the resistor R124 burned out.

Another malfunction of the same amplifier.

The car amplifier already familiar to us was brought in for repair SUPRA SBD-A4240 (V1M07) with "torn out" electrolytes in the secondary circuits of the converter. To my question: "How did this happen?" - the owner replied that the amplifier was in the car that had an accident. As a result, the amplifier worked properly, but there was a terrible background in the speakers - impulse noise from the converter did its job. In place of native capacitors, new ones were installed with a capacity of 2200 uF * 35V. The background is gone.

If possible, then, of course, it is better to install electrolytes with a larger capacity (2200 - 4700 microfarads).

There are times when it is quite difficult to find a large capacity electrolytic capacitor. No problem! You can make a composite capacitor from several, the capacity of which is small. Read about how to connect capacitors correctly.

Other little things.

All active elements - transistors, both field and powerful complementary pairs of transistors are installed on the radiator through an insulating mica gasket. Heat transfer paste is used to improve heat transfer.

In some cases, it is necessary to dismantle the printed circuit board from the amplifier case, which is also a heatsink. Naturally, heat-conducting paste is smeared, soils everything around, dust and dirt stick to it. Therefore, you have to remove it from the radiator and transistor cases, clean the insulating mica gaskets from it. The job is not pleasant.

After the repair, everything needs to be restored as it was. Have heat conductive paste handy KPT-8 or KPT-19. It is better to apply the paste on both sides, both on the metal substrate of the transistor and on the radiator. In this case, the mica will be in the middle and covered on both sides with a layer of thermal paste. I do not advise applying a lot of paste, the main thing is that an even, thin layer of paste forms on the surface.

I also advise you to buy mica on occasion. For example, I bought a mica plate 10 * 5 cm in size and about 1 mm thick. Mica can be easily "flaked" with a sharp knife blade. Get a few insulating gaskets from mica. They can be used to replace broken, damaged, lost insulation pads. Mica is easily cut with a knife into slices of suitable size.

Where can I get repair parts?

When repairing a car amplifier, parts are often required to replace faulty ones. It happens that you can't find them. Where could I buy? Can buy radio components online. I, for example, ordered from AliExpress. It is not always possible to find what you need in our online stores.

Of course, it is not possible to cover all the cases encountered in repair practice, however, if you follow a certain algorithm, then in the vast majority of cases it is possible to restore the device to working capacity in a quite acceptable time. This algorithm was developed by me from the experience of repairing about fifty different UMZCH, from the simplest, for a few watts or tens of watts, to concert "monsters" of 1 ... 2 kW per channel, most of which were sent for repairwithout circuit diagrams.

The main task of repairing any UMZCH is to localize a failed element, which resulted in the inoperability of both the entire circuit and the failure of other cascades. Since there are only 2 types of defects in electrical engineering:

1. The presence of contact where it should not be;
2. Lack of contact where it should be,

then the “super task” of repair is to find a broken or torn element. And for this - to find the cascade where it is located. Next - "a matter of technology." As doctors say: "A correct diagnosis is half the cure."

The list of equipment and tools necessary (or at least highly desirable) for repairs:

1. Screwdrivers, side cutters, pliers, scalpel (knife), tweezers, magnifier - that is, the minimum required set of conventional mounting tools.
2. Tester (multimeter).
3. Oscilloscope.
4. A set of incandescent lamps for various voltages - from 220 V to 12 V (2 pcs each).
5. Low-frequency sinusoidal voltage generator (highly desirable).
6. Bipolar regulated power supply for 15 ... 25 (35) V with output current limitation (highly desirable).
7. Capacitance and Equivalent Series Resistance Meter (ESR) capacitors (highly desirable).
8. And finally, the most important tool is the head on the shoulders (required!).

Consider this algorithm using the example of repairing a hypothetical transistor UMZCH with bipolar transistors in the output stages (Fig. 1), which is not too primitive, but not very complicated either. Such a scheme is the most common "classic of the genre." Functionally, it consists of the following blocks and nodes:

• bipolar power supply (not shown);
• transistor differential input stageVT 2, VT5 with current mirror on transistorsVT 1 and VT4 in their collector loads and their emitter current stabilizer onVT 3;
• voltage amplifierVT 6 and VT8 in cascode connection, with a load in the form of a current generator onVT 7;
• node of thermal stabilization of the quiescent current on the transistorVT 9;
• node for protecting output transistors from overcurrent on transistorsVT 10 and VT 11;
• current amplifier on complementary triplets of transistors connected according to the Darlington circuit in each arm (VT 12 VT 14 VT 16 and VT 13 VT 15 VT 17).

1. The first point of any repair is an external examination of the subject and its sniffing (!). This alone allows sometimes at least to assume the essence of the defect. If it smells burnt, it means that something is clearly on fire.
2. Checking the presence of mains voltage at the input: the mains fuse has blown stupidly, the fastening of the wires of the mains cord in the plug has become loose, a break in the mains cord, etc. The stage is the most banal in nature, but at which the repair ends in about 10% of cases.
3. We are looking for a circuit for an amplifier. In the instructions, on the Internet, from acquaintances, friends, etc. Unfortunately, more and more often in recent years - unsuccessfully. We didn’t find it - we sigh heavily, sprinkle ashes on our heads and set about drawing a circuit for the board. You can skip this step. If the result is unimportant. But it's better not to miss it. It's dreary, long, disgusting, but - "It is necessary, Fedya, it is necessary ..." ((C) "Operation" Y "...).
4. We open the subject and make an external examination of its "offal". Use a magnifying glass if needed. You can see the destroyed cases of p / n devices, darkened, charred or destroyed resistors, swollen electrolytic capacitors or electrolyte leaks from them, broken conductors, printed circuit board tracks, etc. If one is found, this is not yet a reason for joy: the destroyed parts may be the result of the failure of some “flea”, which is visually intact.
5. We check the power supply.We unsolder the wires going from the PSU to the circuit (or disconnect the connector, if any). Remove the mains fuse andWe solder the lamp for 220 V (60 ... 100 W) to the contacts of its holder. It will limit the current in the primary winding of the transformer, as well as the currents in the secondary windings.

We turn on the amplifier. The lamp should blink (during the charging of the filter capacitors) and go out (a weak glow of the thread is allowed). This means that K.Z. there is no mains transformer on the primary winding, just as there is no obvious short circuit. in its secondary windings. With a tester in alternating voltage mode, we measure the voltage on the primary winding of the transformer and on the lamp. Their sum must be equal to the network. We measure the voltage on the secondary windings. They must be proportional to what is actually measured on the primary (relative to the nominal). You can turn off the lamp, put the fuse back in place and turn on the amplifier directly to the network. We repeat the voltage test on the primary and secondary windings. The ratio (proportion) between them should be the same as when measuring with a lamp.

The lamp burns constantly at full incandescence - which means we have a short circuit. in the primary circuit: we check the integrity of the insulation of the wires coming from the network connector, the power switch, the fuse holder. We solder one of the reasons going to the primary winding of the transformer. The lamp went out - most likely the primary winding (or interturn short circuit) failed.

The lamp burns constantly in an incomplete glow - most likely, a defect in the secondary windings or in the circuits connected to them. Solder one wire from the secondary windings to the rectifier(s). Do not confuse, Kulibin! So that later it would not be excruciatingly painful from incorrect soldering back (mark, for example, using pieces of adhesive masking tape). The lamp went out - it means that everything is in order with the transformer. Lit - again we sigh heavily and either look for a replacement for him, or rewind.

6. It was determined that the transformer is in order, and the defect is in the rectifiers or filter capacitors. We call the diodes (it is advisable to unsolder under one wire going to their terminals, or solder it if it is an integral bridge) with a tester in ohmmeter mode at the minimum limit. Digital testers in this mode often lie, so it is advisable to use a pointer device. Personally, I have been using a “beeper” dialer for a long time (Fig. 2, 3). Diodes (bridge) are broken or broken - we change. Integers - “call” the filter capacitors. Before measuring, they must be discharged (!!!) through a 2-watt resistor with a resistance of about 100 ohms. Otherwise, you can burn the tester. If the capacitor is intact, when closing, the arrow first deviates to the maximum, and then rather slowly (as the capacitor charges) “creeps” to the left. We change the connection of the probes. The arrow first goes off scale to the right (there is a charge left on the capacitor from the previous measurement) and then creeps to the left again. If there is a capacitance and ESR meter, then it is highly desirable to use it. Broken or broken capacitors are changed.

7. Rectifiers and capacitors are intact, but is there a voltage stabilizer at the output of the power supply? No problem. Between the output of the rectifier(s) and the input(s) of the stabilizer(s), we turn on the lamp(s) (a chain(s) of lamps) for a total voltage close to that indicated on the filter capacitor housing. The lamp caught fire - a defect in the stabilizer (if it is integral), or in the circuit for generating the reference voltage (if it is on discrete elements), or the capacitor at its output is broken. A broken control transistor is determined by ringing its outputs (solder out!).

8. Is everything okay with the power supply (are the voltages at its output symmetrical and nominal)? Let's move on to the most important thing - the amplifier itself. We select a lamp (or chains of lamps) for a total voltage not lower than the nominal voltage from the PSU output and through it (them) we connect the amplifier board. Moreover, it is desirable to each of the channels separately. Turn on. Both lamps lit up - both arms of the output stages were broken. Only one - one of the shoulders. Although not a fact.

9. The lamps do not light up or only one of them burns. This means that the output stages are most likely intact. We connect a 10 ... 20 Ohm resistor to the output. Turn on. The lamps should blink (there are usually more power capacitors on the board). We apply a signal from the generator to the input (gain control - to the maximum). Lamps (both!) lit up. This means that the amplifier amplifies something (although it wheezes, phonitis, etc.) and further repair consists in finding an element that brings it out of the mode. More on this below.

10. For further verification, I personally do not use the standard amplifier power supply, but use a 2-polar stabilized PSU with a current limit of 0.5 A. If there is none, you can also use the amplifier PSU connected, as indicated, through incandescent lamps. You just need to carefully isolate their bases so as not to accidentally cause a short circuit and be careful not to break the flasks. But an external PSU is better. At the same time, the consumed current is also visible. A well-designed UMZCH allows fluctuations in supply voltages within fairly large limits. After all, we don’t need its super-duper parameters when repairing, just working capacity is enough.

11. So BP is fine. Let's move on to the amplifier board (Fig. 4). First of all, it is necessary to localize the cascade(s) with broken(s)/broken(s) component(s). For thisextremely desirablehave an oscilloscope. Without it, the efficiency of repair drops significantly. Although with the tester you can also do a lot of things. Almost all measurements are madewithout load(at idle). Let's say that at the output we have a "skew" of the output voltage from a few volts to the full supply voltage.

12. To begin with, we turn off the protection unit, for which we unsolder the right terminals of the diodes from the boardVD 6 and VD7 (in my practice it wasthreethe case when the cause of inoperability was the failure of this particular node). We look at the voltage is not output. If it has returned to normal (there may be a residual skew of a few millivolts - this is the norm), we callVD 6, VD 7 and VT 10, VT11. There may be breaks and breakdownspassive elements. We found a broken element - we change and restore the connection of the diodes. Zero output? Is there an output signal (when a signal from the generator is applied to the input)? Repair completed.

Rice. 4.

Has anything changed with the output signal? Leave the diodes disabled and move on.

13. We solder the right output of the OOS resistor from the board (R12 together with the right outputC6), as well as the left conclusionsR 23 and R24, which we connect with a wire jumper (shown in red in Fig. 4) and through an additional resistor (without numbering, about 10 kOhm) we connect to a common wire. We bridge with a wire jumper (red color) collectorsVT 8 and VT7, excluding the capacitor C8 and the quiescent current thermal stabilization unit. As a result, the amplifier is separated into two independent nodes (an input stage with a voltage amplifier and a stage of output followers), which must work independently.

Let's see what we have in the end. Is there voltage fluctuation? This means that the transistor (s) of the “skewed” shoulder is broken. Solder, call, replace. At the same time, we also check passive components (resistors). The most common variant of the defect, however, it should be noted that very often it isconsequencefailure of some element in the previous cascades (including the protection node!). Therefore, the following points are still desirable to perform.

Is there no crossover? So the output stage is presumably intact. Just in case, we send a signal from the generator with an amplitude of 3 ... 5 V to point "B" (connection of resistorsR 23 and R24). The output should be a sinusoid with a well-defined "step", the upper and lower half-waves of which are symmetrical. If they are not symmetrical, it means that one of the shoulder transistors, where it is lower, has “burned out” (lost parameters). We drink, we call. At the same time, we also check passive components (resistors).

Is there no output at all? This means that the power transistors of both arms "through" flew out. It's sad, but you have to solder everything and ring with a subsequent replacement.

Component breaks are not ruled out. Here it is necessary to include the "8th tool". Checking and replacing...

14. Have you achieved symmetrical repetition at the output (with a step) of the input signal? The output stage has been repaired. And now you need to check the operability of the quiescent current thermal stabilization unit (transistorVTnine). Sometimes there is a violation of the contact of the variable resistor engineR22 with resistive track. If it is included in the emitter circuit, as shown in the above diagram, nothing bad can happen to the output stage, because. at the connection point of the baseVT 9 to divider R 20– R 22 R21, the voltage simply rises, it opens more and, accordingly, the voltage drop between its collector and emitter decreases. A pronounced “step” will appear in the idle output signal.

However (very often), a tuning resistor is placed between the collector and the VT9 base. Extremely "fool-proof" option! Then, when the engine loses contact with the resistive track, the voltage at the base of VT9 decreases, it closes and, accordingly, the voltage drop between its collector and emitter increases, which leads to a sharp increase in the quiescent current of the output transistors, their overheating and, of course, thermal breakdown. An even more stupid version of this cascade is if the VT9 base is connected only to the variable resistor engine. Then, if contact is lost, anything can be on it, with corresponding consequences for the output stages.

If possible, it is worth rearrangingR22 into the base-emitter circuit. True, in this case, the adjustment of the quiescent current will become expressed non-linear from the angle of rotation of the engine, butIMHOit's not such a big price to pay for reliability. You can just replace the transistorVT9 on the other, with the reverse type of conductivity, if the layout of the tracks on the board allows. This will not affect the operation of the thermal stabilization unit in any way, because. he isbipolarand does not depend on the type of conductivity of the transistor.

Verification of this cascade is complicated by the fact that, as a rule, connections to collectorsVT 8 and VT7 are made by printed conductors. You will have to lift the legs of the resistors and make connections with wires (Fig. 4 shows breaks in the conductors). Between the buses of positive and negative supply voltages and, accordingly,collector and emitterVT9, resistors of approximately 10 kΩ are turned on (without numbering, shown in red) and the voltage drop across the transistor is measuredVT9 when rotating the trimmer sliderR22. Depending on the number of cascades of repeaters, it should vary within the range of approximately 3 ... 5 V (for “triples, as in the diagram) or 2.5 ... 3.5 V (for “twos”).

15. So we got to the most interesting, but also the most difficult - a differential cascade with a voltage amplifier. They work only together and it is fundamentally impossible to separate them into separate nodes.

We bridge the right terminal of the OOS resistorR12 with manifoldsVT 8 and VT 7 (dot " AND", which is now his "exit"). We get a “stripped down” (without output stages) low-power op-amp, which is fully operational at idle (no load). We apply a signal with an amplitude of 0.01 to 1 V to the input and see what will happen at the pointAND. If we observe an amplified signal of a form symmetrical with respect to the ground, without distortion, then this cascade is intact.

16. The signal is sharply reduced in amplitude (low gain) - first of all, check the capacitance of the capacitor (s) C3 (C4, because manufacturers very often put only one polar capacitor for a voltage of 50 V or more to save money, counting that in reverse polarity, it will still work, which is not gut). When it dries up or breakdown, the gain decreases sharply. If there is no capacitance meter, we simply check it by replacing it with a known good one.

The signal is skewed - first of all, check the capacitance of capacitors C5 and C9, shunting the power buses of the preamplifier after resistors R17 and R19 (if these RC filters exist at all, because they are often not installed).

The diagram shows two common options for balancing the zero level: resistorR 6 or R7 (there may be, of course, others), if the contact of the engine is broken, the output voltage may also be skewed. Check by rotating the engine (although if the contact is “majorly” broken, this may not work). Then try to bridge their extreme conclusions with the output of the engine with tweezers.

There is no signal at all - we look to see if there is one at all at the input (open R3 or C1, short circuit in R1, R2, C2, etc.). Only first you need to unsolder the VT2 base, because. on it the signal will be very small and look at the right terminal of the resistor R3. Of course, the input circuits can be very different from those shown in the figure - include the "8th tool". Helps.

17. Naturally, it is not realistic to describe all possible causal variants of defects. Therefore, further I will simply state how to check the nodes and components of this cascade.

Current stabilizersVT 3 and VT7. Breakdowns or breaks are possible in them. Collectors are soldered from the board and the current between them and the ground is measured. Naturally, you first need to calculate the voltage at their bases and the values ​​\u200b\u200bof the emitter resistors, what it should be. (N. B.! In my practice, there was a case of self-excitation of the amplifier due to an excessively large resistor valueR10 supplied by the manufacturer. It helped to adjust its value on a fully working amplifier - without the above division into cascades).

Similarly, you can check the transistorVT8: if you bridge the collector-emitter of the transistorVT6, it also stupidly turns into a current generator.

differential stage transistorsVT 2 V 5 Tand current mirrorVT 1 VT 4 and also VT6 are checked by their continuity after soldering. It is better to measure the gain (if the tester has such a function). It is desirable to choose with the same gain.

18. A few words "off the record." For some reason, in the vast majority of cases, transistors of more and more power are put into each subsequent cascade. There is one exception to this dependence: the transistors of the voltage amplification stage (VT 8 and VT 7) dissipate 3...4 times more power than on pre-driver VT 12 and VT 23 (!!!). Therefore, if there is such an opportunity, they should be immediately replaced with medium power transistors. A good option would be KT940 / KT9115 or similar imported ones.

19. Quite common defects in my practice were non-soldered ("cold" soldering to the tracks / "patch" or poor maintenance of the leads before soldering) component legs and broken transistor leads (especially in a plastic case) right next to the case, which were very difficult to see visually . Shake the transistors, carefully observing their conclusions. Worst case, unsolder and re-solder.

If all active components have been checked, and the defect persists, you need (again, with a heavy sigh), to remove at least one leg from the board and check the ratings of the passive components with a tester. There are frequent cases of breaks in fixed resistors without any external manifestations. Non-electrolytic capacitors, as a rule, do not break through / break, but anything can happen ...

20. Again, from the experience of repair: if darkened / charred resistors are visible on the board, and symmetrically in both arms, it is worth recalculating the power allocated to it. In the Dominator amplifier in Zhytomyr, the manufacturer installed 0.25 W resistors in one of the cascades, which burned regularly (there were 3 repairs before me). When I calculated their required power, I almost fell off my chair: it turned out that 3 (three!) Watts should be dissipated on them ...

21. Finally, everything worked ... Restoring all "broken" connections. The advice seems to be the most banal, but how many times forgotten !!! We restore in the reverse order and after each connection we check the amplifier for operability. Often, a cascading check, it seems, showed that everything was in order, and after the restoration of connections, the defect “creeped out” again. The last to solder the diodes of the current protection cascade.

22. Set the quiescent current. Between the PSU and the amplifier board, we turn on (if they were turned off earlier) a “garland” of incandescent lamps for the corresponding total voltage. We connect the load equivalent (4 or 8 ohm resistor) to the UMZCH output. We set the engine of the tuning resistor R 22 to the lower position according to the diagram and apply a signal from the generator with a frequency of 10 ... 20 kHz (!!!) to the input of such an amplitude that the signal at the output is no more than 0.5 ... signal, a “step” is clearly visible, which is difficult to notice on a large signal and low frequency. By rotating the R22 engine, we achieve its elimination. In this case, the filaments of the lamps should glow slightly. You can also control the current with an ammeter by connecting it in parallel with each garland of lamps. Do not be surprised if it differs noticeably (but no more than 1.5 ... 2 times in a larger direction) from what is indicated in the tuning recommendations - after all, it is not “compliance with the recommendations” that matters to us, but the sound quality! As a rule, in the "recommendations" the quiescent current is significantly overestimated, in order to guarantee the achievement of the planned parameters ("for the worst"). We jump the “garlands” with a jumper, increase the output signal level to a level of 0.7 from the maximum (when the amplitude limitation of the output signal begins) and let the amplifier warm up for 20 ... 30 minutes. This mode is the most difficult for the output stage transistors - the maximum power is dissipated on them. If the "step" did not appear (at a low signal level), and the quiescent current increased by no more than 2 times, we consider the setting complete, otherwise we remove the "step" again (as indicated above).

23. We remove all temporary connections (do not forget!!!), assemble the amplifier completely, close the case and pour a cup, which we drink with a feeling of deep satisfaction with the work done. And that will not work!

Of course, within the framework of this article, the nuances of repairing amplifiers with "exotic" stages, with an op-amp at the input, with output transistors connected with an OE, with "two-story" output stages, and much more ...

That's why TO BE CONTINUED…