Electrolytic capacitors often fall into the hands of radio amateurs, the quality of which is questionable. The fact is that over time, the electrolyte in them dries out and their capacity decreases. Sometimes almost to zero. Of course, it is impossible to install such capacitors in a circuit. But how to check them? How do you know if this capacitor is good or not? Instruments designed to measure the capacitance of electrolytic capacitors are complex and expensive. In amateur conditions, it is quite possible to get by with a simple device, the description of which is given in this article. It allows you to check the performance of capacitors, including electrolytic ones, with an operating voltage of more than 4.5 V and a capacity of 0.5 to 1000 μF. In this way, you can determine a breakdown in the capacitor, the presence of a large leak, and even roughly estimate its capacity.
Of course, the accuracy of determining the capacitance is small, but it is quite sufficient to answer whether a given capacitor can or cannot be installed in the circuit.
The schematic diagram of the device is shown in Figure 1.
As can be seen from the diagram, the device is an asymmetrical multivibrator assembled using transistors of different conductivities.
The operating principle of the device is based on the fact that its frequency depends on the capacitance of parallel-connected capacitors C1 and Cx. The oscillation indicator is an incandescent lamp H1. The device is powered by battery B1.
When the power is turned on, both transistors open. The light bulb flashes, and capacitor C1 begins to charge through resistor R1. The charge current passes through the base-emitter circuit V1, opening it. when the capacitor is charged, the charge current that opened transistor V1 drops to zero. The transistors turn off. The light goes out. The circuit will be in this state until capacitor C1 is discharged through resistors R2, R3. Then this process will repeat all over again.
When connecting the capacitor being tested in parallel with C1, their total capacity increases and the discharge time will become longer. The light will start blinking less frequently. If the capacitance of the connected capacitor is small, then this change will be insignificant. And when connecting a capacitor with a capacity of 1000 μF, the light bulb will flash in about twenty seconds. If the capacitor is broken or has a high leakage current, the light bulb will burn continuously.
Transistor V1 - KT315 or other similar n-p-n structure. You just need to select specimens with Jko no more than 1 μA and a gain of at least 50.
Transistor V2 - MP39 or other similar p-n-p structure with a gain of at least 50.
Capacitor C1 is paper or ceramic of any type. Resistors are also of any type.
Bulb H1 is a regular one, from a flashlight, with a voltage of 2.5 V and a current of 0.15 A. Bulbs with high current and voltage cannot be used.
INSTALLING THE DEVICE, start by setting the maximum value of resistor R3, placing its slider in the lower (according to the diagram) position. To begin, install resistor R1 with a value of 680 Ohms. Turn on the power and check the operation of the multivibrator. If it works, the light should blink. Otherwise, increase the value of resistor R2. Having achieved the operation of the multivibrator, select the value of R1. It can be selected within the range of 680 ohms -4.7 kohms. At larger values, the light bulb burns longer, but the multivibrator operates less stable. Therefore, it is necessary to set the value of resistor R1 at which the generator operates stably and the light bulb shines brightly enough at the maximum frequency. This frequency is set by resistor R3. In the mounted sample it is approximately 10 Hz.
A flashing light serves as a good indicator that the device is turned on. Connecting the capacitor being tested reduces the blinking frequency of the light bulb. To an experienced eye, the change in frequency is noticeable even when a 0.05 µF capacitor is connected. Connecting a broken capacitor or a capacitor with a large leak causes the light bulb to glow continuously. The light stays on for quite a long time when high-capacity capacitors are connected - 100 - 1000 µF. Therefore, in order to use the device, you must first practice by connecting known-good capacitors of 5, 10, 20, 50 or more microfarads to the device. The device, of course, can also test non-electrolytic capacitors.
In conclusion, I would like to note that electrolytic capacitors that have not worked for a long time with a large leakage should be connected for some time to a direct current source with a voltage equal to the operating voltage of the capacitor. After a short period of operation in this mode, the leakage current will noticeably decrease, and the capacitor can be used again.
The device allows you to measure the ESR of electrolytic capacitors with indication of the measured value on the linear scale of a dial gauge or on the indicator of a digital multimeter.
The device circuit is assembled on four op-amps. A generator with a frequency of 120 kHz is assembled on OR 1. The voltage from this generator is supplied to the inverting amplifier at OP 2, in the feedback circuit of which the capacitor under test is connected. Since the gain value of the inverting amplifier at the op-amp is directly proportional to the resistance value of the resistor in the OOS circuit, its output voltage will be directly proportional to the measured value. Next comes the normalizing amplifier OR 3. By changing its gain and switching the feedback resistor, we can easily change the measurement range. Next comes a linear voltmeter on OR 4. If instead of a microammeter you include a resistor several kilo-ohms in size, then the voltage across it can be measured with a digital multimeter. For example, FLUKE has a very convenient sub-range - 300 mV.
Rice. 2 Schematic diagram of an ESR meter for electrolytic capacitors
The device diagram is shown in Fig. 2, and has two measurement limits of 1 Ohm and 5 Ohm. But there can be as many of them as you like. By switching on, for example, 9 kOhm instead of resistor R9, we get a limit of 10 Ohms.
In general, it seems to me that the use of this device for the purpose of identifying faulty capacitors during repairs of electronic equipment is no better than the use of a device for measuring ESR on a transformer. But, when you are interested in the exact ESR value, when selecting capacitors, for example, then its use is advisable.
It should be taken into account that the presence of even a very small inductance (a ferrite bead, for example, placed on a wire) causes a noticeable (at the limit of 1 Ohm - more than half the scale) deflection of the needle. This way you can easily distinguish between wire and film resistors, for example, if it is difficult to determine by appearance.
You should focus on the design of the probes. The best results were shown by twisted probes made of four wires, with an insulated diameter of about one millimeter. Two wires are twisted together, and then two pigtails are twisted together. With a length of 40 cm, the introduced error is about 0.2 Ohm. The same pigtail of four wires, only short, is connected to the terminals on the device body. It is convenient to use terminal blocks for connecting sound speakers.
The values of the parts, with the exception of the values of resistors R7, R8 and R9, which determine the boundaries of the ranges, are not critical. The device is powered by 12 disk batteries with a capacity of 0.28 Ah.
The setup is done like this. We insert a known resistance into the block, for example, 3 Ohms. By rotating the trimmer R11, set the arrow to 30 (if the head is 50 microampere). That's all. Tests of the device on capacitors with a capacity of 820-4700 μF from manufacturers SXE, SAMHWA, KELNA, LXY and others, with an ESR value of less than 0.1 Ohm, confirmed its fairly high efficiency.
All the best, writeto © 2005
Recently, in amateur radio and professional literature, a lot of attention has been paid to such devices as electrolytic capacitors. And it’s not surprising, because frequencies and powers are growing “before our eyes,” and these capacitors bear a huge responsibility for the performance of both individual components and the circuit as a whole.
I would like to warn you right away that most of the components and circuit solutions were gleaned from forums and magazines, so I do not claim any authorship on my part; on the contrary, I want to help novice repairmen figure out the endless circuits and variations of meters and probes. All the diagrams provided here have been assembled and tested more than once, and appropriate conclusions have been drawn regarding the operation of this or that design.
So, the first scheme, which has become almost a classic for beginner ESR Metrobuilders “Manfred” - this is how forum users kindly call it, after its creator, Manfred Ludens ludens.cl/Electron/esr/esr.html
It was repeated by hundreds, and maybe thousands of radio amateurs, and were mostly satisfied with the result. Its main advantage is a sequential measurement circuit, due to which the minimum ESR corresponds to the maximum voltage on the shunt resistor R6, which, in turn, has a beneficial effect on the operation of the detector diodes.
I did not repeat this scheme myself, but came to a similar one through trial and error. Among the disadvantages, we can note the “walking” of zero on temperature, and the dependence of the scale on the parameters of the diodes and op-amp. Increased supply voltage required for device operation. The sensitivity of the device can be easily increased by reducing resistors R5 and R6 to 1-2 ohms and, accordingly, increasing the gain of the op-amp; you may have to replace it with 2 higher speed ones.
1. Powered by mobile phone lithium battery
2. The stabilizer is excluded, since the operating voltage limits of the Lithium Battery are quite narrow
3. Transformers TV1 TV2 are shunted with 10 and 100 Ohm resistors to reduce emissions when measuring small capacities
4. The output of 561ln2 was buffered by 2 complementary transistors.
In general, the device turned out like this:
For greater versatility, I added additional functions:
1. infrared radiation receiver, for visual and auditory testing of remote controls (a very popular function for TV repairs)
2. illumination of the place where the probes touch the capacitors
3. “vibrick” from a mobile phone, helps to localize bad soldering and microphone effects in details.
Remote control video
And recently on the “radiokot.ru” forum, Mr. Simurg posted an article dedicated to a similar device. In it, he used a low-voltage supply, a bridge measurement circuit, which made it possible to measure capacitors with ultra-low ESR levels.
I wanted something synchronous, I started thinking about the implementation scheme, and now in the magazine “Radio 1 2011”, as if by magic, an article was published, I didn’t even have to think. I decided to check what kind of animal it was. I assembled it and it turned out like this:
Well, lastly, on the website monitor.net, member buratino posted a simple project on how you can make an ESR probe from an ordinary cheap digital multimeter. The project intrigued me so much that I decided to try it, and this is what came out of it.
A capacitor is an element of an electrical circuit consisting of conducting electrodes (plates) separated by a dielectric. Designed to use its electrical capacity. A capacitor with a capacitance C, to which a voltage U is applied, accumulates a charge Q on one side and Q on the other. The capacitance here is in farads, the voltage is in volts, the charge is in coulombs. When a current of 1 A flows through a capacitor with a capacity of 1 F, the voltage changes by 1 V in 1 s.
One farad has a huge capacitance, so microfarads (µF) or picofarads (pF) are usually used. 1F = 106 µF = 109 nF = 1012 pF. In practice, values ranging from a few picofarads to tens of thousands of microfarads are used. The charging current of a capacitor is different from the current through a resistor. It depends not on the magnitude of the voltage, but on the rate of change of the latter. For this reason, measuring capacitance requires special circuit solutions based on the characteristics of the capacitor.
The easiest way to determine the capacitance value is by the markings on the capacitor body.
Electrolytic (oxide) polar capacitor with a capacity of 22000 µF, designed for a nominal voltage of 50 V DC. There is a designation WV - operating voltage. The marking of a non-polar capacitor must indicate the possibility of operation in high voltage alternating current circuits (220 VAC).
Film capacitor with a capacity of 330000 pF (0.33 µF). The value in this case is determined by the last digit of a three-digit number, indicating the number of zeros. The following letter indicates the permissible error, here - 5%. The third digit can be 8 or 9. Then the first two are multiplied by 0.01 or 0.1, respectively.
Capacitances up to 100 pF are marked, with rare exceptions, with the corresponding number. This is enough to obtain data about the product; the vast majority of capacitors are marked this way. The manufacturer can come up with his own unique designations, which are not always possible to decipher. This especially applies to the color code of domestic products. It is impossible to recognize the capacity by erased markings; in such a situation, you cannot do without measurements.
The simplest RC circuit consists of a resistor and a capacitor connected in parallel.
After performing mathematical transformations (not given here), the properties of the circuit are determined, from which it follows that if a charged capacitor is connected to a resistor, it will discharge as shown in the graph.
The product RC is called the time constant of the circuit. When R is in ohms and C is in farads, the product RC corresponds to seconds. For a capacitance of 1 μF and a resistance of 1 kOhm, the time constant is 1 ms, if the capacitor was charged to a voltage of 1 V, when a resistor is connected, the current in the circuit will be 1 mA. When charging, the voltage across the capacitor will reach Vo in time t ≥ RC. In practice, the following rule applies: in a time of 5 RC, the capacitor will be charged or discharged by 99%. At other values, the voltage will change exponentially. At 2.2 RC it will be 90%, at 3 RC it will be 95%. This information is sufficient to calculate the capacity using simple devices.
To determine the capacitance of an unknown capacitor, you should include it in a circuit consisting of a resistor and a power source. The input voltage is selected slightly lower than the rated voltage of the capacitor; if it is unknown, 10–12 volts will be sufficient. You also need a stopwatch. To eliminate the influence of the internal resistance of the power source on the circuit parameters, a switch must be installed at the input.
The resistance is selected experimentally, more for the convenience of timing, in most cases within five to ten kiloohms. The voltage across the capacitor is monitored with a voltmeter. Time is counted from the moment the power is turned on - when charging and turning off, if the discharge is controlled. Having known resistance and time values, the capacitance is calculated using the formula t = RC.
It is more convenient to count the discharge time of the capacitor and mark the values at 90% or 95% of the initial voltage; in this case, the calculation is carried out using the formulas 2.2t = 2.2RC and 3t = 3RC. In this way, you can find out the capacitance of electrolytic capacitors with an accuracy determined by the measurement errors of time, voltage and resistance. Using it for ceramic and other small capacitances, using a 50 Hz transformer and calculating capacitance, gives an unpredictable error.
The most accessible method for measuring capacitance is a widely used multimeter with this capability.
In most cases, such devices have an upper measurement limit of tens of microfarads, which is sufficient for standard applications. The reading error does not exceed 1% and is proportional to the capacity. To check, just insert the capacitor leads into the intended sockets and read the readings; the whole process takes a minimum of time. This function is not present in all models of multimeters, but it is often found with different measurement limits and methods of connecting the capacitor. To determine more detailed characteristics of the capacitor (loss tangent and others), other devices are used, designed for a specific task, often stationary devices.
The measurement circuit mainly implements the bridge method. They are used limitedly in special professional areas and are not widely used.
Without taking into account various exotic solutions, such as a ballistic galvanometer and bridge circuits with a resistance store, a novice radio amateur can make a simple device or an attachment for a multimeter. The widely used 555 series chip is quite suitable for these purposes. This is a real-time timer with a built-in digital comparator, in this case used as a generator.
The frequency of rectangular pulses is set by selecting resistors R1–R8 and capacitors C1, C2 using switch SA1 and is equal to: 25 kHz, 2.5 kHz, 250 Hz, 25Hz - corresponding to switch positions 1, 2, 3 and 4–8. The capacitor Cx is charged at a pulse repetition rate through the diode VD1, to a fixed voltage. The discharge occurs during a pause through resistances R10, R12–R15. At this time, a pulse is formed with a duration depending on the capacitance Cx (the larger the capacitance, the longer the pulse). After passing through the integrating circuit R11 C3, a voltage appears at the output corresponding to the pulse length and proportional to the value of the capacitance Cx. A multimeter (X 1) is connected here to measure voltage at a limit of 200 mV. The positions of switch SA1 (starting from the first) correspond to the limits: 20 pF, 200 pF, 2 nF, 20 nF, 0.2 µF, 2 µF, 20 µF, 200 µF.
Adjustment of the structure must be done with a device that will be used in the future. Capacitors for adjustment must be selected with a capacity equal to the measurement subranges and as accurately as possible, the error will depend on this. Selected capacitors are connected one by one to X1. First of all, the subranges of 20 pF–20 nF are adjusted; for this, the corresponding trimming resistors R1, R3, R5, R7 are used to achieve the corresponding multimeter readings; you may have to slightly change the values of the series-connected resistances. On other subranges (0.2 µF–200 µF) calibration is carried out with resistors R12–R15.
When choosing a power source, it should be taken into account that the amplitude of the pulses directly depends on its stability. Integrated stabilizers of the 78xx series are quite applicable here. The circuit consumes a current of no more than 20–30 milliamps and a filter capacitor with a capacity of 47–100 microfarads will be sufficient. The measurement error, if all conditions are met, can be about 5%; in the first and last subranges, due to the influence of the capacitance of the structure itself and the output resistance of the timer, it increases to 20%. This must be taken into account when working at extreme limits.
R1, R5 6.8k R12 12k R10 100k C1 47nF
R2, R6 51k R13 1.2k R11 100k C2 470pF
R3, R7 68k R14 120 C3 0.47mkF
R4, R8 510k R15 13
Diode VD1 - any low-power pulsed, film capacitors, with low leakage current. The microcircuit is any of the 555 series (LM555, NE555 and others), the Russian analogue is KR1006VI1. The meter can be almost any voltmeter with a high input impedance, which is calibrated for it. The power source must have an output of 5–15 volts at a current of 0.1 A. Stabilizers with a fixed voltage are suitable: 7805, 7809, 7812, 78Lxx.
PCB option and component layout
As part of my job I have to repair industrial equipment. Analysis of faults shows that a significant proportion of them are due to failed electrolytic capacitors. Using an ESR meter greatly simplifies the search for such capacitors. My first one helped a lot in this matter, but over time I wanted to have a device with a more informative scale, and at the same time “test” other circuit solutions.
You may ask, why analog again? Of course, I have an ESR meter with a digital indicator for a detailed study of large capacitors, but this is not required for operational troubleshooting. In addition, there is a long-standing sympathy for pointer indicators, inherited from the Soviet past, so I wanted something a little vintage.
As a result of prototyping, I settled on ludens, which allows you to experiment widely with measuring scales.
Another time-consuming part of the scheme is transformer. I used a magnetic core from a matching transformer from an ATX power supply. Considering that this is a standard W-shaped core, winding should not pose any particular difficulties.
The primary winding contains 400 turns of wire with a diameter of 0.13 mm, the secondary winding contains 20 turns of wire with a diameter of 0.2..0.4 mm. My secondary winding is located between two layers of the primary, I don’t know how important this is here, just out of old habit.
Before calibration, instead of resistors R10, R12 (R11, R13), variable resistors with values close to the expected values are installed, and the resistor slider R14 is set to the middle position. Then a resistor with a resistance corresponding to the end of the measuring range is connected to the measuring probes, and resistor R10 (R11) sets the arrow closer to the left side of the scale, where the last point of the measuring range will be. For obvious reasons, it cannot be in place of the mechanical zero of the microammeter.
Next, short-circuit the probes and use resistor R12 (R13) to set the arrow to the far right mark of the scale. These operations are repeated several times until the arrow accurately positions itself at the start and end points of the range without our help. Now that we have “found” the boundaries of the measuring range, we measure the resistance of the corresponding variable resistors and solder constant ones in their place.
We find the intermediate points of the scale by connecting resistors of the corresponding resistances to the probes. To simplify the process, it is permissible for these purposes to use a resistance store with bifilar winding of coils. Subsequently, I checked the assembled device with the P33 magazine - the deviations in the readings turned out to be insignificant. To remember the location of intermediate points, it is not necessary to mark the scale with a pencil; it is enough to write down the numerical values obtained according to the factory scale on a piece of paper, then put the marks on the corresponding place of the template in the program.
Attached are my scale options made in Sprint. The file already contains a factory scale template, which can be enabled by checking the “display” box.
The scale obtained in this way is glued to the factory scale using an adhesive stationery pencil.
The connecting wires are soft to bend, with a cross-section of 0.5..1.0 sq.mm., it is not advisable to make them too long. Factory probes need to be lightly sanded to reduce contact resistance and pierce the varnish coatings on the board.
How to check a capacitor. Theoretical information about capacitors |
Basically, according to their design, capacitors are of two types: polar and non-polar. Polar capacitors include electrolytic capacitors, while non-polar capacitors include all others. Polar capacitors get their name from the fact that when using them in various homemade products, it is necessary to maintain polarity; if it is accidentally broken, the capacitor will most likely have to be thrown away. Since the explosion of a container is not only beautiful in its effects, but also very dangerous.
But don’t be alarmed right away: only Soviet-type capacitors explode, but they are already hard to find, and the imported one only “farts” a little. For capacitor checks you will have to remember, namely: the fact that the capacitor passes only alternating current, it passes direct current only at the very beginning for a few microseconds (this time depends on its capacitance), and then it does not pass. In order to check the capacitor using a multimeter, you need to remember that its capacitance must be from 0.25 µF.
How to check a capacitor. Practical experiments and experiments |
We take a multimeter and set it to test continuity or measure resistance, and connect the probes to the terminals of the capacitor.
Since direct current is supplied from the multimeter, we will charge the capacitor. And since we charge it, its resistance begins to increase until it becomes very large. If, when we connect the probes to the capacitor, the multimeter starts beeping and shows zero resistance, then we throw it away. And if we immediately see a 1 on the multimeter, then there is a break inside the capacitor and it should also be thrown out
PS: You cannot test large containers this way. :(
In modern circuits, the role of capacitors has increased noticeably, as the power and operating frequencies of devices have increased. And therefore it is very important to check this parameter for all electrolytes before assembling the circuit or when diagnosing a malfunction.
Equivalent Series Resistance - equivalent series resistance is the sum of the series-connected ohmic resistances of the contacts of the leads and the electrolyte with the plates of the electrolytic capacitor.
ESR meter based on Sunwa YX-1000A dial multimeter
The circuit operates on the principle of testing a capacitor with alternating current of a given value. Then the voltage drop across the capacitor is directly proportional to the modulus of its complex resistance. Such a device will detect not only increased internal resistance, but also loss of capacity. The circuit consists of three main parts: a square pulse generator, a converter and an indication
The rectangular pulse generator is assembled on a digital chip consisting of six NOT logic elements. The role of the AC-DC voltage converter is performed by DA2, and the indication is on the DA3 chip and 10 LEDs.
The ESR meter scale is non-linear. To expand the measurement range there is a range switch. made in the Sprint Layout program is also available.
An oxide electrolyte can be simplified in the form of two aluminum strip plates separated by a spacer made of porous material impregnated with a special composition - electrolyte. The dielectric in such elements is a very thin oxide film that forms on the surface of aluminum foil when a voltage of a certain polarity is applied to the plates. Wire leads are attached to these tape covers. The tapes are rolled into a roll, and the whole thing is placed in a sealed housing. Due to the very small thickness of the dielectric and the large area of the plates, oxide capacitors have a fairly large capacity despite their small dimensions.
The basis of this circuit is made up of eight operational amplifiers with negative feedback and occupy a stable operating position if their two inputs match the applied voltage. Amplifiers 1A and 1B generate oscillations at a frequency of 100 kHz, which is set by the chain C1 and R1. Diodes D2 and D3 are designed to limit the lower and upper amplitude of the output signal, so the level and frequency are resistant to changes in battery supply voltage.
This amateur radio circuit allows you to control EPS in circuits up to 600 volts, but only if the circuit does not have an alternating voltage with a frequency of more than 100 Hz.
The output of op amp 1B is loaded onto resistor R8F. The capacitor under test is connected through the probes. Capacitor C3 is blocking. Diodes D4 and D5 protect the device from the charging current of capacitor C3. Resistor R7 is designed to discharge C3 after the measurement. The DC bias voltage from diode D1 and the signal from resistor R9F are summed at the input of operational amplifier 1D. Each of the three stages has a gain of 2.8.
Details: 1. Op-amp chip LM324N. 2. "F" resistors 1% accuracy; all others - 5% 3. R7 from 0.5 watts, the rest 0.25 watts. 4. R21 sets linearity in the middle of the scale: 330 to 2.2 ohms. 5. R24 corrects the DC offset at infinity ESR. 6. R26 helps set zero (full scale): 68 to 240 ohms. 7. R6F=150 Ohm, R12F=681 Ohm
ESR meter on available radio components |
The probe circuit consists of: a generator, a measuring circuit, an amplifier, and an indicator. T1 is a composite transistor. A homemade LED scale is used as an indicator.
To speed up the assembly process, a probe for testing capacitors is made on a breadboard and placed in a housing made from a piece of cable channel. The pins are made of copper wire
The delivery set includes the measuring device itself, three probes for it and four legs for the board. The Esr meter is designed to run on a 14500 lithium battery with a voltage of 3.7 volts, but you can not order it, but take it from an old laptop battery, and it doesn’t matter that it is larger in size.
About controlling the ESR meter.
1 - USB for power supply and battery charging. The device for testing electrolytic capacitors can be used without a lithium battery, using external power, but then the error of the device increases slightly.
2 - turn on the device
3 - Operation indicator. Begins to glow after the probe enters test mode
4 - Button to start the measurement process. We press it only after connecting the measured capacitance to the contacts
5 - Connectors for connecting measuring probes or transistors of suitable size
6 - Panel for measuring small radio components, the legs of which can fit into the hole
7 - Contact pads for testing SMD.
MG328 is designed to operate on a 14500 battery, but I decided to install an 18650 battery there. To do this, I unsoldered the original holder and directly soldered an 18650 element in its place. In terms of dimensions, everything fit into the standard dimensions of the finished board.
After power is supplied to the board from USB, the charging indicator starts to light. The device has a self-testing mode. To start it, you need to connect all three probes together and press the test button. After this, DIY MG328 will switch to self-test mode. In addition, this mode can be accessed through the menu. To do this, you will need to press the test button for two seconds.
To navigate the menu, you need to press the test button to select any of the items, and then hold down the same button for a few seconds. A pleasant surprise was the found menu item - frequency generator.
The photographs below show examples of measuring various types of radio components.
In general, I'm as happy with the measuring device as an elephant. Already in many of my repairs I found dead capacitors, without external signs of problems.
