All You Need To Know About AMPLIFIERS To Fix Stuff - A Beginners Guide To Audio Amplifier Repair

hi guys welcome to another work electronics repair video i have on the workbench today quite a large audio amplifier a powerapp pa it's uh i believe a three kilowatts one so it's quite high powered and this is the sort of thing that they use in clubs and concerts and on this island for the carnivals so i need to repair this amplifier and i thought it would be a good opportunity because of some of the comments i've had recently to other videos i know that a lot of you guys would like to see a video all you need to know about audio amplifiers to fix stuff and you know what guys that's a really really big subject and i'm not quite sure at the moment i can actually put all that together in one but what i thought it would do is make this video which is a beginner's guide to audio amplifier repair this is to get you started so first of all let's have a look at the amplifier we have a quick look around it then let's have a look at the theory of amplifier design the types of amplifier you're commonly likely to find no mathematics just the difference between the common types of amplifier you're going to find audio amplifying and when we've done that let's do a practical so we're going to attempt to repair this amplifier i hope you're looking forward to this so let's get started um i have an amplifier here this thing's so heavy oh they pick the thing up must weigh about 25 30 kilos i mean hill uh dx 3000 i think it's a three kilowatt power amplifier and if you guys wonder i mean it's like 1500 watts per channel if you look at these little amplifiers you make these days and they said 1500 watts per channel why do you think it takes this much weight of the electronics to make an amplifier that can deliver that power yeah that's heavy i think the guy this uh belongs to he has a company so they do uh pa systems and such like he does carnivals you have these carnivals here in the street with big uh articulated wagons coming down trucks with dance floors on them and you know pa system so powerful they actually shake the ground and that's i think what this comes from so uh i've taken the end off you see there's lots and lots of transistors in here um two four six eight ten down its size and uh obviously two channels and this i think is actually the rear side of this so you see the transformer the electrolytic capacitors in here 18 000 micro 525 volts each of these so you don't want to be touching them if you're worried about capacitors with charging that's for sure and that actually means that when this is powered up if there's you know there's gonna be a lot of voltage in there so although you think about it's an amplifier it's got a transformer doesn't mean it's not necessarily hazardous to work on in some respects um yeah this has been added on here this is uh something i actually i actually did this a few years ago so there was a thermal triple like a kind of like a fuse trip on the back that was faulty and uh let's see it's only somewhere i can't quite see it i know when i looked at this before it was working then so there was a trip in here somewhere that was faulty i couldn't get one of the physical size uh to replace poison so i actually wired one inside here you can see this thing here does the job i think yes i remember now so let's have a look on the front of this the on off switch which i can probably get into the shot weight of the stuffing the on off switch in fact we had two poles on one side of it was like a thermal chip that would drip out and it was open circuit so i wired this in so we still have a thermal trip and i mounted this here so that's actually on the main power coming in yeah it comes through this and then goes off down to the transformer i've turned the amplifier over so you can have a look at the other side there was two metal plates here which haven't been moved and now we can see the other side of the amplifier modules now so i mean straight where i can see some couple of transformers a couple of relays i notice and um i'm not sure how easily i can actually get these amplifier modules out of here but we can see a positive and negative supply coming in and there's two of them on each of the amplifier modules this is probably on ground i'm guessing so it'll be the plus and minus supply we can see this on those capacitors on the other side as well i'll show you again in a moment so this is basically what we have to work on with this yes we have on this side we can see again we should see the red and the black and then i'm guessing that the these are all connected together these parts these capacitors we can just have a loop i'm just going to use my flue committer for a moment because somebody said that i needed to clean my multimeter so i actually did this most sensible thing which was to take the casing off this plastic take it home and ask the wife to watch it that way quite well so let's just have a look so i'm pretty much assuming that both ends of these are actually connected together yep that looks like it is and that looks like it says yeah and then we'll have the supplies so we can just check there's no short circuits on here that that's a good thing let's go to the other ones this is the negative end that will go the other clarity just to check it yeah you see the capacitors charging up okay yeah so there's no short circuits on it that's nice to see um also just looking down here i can see two bridge rectifiers this one and this one this one has the um red wires going to yeah well yeah one red one black so this bridge rectifier is effectively across the two ends of here and this is this ground this will be a center tap on the transformer and the same here we have another bridge rectifier which is the red and black here and again these will go to a center tap on the transformer and down in the middle is a third bridge rectifier which comes from two orange wires here and um that has down some thin black and red wires on so i'm assuming this is like a low voltage supply to the the uh circuitry not the main amplifier section but the sort of the components before that like the preamplifier if you like so that's what we appear to have um but let's have a look first so i'll draw this out what we have and while we're doing that let's have a look at amplifier design in general and amplifier repair in general so we can make this a beginner's guide to audio amplifier repair i doubt it's possible to do it all you need to know it's such a big topic but let's get the basics first let's get a good grounding with this and then we can start repairing amplifiers probably the first thing you need to know about audio amplifiers is they are not all the same in the same way that power suppliers come in different types you get a linear power supply switch mode power supplies or even something like an atx power supply if you saw the recent video i made there's a number of common designs or topologies and that's also true of audio amplifiers with amplifiers these designs are called classes so you may have seen this you get things like a class a amplifier a class d amplifier and so on this class has nothing to do with the actual quality of the amplifier it's not a case that a class a amplifier has better performance or better sound quality than the class b and so on these classes are the topology the design of the amplifier amplifier classes go from a b c d e f and so on but not all of these designs or topologies or classes are actually audio amplifiers there are some classes that are rf radio frequency amplifiers the classes that you will find with audio amplifiers are these ones you have class a you have class b you have class a b which is basically a combination of these two a kind of a halfway house then you have class d you have class g and you have class h so these are the different topologies the types of audio amplifiers that you're likely to come across if you're doing repair work so of course you're going to want to know what is the difference between all these classes so let's have a look at each one in turn and i'll explain briefly how each one works and then after we've been through this theory we can look at our amplifier see which class ours is and then we can get on with the repair the class a amplifier is the most simple i'll draw you a basic circuit this is using the mosfet but you can also do this with bipolar transistors as well but there's one principle that is true of all classic amplifiers so with the mosfet um we'll have a 12 volt supply and i'll just draw you a circuit so we have a mosfet to ground okay this is the gate of the mosfet and we have then um coming in through a capacitor this is your audio in okay and we have audio other side sound to the ground so this is where you feed your audio in you have your mosfet and then you need a couple of resistors so what you basically need to do we'll put a variable resistor to ground here another resistor to the gate and this sets the bias which i'll just explain in a moment your output comes from here your capacitor g output so this is your factory where you would connect your speaker okay the idea of this bias is that you have to set the gate voltage so the voltage at this point is half of the supply voltage so if you have a supply voltage of 12 volts by adjusting this resistor you're effectively adjusting the voltage on the gate okay and you adjust that so the voltage here is six volts the idea of the other resistor here by the way is that even if you put the variable resistor all the way to one end it can't connect directly there's always some resistance between the drain and the the gate so this is quite a simple circuit you adjust this so you've got six volts here okay when you feed an audio signal in ac the capacitor lets the ac come through the dc is blocked so this will not affect the dc voltage here so on the positive half cycle you increase the voltage on the gate the fet turns on more and the voltage on the drain drops down towards zero okay when this goes negative to the negative half cycle you decrease the voltage on the gate the fat turns a bit more off and the voltage on the drain goes up towards the supply and if you have the correct amplitude coming in here you can effectively change the voltage here from six up to 12 or down to zero so effectively your output here is about six volts peak to peak okay this capacitor here is to stop dc coming through your speaker all the time because your speaker diaphragm naturally sits in the middle okay the coil in your speaker so your speaker has a coil inside it which effectively is moving uh in a magnet so you have a magnet around this coil and the coil is attached to the speaker cone okay so as you pass coming through here one direction will deal with it it magnetizes in one direction on the other and that's what actually makes the speaker current move in and out so that's how the circuit is actually where it sort of ended the coil obviously comes to here so you don't want a dc voltage going through the speaker because the speaker code would move to one end and stop there effectively when there were six volts on the ear that means that it can't move further that way so it can't reproduce the sound correctly and anyway having a dc voltage flowing through your speaker coil is not a good thing so that's how this circuit works basically the advantage of it's very very cheap if you like to make is a very simple circuit the disadvantage is that this mosfet is effectively always passing currents so even when there's no audio coming in this is passing current the voltage here there's six volts means there must be a voltage drop across this resistor and the only way there can be a voltage drop across this resistor is if current's flowing through it so the current flows through the resistor and through the fat and that's the disadvantage of this design these things are used in very low powered amplifiers maybe little transistor radios and such like but even so it's not a particularly good design because even if you had no sound it's draining from the battery you know it's it's very unefficient the efficiency of this circuit that's the the wattage that's turned into sound rather than the wattage that's just used to pass through this fat all the time is about 25 you can find similar circuits with bipolar transistors and more complicated ones with multiple stages but the main thing you need to know about this one is that the output device is always passing current even with no audio input you can increase the output power of this by decreasing the value of this resistor and if you like putting more mosfets in parallel with each other so they share the load they share the current flowing through them but because it's always passing current it will just get hot so you'll need large heat sinks you'll need to watch power supplies because of this quiet comment all the time so effectively as well as making a powerful amplifier you're also making a rather good foot wall or room heater yeah there is one other advantage to this design and what this is why these are often used for things like head filled amplifiers and such like and some audio purists are these amplifiers and this is because this device is effectively handling the entire waveform the plus and minus all 360 degrees of the waveform has been handled by this therefore there's no distortion you can get a very pure signal here so you can have a small input coming in a large input going out and there's nothing really to add any distortion to the waveform so this gives you if you like the purest audio quality but due to the reasons i was saying it's only really used for low power amplifiers now let's have a look at the class b amplifier design and the whole point of the class b amplifier is to get around this problem of wasted power to improve the efficiency of the amplifier so the class b amplifier you have a positive and negative supply rail we'll put 12 volts in again okay and um we'll have a minus 12 volts the way you get these by the way normally is in the power supply to have a transformer very dodgy transformer okay i'll draw the transformer as well the diodes so you have a power supply basically something like this with a center tap on the transformer so the center tap is basically halfway down the winding it's just brought to a connection and that's connected to ground you then have capacitors here and you also have capacitor here we'll capacity tools and that basically gives you a positive and negative supply so that's how you actually get the positive and negative supply now the amplifier the class b effectively i'll just show you here so we know we have a plus and it could be any voltage which is 12 volts just to represent what we can do okay with the amplifier itself we're using npn in the pmp transistor again it could be mosfets it could be an n-type and p-type or it could be transistors i'll use transistors in this one so these are your two transistors this one's an npn to understood this the pmp transistor put the base on here and then from the base we have some bias resistors so i'll just draw them in okay and these resistors are set basically so each transistor is off okay it sets the base voltage so it's not on it it's off it's not doing anything it's not passing any current okay and the audio input comes here we use two capacitors and this is where we feed the audio in okay and again the audio goes to ground and our speaker actually goes from here and the other end of the speaker is connected to ground so this center point this is effectively our ground now let's have a look how this works we feed the ac waveform in here the audio okay when we're on the positive half cycle through this capacitor the ac increases the voltage on the base okay it won't affect the dc voltage here because the capacitor only passes ac not dc so we increase the voltage on the base and as that increases we turn the transistor on so this transistor starts to conduct current down into the speaker to grout okay so there's this voltage going up the voltage on this emitter is also going up okay notice this doesn't invert the signal now on the other one the half cycle the voltage vector comes to zero and goes below zero and this be the pmp transistor the base has to be below the voltage of the emitters are turned on so this one now turns on this one turns off because the voltage is below the emitter and it's an npn the same as when this was conducted the voltage here was above the emitter okay so this handles the negative half cycle this conducts and it pulls the voltage on the emetic down towards minus 12 volts so this conducts a negative half cycle and this conducts a positive half cycle at no point are both the transistors conducting at the same time so the advantage of this is that when there's no audio coming in there's basically no output current there's no output that's not doing anything yeah no apart from the little bit of current that flows through the resistors of course so that's how this circuit works and you can see it's much more efficient than the class a amplifier your output waveform now is basically go we assuming we have the correct uh level of input so we can fully drive this the output voltage of this which we can go up to plus 12 or minus 12 on this point so effectively you have about 12 volts peak to peak and that's for the given voltage that's the actual maximum output the disadvantage of this circuit this class b amplifier is that if you watch my video on transistors all you need to know about transistors you'll know that until the base voltage is about 0.6 volts above the emitter voltage the transistor will not switch on and the same with the pmp the base has to be about 0.6 volts below the emitter before it will start to conduct so in actual fact you get a situation when the voltage audio waveform is near to zero either side of zero you get a point where this transistor is already switched off because the voltage g is now below 0.6 but this one isn't switched on yet because the voltage hasn't got to minus 0.6 and this is called crossover distortion so what happens with your output waveform is effectively as it goes through zero it kind of distorts like so yeah this is crossover distortion at this point one transistor is switched off but the one hasn't turned on yet another cause of distortion with this these transistors are called complementary so the npn and the pnp transistor are selected to have matching characteristics but no matter how well you select the transistors will never be exactly the same so you may find that this one turns on it may be 0.55 positive but this one doesn't turn until maybe 0.58 negative so the distortion actually is not the same for either half phase so that's the advantage efficiency and the disadvantage of class b is crossover distortion for this reason you will not really find true class b amplifiers these days what you find is a halfway house between the two and that is class a b the way the class a b works is based on this circuit so what we do is we replace this resistor with a diode okay and we also replace this resistor with a diode semiconductor diode now a diode like a transistor junction has a voltage drop across it if you measure with your semiconductor analyze you can see about 0.6 volts so what actually happens now with the two diodes in series is that this base will be about 1.2 volts higher than this base 0.6 on that semiconduction semiconductor junction and 0.6 on this one the two added together there's 1.2 volts okay so now by selecting the value of the resistors you can actually modify the performance of this so that both transistors are just turned on just only just but they just turned on in the quiescent state okay the base voltages will be different because of this and they'll be different by 1.2 volts so we now have a situation where on the positive half cycle this transistor was already just slightly turned on so as the voltage goes positive we don't have to wait till it reaches 0.7 revolt this starts to conduct straight away and it continues to conduct taught you vc is a zero point again at the zero point this one starts to conduct straight away because the base voltage again was set to point where it was just turned on so effectively by doing that we get rid of the crossover distortion okay and that is called class a b this is a very very efficient amplifier type it can be scaled up easily so that you can have many transistors in power we can do this sort of thing so we can add more transistors in parallel on both halves okay and the same obviously on the other half so we can do this sort of thing with it okay and we can make high powered amplifiers that don't generate so much heat as the class a they're much more efficient they're not wasting family because of the efficiency and the simple design of a class a b amplifier almost all consumer equipment with amplifiers in are class a b or at least they were until class d came out which we'll talk about a little bit later i'll just talk now because it's just a good time to mention this about this business of connecting lots of transistors in parallel and this is true whether the mosfets or whether the bipolar transistors when we have lots of transistors wide and power like this so effectively they're all wide across each other okay to share the load they're all driven from the same signal but although the transistors can be matched i mean some amplifiers you'll find that the transistors the output transistors were chosen specifically maybe from batches so the amplifier manufacturer bought large quantities of transistors and then they batched them so they put them in bins they bind them each bin having transistors have a very similar gain and a very similar base emitter voltage drop but even by doing that you can never absolutely match the transistors or mosfets so what you normally find in this type of arrangement is that you have a small value resistor on the emitter of each transistor okay this then goes to your loudspeaker to ground and then the other side you'll have again small value resistors on the emitter of the pmp transistor going to minus 12 or whatever voltage we're using okay so you will normally find in fact i would say you will always find in transistors where they have in amplifiers where they have multiple transistors on the output stage you will find these low value emitter resistors and this is quite important when it comes to fault finding now let's talk about class g and class h amplifiers because they're very closely related to the class ap amplifier so we'll we could get the basic circuit that we just had speaker to round minus volts plus volts because i will resist isn't that where it did so there's our basic circuit the same we just had okay that way the input through the capacitors i'm sure you can draw one of these yourselves now the basic topology of this okay so how much power how much wattage are these transistors dissipating yeah that's the question and here's the question are they dissipating the most wattage when they switched off so no current is flowing through them all the dissipating the most wattage when they switched fully on so all the current is flowing through them or are they disappointed are they dissipating the highest wattage when they're half on half off which do you think it is guys which do you think is the time when they dissipate the most wattage well the answer is not when the fully conduction where they carry most current every daughter it was and the reason for that is basically this when the transistor is switched off okay when it's switched off there's no audio input there's no drive it's switched off there's no current flowing through the transistor now wattage equals voltage times current so when it's switched off effectively you've got 12 volts on this end and via the speaker to ground you've actually got naught volts on this end because this is always resistance maybe four ohms so the voltage across your transistor is 12 volts but the current is zero so 12 times zero 12 volts times zero amps equals zero so basically when the transistor is off the wattage is zero zero watts and that makes sense that's kind of intuitive it's not passing any current there's no wattage what happens when the transistor is fully turned on well when it's fully turned on the emitter is basically shorted to the collector the resistance connected to emitter is very low maybe point one of a volts point one point two something like that yeah so all the voltage appears across the speaker so at this point you've got 12 volts across your speaker to ground if we'll use 12 volts again okay but you've got almost no voltage across the transistor so where's the wattage being dissipated well the transistor now has almost zero volts which has a little bit let's call it point one of a volt and the currents what's the currents well this is a 12 volts supply and your speaker is 4 ohms so ohm's law says if this was short that the maximum current can flow is 3 amps so ohm's law says i equals v over r which is 12 over four which used to be three fours and twelve so the current flowing is three amps what's the wattage dissipated by the transistor 0.1 times 3 which is like 0.3 watts okay so when the transistor is fully turned on the wattage dissipated by it is very low the closer this is to zero the closer the wattage is to zero all the wattage is actually being now dissipated by your speaker yeah which is three times 12 equals 36 watts if we're talking dc which we are at the box so that's what you have so in both cases when the transistor is on or the transistor is off the wattage dissipated it's basically zero what happens when it's half on a half off well now effectively we have six volts here okay whichever one plus or minus is it's still six whether it's positive or negative okay this applies to either positive or negative apart from your cards have minus watts okay so when it's half on and half off you've got six volts across the transistor and you've got six volts across the speaker yeah the resistance of the two is the same four m four ohms it's a voltage divider so we've now got six volts across our speaker okay what's the current flowing well it's half of what it was when it was 12. yeah you can do the maths again six over four equals 1.5 amps okay so there's now one and a half amps floating through our transistor what's the voltage across the transistor six so the wattage now is 6 volts times 1.5 amps equals 9 watts yeah you can see that and the other 9 watts by the way has been dissipated by the speaker if it's dc so this is why the transistors get hot in an amplifier this is why they have heat sinks on them okay the voltage is always changing the wattage is always changing but on average your transistors are probably dissipating half the wattage of the speaker you know half of it each if you like because it on off the timing so a quarter now a quarter now and half of it there but they're all going to be on the same heatsink so at the end of the day half the wattage is going into the heatsink now this problem only becomes worse if you decrease the resistance of the speaker the two ohms you double the current you double the wattage if you increase the voltage here to a higher voltage say 24 and minus 24 you're doubling the voltage you jump in the wattage again okay so this situation of wasted heat will increase with the wattage you can put more transistors in parallel like i showed you and bigger heat sinks than cooling fans but you're wasting a lot of power so you might think well why not keep the voltage down keep the voltage down well this comes down to the maximum wattage that the speaker can produce yeah so with the 12 volt supply i showed you even when the transistors are fully on in dc the maximum wattage is about 36 watts yes there's a rms formula but the mass don't matter you could only get a certain wattage out of a speaker with a plus or minus 12 volts apart if you want more you either have to reduce the resistance of the speaker or you have to increase the voltage you can only reduce the resistance of the speaker so far so at the end of the day the only practical way to get more wattage out is to increase the voltage and as i've just shown you increasing the voltage increases the power the wattage dissipated by the transistors and the hotter it runs so this is where i know there's a bit of theory there but this is where your class g in class h amplifier is coming so this is a class g amplifier we have the same setup the class a b the amplification is the same is working the same as this was working and typically this is used on hard powered amplifiers when you get into the hundreds of watts up to the kilowatt and so this is where you use these sort of circuits so typically for an amplifier of this sort of power you need a supply voltage maybe a plus or minus 80 volts plus or minus 90 volts but because of the high supply voltage it dissipates a lot of heat so you have to make very big heavy amplifiers to get around that what they normally do is they use a lower voltage so for instance this is quite typical plus 48 and minus 48 volts supply okay and your speaker's connecting to ground so the waveform effectively on here can go peak to peak plus or minus 48 yeah which is like 96 volts okay peak to be what they then have is another voltage rail at double this one so you'll have a 96 volts positive and you'll have a 96 volt negative supply and here the fit transistors or mosfets and these could be mosfets but we'll use transistors why not quite typically these use mosfets even transistors here i will say so basically we have a transistor that's effectively wide here and we have another transistor that's effectively wired here npn pmp and this transistor can basically supply the higher voltage so the circuits effectively monitors the amplitude of the signal and when the amplitude is low it's just using the 48 volt supplies okay your sine waves here if the sine wave gets the point where it's going to go above 48 on the peak yeah it turns on these transistors and then gives you twice the voltage to the transistor so now you can go up to 96 volts just about and the same on the negative and when you get to quieter passages of music again it goes down and this can work very quickly i mean normally the big peaks are unlike the bass dumpster boom boom boom they're music yeah especially the good stuff we play so these are switching on and switching off as and when they need it and that increases the efficiency again and it means you can make an amplifier with smaller heatsinks less weight the disadvantage is that it's more circuitry you've now effectively got four devices fit on the output and again there may be more of them in parallel so like i was showed you previously you might have like four of these in parallel four of these and parallel and so on lots more devices more circuitry more complex circuit because that's the monitor the voltages and know when to switch these on and off but that is a class g amplifier okay class h is so similar to this we'll use the same diagram with a class h amplifier instead of this switching between a low voltage and a high voltage as and when is needed effectively a class h amplifier modulates the power supply voltage so the power supply can go anywhere between plus 48 and plus 96 minus 48 and minus 46 or whatever 96 or whatever values that you use but these are quite typical so with the class h you're not switching between two voltage rails you're actually changing the power supply the voltage rail in real time you're modulating the power supply voltage to suit the output waveform and that is a class hm right remember i said with this circuit that the only way to increase the wattage the power output is to either decrease the resistance of the speaker or increase the voltage supply that's not actually totally true there's another way to do this and you'll see this and you may have seen this and wondered what it was or maybe you knew what it was bridge mode high powered amplifiers can run in bridge mode so in bridge mode you take a stereo amplifier and you run it in mono mode and auto magically by doing so you can increase the power output of the amplifier and the way that actually works i'll just show you briefly as this so we'll use the 12 volt supply although this is used on higher powered amplifiers with higher voltages but we'll do it so again we have our class a b amplifier i won't bother with all the resistors but this is all there yeah this is your left hand channel and on the same amplifier you take your right hand channel so again you have your uh transistor let me go to the other one plus minus plus minus yeah plus 12 volts minus 12 volts and what you do in bridge mode these are two stereo channels you feed them with the same audio signal but out of phase so effectively when this one's going positive half cycle like so the other one is actually getting the same waveform upside down okay so this is actually getting the negative and then the positive that way okay so what actually happens now is on the positive half cycle this transistor turns on and conducts yeah and you put your speaker here okay that's where your speaker goes so it conducts through here okay and on this side remember i said this is inverted so when this is getting the positive offside well this amplifier is getting a negative off cycle upside down so the pnp transistor turns on so the current flows like ah okay and then opposite way negative off cycle this one's conducting and this is upside down so this one's conducted so the current flows right up okay now with the same supply voltage look what you've got you can actually now double the voltage across the speaker so rather than being on this where it can be plus or minus 12 one end of speed is connecting to ground so this end can only be plus 12 or minus 12. yeah it can only be plus or minus 12 this is ground in this case when this is plus 12 this is minus 12 on the other end of the speaker and when this is plus 12 this is -12 so now you've got plus or minus 24 volts peak to peak okay so you've double the supply voltage to speaker and therefore for the same resistance of speaker you've now increased the wattage yeah you doubled the wattage okay so that is bridge mode i think i can fit this here class d so with the class d these are sometimes called digital amplifiers but these they're not digital they're actually pulse width modulators and this circuit is very similar to a buck converter so with a book converter you've probably seen this after sharing this before we have a voltage supply okay and we have two mosfets coming to ground okay this is your high side mosfet and this is your website mosfet you see these in vrms all the time okay and if you've seen me draw these this comes from a pulse width modulator pulse width my pens given up let me find a new one one moment we have on the book converter effectively capacitor to ground okay and this is your output voltage and with the pulse width modulator by varying the on time and off time at very high frequencies of the mosfets you can vary the voltage that's here okay and you normally do this to make a stable voltage of vrm but think about it if we have an audio signal coming in here okay and we sample this very very fast maybe into the megahertz millions of times a second and we sample the voltage at that point yeah over there or there or there the voltage will vary and we can modulate the width of the pulses to match the amplitude of the signal okay so when the wave forms up here we get a very wide pulse relatively down here when it's down here we got a very narrow pulse okay and that's what this is doing so it's modulating using the incoming waveform so the voltage here is varying depending at that instance on the amplitude of the input so we have that there and then normally in a um class d amplifier you have two of these so i have another one here okay to ground base again these can be controlled effectively by the same pulse width modulator over here controlling everything forwards okay and we have coming out same thing inductive oil capacity to ground okay and an output voltage with the amplifier what we do is we connect our speaker to here okay these capacitors are not large electrolytic capacitors these small capacitors for the simple reason we don't want a stable voltage here we want the voltage to be varying depending on the amplitude of the vehicle we wait for but we need the capacitors to filter out the switching noise this is switching very rapidly so these coils and these capacitors effectively filter out the very high frequency switching the pulses and what we get coming out is a smooth waveform that represents the input one and this is the most efficient type of amplifier is a class d these are often used again in pa systems commercial amplifiers club disco amplifiers and such like some high-end home audio amplifiers and also these are used in car stereos particularly because of the high efficiency and what you can do with an amplifier like this often they're combined with a switchboard power supply so for instance on your car stereo the high powered ones the amplifiers um you could only get as we explained a certain amount of wattage out of a 12 volt supply so those type will commonly take the 12 volts coming in switch mode like dc to dc converter creating a much higher voltage which then feeds into this this is also similar to that bridge circuit if it looks like it's very similar yeah so that's a class d amplifier let's have a look at the test equipments we need to repair amplifiers now there is a lot of very specialist equipments around and quite expensive equipment if you actually want to test amplifiers and as much as you effectively measure in the distortion uh there's often little trimmer parts and adjustments you can make in the amplifier that you know will balance the output between the two channels that will effectively affect the amount of distortion that's generated that will balance within the circuit to the voltage between the various transistors and for sure that specialist test equipment has a use if you're repairing amplifiers on a professional basis and you're doing lots of them but probably like me you just want to fix amplifiers and really you just want to get them working your customer wants it working and if that's the case then you don't really need a lot of specialist equipment so i'm sure those will shout out to the screen and say they disagree with that and that's fine that's fair this is just from my experience of repairing amplifiers and what i needed to do the job okay so the first thing you're going to need is a multimeter nothing special about your multimeter any decent or half decent multiple will do the job and this will probably get you to the cause of lots of lots of faults just with a multimeter but there's more equipment we can add to make life easier and the next item i would pretty much say this is essential as well when you're repairing amplifiers and it's something very simple you can build this yourself so this is a current limited using two light bulbs basically the way this works is i plug the mains power in here i put the amplifier in here and i can set it to limit and the current that's drawn by the amplifier then flows through the light bulbs i'll just show you very quickly how you can actually build one of these this is the current limited circuit we have live and neutral coming in and the neutral connection goes straight to the output so this is the socket on the output that goes to where i put the amplifier in the neutral just goes directly to there i then have two light bulbs and my white bulbs are wired like so this is the first one and the second one is just wide across the first one and that's how you wire two eyeballs on mine one of them is a 60 watt and the other one is a 100 watt from here then this connection goes to the other output the only other thing i have on mine is a switch connected from here to here that i can use to short the light bulbs out which that means i have a direct connection so when it's closed online this is called mains and when it's open this is called limit and that's basically the circuit of my cover limiter the reason this is essential is due to the way the amplifiers tend to fail so one of the most common faults with an amplifier is that one or both channels have blown that means the output transistors have run short circuits and possibly other devices as well now it's quite common for you to come along you'll get your multimeter you'll find the short-circuit output transistors you'll replace them and you might even figure out that the pre-driver is the driver that's driving the output transistors also boiling you might change them but what you probably didn't spot is that the base emitter resistance have gone open circuit or some similar problem in the circuit so you then pull your amplifier back in switch it on only to finally to instantly commit suicide due to the fact that you didn't find it all the components that were false so yeah and you've now blown all the parts you put back and you have to start again when we use this we can power the amplifier through here we can set it to limit we can switch it on and if that sort of situation occurs the light bulbs will come on because it's trying to draw a lot of power but that means that the power it can draw is limited to these light bulbs in my case 160 watts so instantly you'll know there's a problem you can switch it off and secondly it's very likely that the components the transistors that you replaced are okay because this stopped a massive amount of current flowing through them the reason mine has two light bulbs by the way is because on some devices i only want to put 60 watts some 100 160 depending effectively the more code i think is going to draw i might use to use higher powered light bulbs and the way i switch between the two is very simple i just unscrew the light bulb i don't need okay so i can have one the other or both uh you can build these in lots of sort of ways i tend to use this sort of thing where i use effectively just a backing box and a couple of sockets in the light bulb holder i have another one i'll show you the other one as well and it's built in a very similar way so get yourself one of these if you want to save yourself a lot of pain unless they get one of these i mean build one of these save yourself a lot of pain makeups repairing amplifiers in order of usefulness but these next two devices are both pretty damn useful one is a component analyzer this is a dca 55 there are other components otherwise is available this will obviously test transistors for you out of circuit but it will allow you particularly to measure the gain of a transistor and also to see if there's any leakage currents these type of faults occur in amplifiers when effectively due to age the transistors become worn out we could say and they start to leak or the gain goes and that can cause all sorts of problems with amplifiers and a lot of cases these circuits have to be balanced they have to be matched the whole performance of the amplifier depends on the components being within specification so one of these is very useful the other reason it's very useful is if you have blown output devices where you have an amplifier with many transistors in parallel quite often those transistors are matched to each other so the gain is within a certain range and if you fit transistors outside that range then it won't work properly it's quite possible that some of the transistors will blow because effectively due to a misbalance they're not all switching on at the same time so at some point in the waveform always some of them are switched on and they're taking all of the load now so this is fairly essential if you don't want to buy one of these there are numerous product projects and circuits you can find online that will allow you to build a little device to measure transistor gain in fact i may actually do that as a project soon so let's have a look on the way to video if you're going to repair amplifiers a lot or even just a few you're going to find an oscilloscope very very useful it's one of the areas of repair where an oscilloscope will greatly help you to complete those repairs in a way that you wouldn't be able to do without it so an oscilloscope is bordering on the essential for amplifier repair the good news is that because amplifiers work on audio frequencies you don't need an oscilloscope with a wide bandwidth in a lot of cases you just want to know if a signal is getting to a certain point in the amplifier so even one of the cheap oscilloscope kits will be a big help to you and in fact that's another uh topic i'm going to look at in uh or probably videos of what can we get away with cheap to repair amplifiers other than that seeing you find yourself a secondhand one another thing i found very useful in the past by working amplifiers is this power supply this bench power supply is a dual output so it's dual 30 volts and it's a three amp supply it's not particularly high powered one but it has current limit on it and quite often i've have i have faulted amplifier circuits i use this to power the amplifier module out of the amplifier instead of the original power supply and it helps me to trace faults with the current limiter on so that nothing is getting damaged in a way it kind of does what the current limiting light bulb did but a little bit better sometimes so that's a nice thing to have and you don't have to have a dual supply like this you could get away you will get away you could work perfectly well with two of these and just connect them in series um so if that's where you've got to put some minor supply that's the main point with amplifiers they have plus and minus symmetrical power supply rails here's another one um i don't use as much um it's nice to have which is certainly nowhere essential this is a very i can mount mounted on top you'll see my other current limiter very similar to the first one but with one light bulb on this and this basically just allows me to vary the mains voltage from zero to full power which also allows me to see the current drill so on here i can just effectively just turn the knob and you can see the voltmeter going down there yeah so this allows me to run amplifiers on a low voltage supply again it can help in full finding where you have an amplifier which effectively would just go bang if it had the full voltage going in it kind of does again what the dim light bulb tester does in a similar way although it doesn't limit the current in the same way that's one of these nice to have things uh but okay if you managed to get your hands on one fine otherwise i wouldn't worry about it there's other ways to do what we can do with this for the main part one or the meter that you don't need until you do and when you do this will save you a lot of time and effort this is my esr meter it's for testing electrolytic capacitors and this will make life easier if you have vintage amplifiers in particular but any amplifiers have 40 electrolytic capacitors and finding them can be quite difficult so if you have the funds get yourself in the sr meter this is about 45 euros again there's other ones available but the ones that are built into the little uh component analyzers by which i mean these things these are not suitable really for measuring esr if you're going to get any sr meter get one of these okay these are nice little devices but esr measurement nah not really one other item that you may find useful for amplifier repair is a signal generator now this is a little standalone signal generator it's good for generating sine and other waves for that matter at varying frequencies so we can use uh one of these to make a sine wave at whatever frequency we want inject that into the amplifier and effectively monitor the output this becomes more useful when you're trying to measure output power and distortion and a lot of cases repairing amplifiers you can just inject any audio source um from a mobile phone from a computer and for that matter if you don't want to buy one of these but you want to inject sine waves you can find uh free programs if you look on the internet uh signal generator programs for your pc that effects will use your sound card to generate frequencies and for audio frequencies that's fine so you can also do this just effectively using the sound card on your pc um this wasn't expensive um actually i've only everybody used it once to be quiet and that was repairing the laser format enough not an amplifier so some sort of signal generator is going to be very handy but it could just be your sound card on your computer or even your mobile phone lastly another quick mention about the oscilloscope i mean oscilloscope really is very very useful but if you don't have one and you're on a tight budget you can get away with a signal tracer these are very simple little circuits that allow you to listen to the signal anywhere within the amplifier circuits i actually made a video a little while back showing how to use a little pair of pc speakers to determine if the bios on the graphics card was being accessed and that same project is fine for signal tracing and audio amplifiers so i will link that video to the end of this one and in the comments as well sorry and in the description as well and have a look at that if you're on the budget in fact another little project i have in mind is to build a little signal injector pen and a signal tracing pen as cheap as possible for those of you out there who just want to do these things on the budget and those things will be coming up in forthcoming videos this is a typical schematic for a class a b power amplifier this schematic is for one channel of a stereo amplifier so the amplifier will actually contain two of these and you can also see if you look at it it's fairly symmetrical you imagine you can sort of draw a line through here okay this side is the positive supply and this side is the negative supply these are your output transistors and you can see them here and you also see that they are complementary so these are npn transistors these are pnp transistors and if you look across the circuit you'll pretty much see that as the case that is a symmetrical circuit with npn and pnp devices the audio comes in here and the speaker goes out here so this is the connection to your speaker and then that connects to ground so that's basically how it is the positive and negative supply can vary i think this one says 56 volts but you'll quite often find amplifiers with around about 90 volts on either rail especially if they're high-powered amplifiers or a kilowatt or something like that so that's a fairly typical circuit now it looks complicated but i wouldn't let that deter you don't worry about it too much i'll show you the main parts of it and i'll show you what mainly would go wrong with these of amplifiers so generally speaking this input circuitry doesn't fail if it does fail then it's probably either the cause of a catastrophic failure over here which you send too much voltage over here or it's a case where you've got uh transistors that are out of spec that the gain is wrong and such like so it doesn't give a symmetrical drive to here in which case usually it'll work but it'll be distorted you'll have some problems you know the sound quality will not be good so normally speaking you don't have a problem over here never say never but that's you know a good rule of thumb these two devices this one here and this one here are voltage regulators basically they're using transistors this is all discrete transistor bear in mind you'll find amplifiers that use integrated circuits and a lot of this and even the entire amplifier can be integrated and also you can find mosfets amplifiers which are usually much simpler than this but very efficient high powered amplifiers back to these voltage regulators so you can see these are mounted on heatsinks it shows your heatsink in the heatsink here and these basically just regulate the supply coming into this part of the circuitry from here onwards all the lower end of it runs at a lower voltage than this end can you see that so effects where there's a divide down here between the high voltage and the lower voltage supply off oh george so sort of down here if you imagine it's kind of like it a divide okay so what's going to go wrong with this well faults come into a few types really one is the common plague we have of faulty capacitors now this amplifier is what's called direct coupled so through the audio path basically there aren't any capacitors between the various stages that gives a better audio quality but it can cause some problems but even said that there are capacitors in the amplifier and capacitors are known to fail so capacitors and audio amplifiers especially vintage ones there's a good place to start it's probably a good idea to remove them and check them giving your esr meter with a capacitance meter or a multimeter with capacitance range that shouldn't be too difficult to do and it's well worth spending the time to do that if it's particularly old it might just be worth replacing all the capacitors the electrolytic ones so that's one of the type of thought you find another common type of fault that's due to age is faulty tremor pots you see this thing here this is a variable resistor and this is being used to balance the voltage between these two transistors some amplifier circuits will have more than one of these trimmers yeah they look like these so so these are a tremendous part as you can see them they come in various styles but these have a habit of going open circuit so that the little slider loses contact to the track just due to age and maybe corrosion and dust so that's quite a common failure if you find this sort of fault it will probably cause damage to other parts of the circuit it's not unlikely if one one of these fails you might blow the output stage so it's something to have a look at and i would suggest if the amplifier has these in it to test them all and if anything looks suspect to change them look for open circuit ones open circuit to the wiper that's the main thing with those so moving towards this end of the amplifier these are your output transistors you can see you have three in parallel one two three and you could have more our amplifier has ten the one we're gonna look at but three four four is quite common six eight of these in parallel and these all share the load so your main current path on this amplifier and we'll take a different colored pen your main current path is from the high voltage supply through the fuse there's a fuse through these transistors into the speaker and to ground yeah that's the main common path and then the other side from the negative supply through the factory flows that way through these transistors and through the fuse okay so that's your main current path and as you probably know with the electronics that's which works hardest fails quickest i think i just made that up yeah work work hard play hard and die yeah not necessarily hard so that's what tends to happen usually because somebody's been over driving the amplifiers and they're putting too much power out of it yeah too much power coming out loud parties okay uh loud club nights and that's quite often if people don't drive an amplifier properly what happens is it gets to a point called clipping so clipping is where you have these your voltage rails 90 volts minus 90 volts yeah this is ground zero volts and your waveform is kind of doing this in between yeah and if you drive it too hard you can get to a point where the waveform wants to go above 90 volts but it can't so at that point it stops yeah comes down and again it hits the the limit yeah and what position i'm sure you can see the waveform is is effectively flattening like this okay yeah these flat bits this is clipping this is very bad for the amplifier because your speaker is an inductor when you're feeding ac through it it has a reactance a resistance is an inductor when you start to clip at this point you're feeding dc through the speaker and more current will flow it's very bad for your speakers you can blur the speaker coils and it's also very bad for amplifiers you can blow the output devices and usually if you blow one of these output devices or more you'll blow these output devices as well so these will go short and when this tries to come on these will take all the current straight through the current now flows not through your speaker but through the fuse directly down through here and back through the fuse okay that's where the current goes there's nothing in the way to stop it and you blow these transistors okay that's a very common failure mode in amplifiers and another reason short circuit load or somebody just too low impedance alone they put too many speakers on the output thinking they can and it's now two-way resistance so that is a very very common and this is what's called a blown amplifier when a transistor blows and goes short circuits you can do it in various ways so your transistor collector emitter base it can go short circuit from the collector to the emitter yeah this is now a short circuit and all the currents the high voltage will flow through here as i've just shown you yeah and probably boiled all those about that they're both short and that will blow the output stage but to make matters worse sometimes it can go short circuit to the base or to the base as well if that happens the high voltages on here say 90 volts comes through your transistor into the base and that base you can see is wired into this other part of the circuit and it's connected directly to these other devices there's no capacitors that block the dc so what happens now more voltage comes into this transistor than this transistor really likes to have yeah and they don't like it up them so basically these transistors blow and you now have a cascade effect which can go back through the amplifier so you can't have a situation where many devices are blowing in the channel if you find blown output transistors shorted out transistors if they used to check i'll show you in a moment on the amplifier how we do it if these are short you need to then check the driver transistors and you need to work your way back it might be worthwhile to start removing devices but one good suggestion is that you may be able to remove these or another way isolate this off of the amplifier from this off of the amplifier and then feed the signal in and test that the signal is okay as far as a certain point yeah that's possibly a good way to test that so that's one of the big problems with amplifiers and why sometimes they can be expensive to repair especially when the parts are hard to get at the vintage ones this by the way is where the dual bench supply comes in useful that you can actually power up part of the circuit some of the voltage rails and effectively makes it easier to fault find without starting to test every component possibly take them out of circuit probably you would have to do so that's one of the times that dual bench power supply would be useful but that isn't the end of the woes with amplifiers there's another problem as well this is quite common when these transistors go short-circuit you'll notice that you'll notice that these transistors have resistors in the emitter connections and these are low value resistors these are 0.3 ohms on each one and these effectively are to balance the transistors because your transistors when you apply a voltage to the base you may not all turn on exactly the same point so these resistors here and these resistors here in the base that's these and these allow you to balance the transistor so that it doesn't matter if one's turning on slightly before the other okay in fact it really is to make the more tunnel at the same time even though the voltage you apply between the base emitters slightly different they'll turn on at the same time so they'll all come on together that's the more truth always these low value resistors quite often will go open circuit so if you have short circuit transistors you're very like open circuit emitter resistors as well and if the voltage comes back into the base you may want open circuit base resistors these are also quite low value resistors 3.3 ohms so once you find the failure here you can replace the transistors but before you do once you've taken these transistors out you need to test all these resistors to see if any of them have got open circuits a common thing you'll also find with an amplifier mentioning is you'll find the blue fuse so you'll find one or both of these fuses of boiling now that's a visual thing but the fuse has blown for a reason and it's probably blown for this reason almost always i accept fuses can just fail but normally if a fusion just failed it's just open circuit and see the wire's broken if this has happened it's probably black inside it it's vaporized yeah so don't just replace the fuses you need to test for shorts first you need to test for all this loss and the same applies when you open an amplifier one of the first things you might notice is that some of the resistors are burnt and you think oh i've got some burnt resistors here i'll change them and you can see the ones on the other side and they're okay so you know the value you change to these burnt resistances because that's obviously what's wrong with it yeah obviously what's wrong with this oh and the fuse is gone as well but the resistors are burnt so i'll change the resistance i'll change the fuse you know where this is going don't you because these are still short so when you plug it in these just burn up again yeah that's all that happens yeah another little fire on the circuit board this again is where that current limiting light bulb trick came in so that it limits mr current this can draw these won't set on fire again but you'll you'll see you've got a short the light bubble come on so that's another uh thing with amplifiers so there's lots of things you might think that were against you when you you're repairing amplifiers but there's two things that are kind of for you yeah one of them as i mentioned this circuit is basically symmetrical yeah which means that if you have burnt components on the positive half and you'll find this sometimes you'll find an amplifier where the positive side is blown but the negative side hasn't okay or components always a burden different ways so quite often you can figure things out by comparing with the other half because of the symmetrical weights designed if something's blown up you all know what it is and even better than that most amplifiers are stereo amplifiers and the stereo amplifier has two of these okay and if you walk in the water cases only one channel is blown because the fault was on one channel the other one didn't have one blue before the other one caught yeah so again you have another channel which you can compare all your tests and your readings with the blown one and work your way through making comparisons just to show you what these transistors look like these are fairly typical with older amplifiers this is an mj2955 this is actually an mj3000 which is not a complementary one of this so this is the pmp and this would have an mj3055 which is the matching complement i just didn't have any to show you um sometimes they look like this which is a slightly different style but basically the same thing this case is called a to3 style transistor and usually these are mounted onto the heat sinks with little microwashes underneath and little plastic inserts in here where the bolts go through and those inserts and the microwashes are essential because this has to be isolated from the heatsink or always some of them do you might find for example all the ones on the positive side have bolted and all there was on the negative side of the washers to insulate them that's because the metal khan is the collector so when replacing these you need to make sure you put the inserts in and also to be quite honest when we place in output devices we get into this issue of matching the gain and this is why you should probably seek some advice on an audio repair for them rather than just put your devices in check with a few experts first even i do this yeah i can trace the photo when it comes to replacing these devices i'll usually ask the experts can i put that in um another type you'll see on more recent amplifiers of these um these are plastic ones again these are the metal tabs so you'll have the little micro washer or a gray heat conduction washer underneath this is typical of a complementary transistor pair this is a tip2955 it's like a plastic version of that metal one and this is the 3055 and you'll see they're both 100 volts uh collector base 60 volt correctly emitted these are maximum ratings five volts emitter bays 15 amp 90 watt transistors so they're complementary although quite often the manufacturer of the amplifier will have also tested gain of them and as i mentioned before they'll bend them so that when they build the amplifier they use transistors or a very similar gain as close as they can get to each other they come in a few others this is another type you might find and obviously you'll find mosfet amplifiers but the devices basically look like these physically so now we know basically what to look for in many cases electrolytic capacitors trimmer pots output transistors signal transistors and resistors quite often power resistors or just normal type resistors which are burnt or just test faulty those are the main things you're going to find wrong with amplifiers now we know that we know what the circuit's looking like we've got a good idea of where we're going to look and how we're going to test it let's have a look at a real-life example we'll get our very heavy amplifier back on the bench and let's see if we can fix it it's very tempting to get hold of the multimeter and start testing around the amplifiers looking for shorts and the various things we just mentioned but actually that isn't the best place to start the best place to start is not to use this but to use these yeah so have a good look first look around the amplifier can you see any obvious problems and for example burnt resistance i was just mentioning but in general have a good look around loose wires anything like that and the first thing i noticed on this before i started to make this long video i was doing the repair video was that there was a problem down here so i'll just show you what i spotted on the visual inspection i already made what i prepared earlier as they say this is what i preferred earlier and then we'll continue with the diagnosis of this amplifier and you can see a little transformer looks like a when you relay and oh what this oh that's interesting this looks like some sort of modification or previous repair there was a resistor here and i just touched it and there's a wire this green one which is soldered onto this that was just fell off the resistor that there um i don't really know what that's doing i mean it looks like uh was it a one with a resistor or something like that um maybe uh one just maybe a 10 in resistor actually um so that's there and that's i'm not quite sure if it's kind of like trapped underneath this thing here i think we need to get the other side off let's see if we can see something similar on the other channel this is the other channel i don't know which is the working channel by the way and this has two resistors down here in basically the same place that we can see on the other one um but they don't appear to be exactly identical this has a slight four-way connection going on here you can see you see that now this is a ground terminal naught volts and then there's two resistors down there all seven and all six if you look at the other side and i wish this thing was easier to move around okay so on this one we only have two wires on this not four but it seems to be basically the same port this is the ground terminal and then these resistors i think the ones under here that somebody's done some work on this at some point so we probably need to have a look to see what that actually is i mean i wonder there but maybe this is just what's wrong with this one wires fell off i didn't do this it works i have no idea what it exactly entails oh i see yeah so we can see down here now that there's a whole bunch of the pcb under there and that's where those resistors go i think probably the best thing to do with this is going to be to get the two amplifier modules out if i can and i can get them on the bench and we can compare the two and then maybe start to figure out what we can do with this actually on second thoughts i think i'll just repair this someone's obviously worked on this at some point in the past and repaired this burnt area of this pcb so it might just be a case if i just solder this back on here because somebody else already figured this out before me that it might just work so because it's probably not going to be too easy getting these amplifier modules out i'm not sure um let's just try this first so let's solder this back on here let's power it up and let's see what it actually is doing i've soldered that wire back onto that resistor i just noticed on here there's a tested by dave uh june 1988 and on the other side it says nigel december 1990 so this is quite an old amplifier i mean you're looking at 34 years old which is getting a little bit for an amplifier um we're just going to show these things were kind of built to last yeah although there's something wrong with it at the moment you can see i've fixed the problem with the broken wire on the resistor so that actually might be all that was wrong with this but we can make some checks before we power it up and then we can test it if the chest passes okay i have a schematic for this amplifier which i downloaded i'll put the link into the description of this video and i'll also put it on the we're not touch repair discord server as well and there's actually quite an unusual design i had to ask uh one well i asked on bad caps for him in the audio repair section some knowledgeable guys there i was asking if this is a class a b amplifier because you can see that all the output devices are npn and it shows two in each bank but there's actually ten in each bank so there's ten of these and ten of these and if you look at the circuit you'll see that there's a transformer here which effectively drives the base of these transistors and the way this works as far as i understand it is that the two windings are out of phase with each other so in the case of this winding the positive half sight was driving these transistors and the output which is connected from here the speakers is being pulled towards the positive supply rail and on the negative half cycle i believe this actually inverts the signal so this sees this as a positive signal switches these transistors on but because they're connected to the negative supply rail it pulls the speaker connection down towards ground so although they're both effectively amplifying what they think is a positive half cycle one's really the negative half cycle and because these are taking the speaker connection in either direction positive or negative you get the correct output that's the way i i understand it i'm sure there's going to be guys watching this who can tell me exactly how this works so the guys on bad caps tell me this is a class a b amplifier or b one with a rather unusual design so we have that that's handy the first thing we're going to check is whether there's any shorts from any of the transistor collectors to ground or to the the voltage rails so let's have a look at that now on our amplifier if we just look at the schematic in a little bit more detail you'll see the positive and negative power rails which are more to the dotted lines here actually go through the relays so these two relays we saw on the other side of the pcb effectively connect the power and i'm assuming that with the amplifier powered off there's no connection the relays are open so although normally you could measure the resistance across the main capacitors to see if there's a short in the output transistors in this case you can't because the relay is open so what i'm going to do is to make sure we have no short circuit transistors as to measure from the collector to the emitter and this is quite easy because the emitters have these resistors in them we can see these resistors here the small value resistor and they're actually if i zoom the camera at one moment those resistors are actually here by the transistors so quite easily if we just move on continuity mode you'll see that all the collectors of the transistors in each bank are connected together okay so that's all one bank and this is the other bank yeah let me just took a second to respond there so all these collectors they're connected together now between the two collectors obviously they'll be open circuit so that's effectively telling you there isn't a short through this one to the collector of the other one that's basically uh a short from here to here but a quick way to tell again is we just go on the bank of transistors anywhere and we go to one of the emitter resistors there's a short circuit this will find the short circuit on any of the transistors because all these emitter resistors are low value so you would only need an ohm or two if there was a short onto any of the transistors so that's fine 560 ohms yeah and we can do the same on this side we can just go to the emitter resistors okay that's fine and we'll just do the same on the other channel as well so far i'm not finding any short circuits okay and that's okay yeah i also found with this schematic with this actually some service notes as well so we know there's no shorts on the output stages well let's have a quick look at these service notes and see if there's anything else we can test so first of all this is telling you what to do if you've replaced any output devices any transistors we haven't done but we can still make some of these checks so it's saying that without the relays if we remove the relays we can check some resistances and it basically is saying that between the emitters and the collectors we should read 500 to 900 ohms so we can check that and between the base and the emitters three to five ohms okay the relays are held in place with little spring eclipse and they're quite easy to remove you just take something with a hook shaped end and just flick it to get under the back of the clip push down slightly and then they should actually just kind of clip out yeah there we go so i can clip those out quite easily yep it's off the relays will now come out so i'll get both of the relays out and then we can measure some resistances okay so i i have the relays um i don't think this actually make any difference on the collector to emit it reading but let's see what we have oh yes it's gone up yeah it's gone up so it's now 840 and it's saying it should be between five and nine hundred so that's good let's go on the other side make a good contact right yeah 843. it doesn't matter which end of the resistor you've eaten because the resistor is a very low value maybe half an ohm so it won't make much difference and we can do the same on the other side as well um so again to one of the resistors and we just go to the yeah 835 slightly different 838 so they're a little bit different but they're not very different so i'm not too concerned about that at the moment another thing it mentions in the instructions here is to test all the emitter resistance this is something i mentioned in the guide earlier you should always do this so the emitter resistors we know are these and they read let's see what they are well point six yeah so it's not short but although about half of them that's what i thought you were so i need to go along and just check that all of the resistors are okay on both banks on both channels so that'll just take me a few minutes so i'll be back with you in a moment okay that's all checked fine so i'm happy with this one um base rail to emitter rail three to five ohms so we need to look at that one now um now the base emitter resistors sorry the base rail resistors these ones are from the collector to the base yeah all the bases you can see there and there's another one on this side there'll be somewhere yeah r12 to there and the same for the other channel so where there was one emitter resistor per transistor there's one base resistor per bank so there's only four of them and we need to check from the what was it saying yeah we need to check basically from the base to the emitter three to five ohms so let's have a look to see where these resistors are the base resistors actually these two large ones i hook them up on the board you can see from the actual part number so um although the component number so there they are but they're a little bit difficult to get the emitter underneath these to actually read them if you look at the schematic you'll see the resistor actually comes down to one of these windings on the transformer there's two windings on the secondaries which i think we can assume is here and here yeah two pairs of windings so it's going to be easiest i think to measure from the end of the transformer winding to the emitter now it doesn't really matter which side of the winding because the resistance is so low anyway and even these resistors are very low so let's see if we can actually do that um so we'll put the emitter back on it's just going to sleep okay i'll put it back into the uh ohms range or even continuity okay so these are the two wines i can see them on the transformer so if i go for them yeah i'm gonna have to stand this up on its side so i can actually get to the emitters as well i can't get to them from this side of the pcb which means basically you can't really see what i'm doing but let's have a go anyway oh actually i did find a way to test it from this side so one of the emitter rails which is here you can see it goes to the positive speaker output on the back of the amplifier so that's easy to get to and the other emitter rail you can see goes to one of the pins on the relay contacts that's also easy to get to so let's now have it to see if we can find these i'm looking between three and five ohms so one of the transistors i'm not sure which so one of the emitted rails one of the banks goes to the positive of the speaker output so yeah it's that one 3.5 ohms it'll probably read the same from both sides yeah three three point five okay so depending which size you read two that's there and then the other one is in the relay contacts so let's have a look so we'll go to here and it's one of these contacts that one it doesn't anywhere else but it doesn't go anywhere else okay go to this one so again that's three point five and is it three from this end yeah very close three okay so that reads okay i'll now just make the same reading on the other channel um you can't see that's too heavy to move this thing around but i'm sure you'll know what i'm doing so positive speaker terminal well this channel actually is a bit higher 4.4 okay and once you read to the relay contacts let's have a look so we gotta go to the same relay contract three three by three so actually one of the emitted rails reads a little bit higher than the other three of them which might indicate a problem um it's not it's the first thing we found actually is anything different it's actually 4.4 rather than three but according to the information here if it's between three and five it's okay so i think now we can continue so once we've made these checks we can now power this up and let's see what it's actually doing i have the amplifier ready to test i've connected it to the light bulb current limiter with both bulbs in i've set it to all limits so now i'm going to switch it on and what should happen is these bulbs should come on and then go dim as the big capacitors charge up if they come on bright we know we've got some sort of short or some sort of problem here so yeah there we go so we come on brighter then we dim down okay so that's fine it's not going to go bang so now we can turn the amplifier on and let's see what happens with this well it made a little bump noise i'm sure the power's on but i don't have any speakers or anything attached to it so we can't really tell if it's working or not yeah so i'll now get some audio wheeze i'll get some speakers on this amplifier let's see if it actually works i've attached some speakers to the amplifier i'll just once again put it onto the current limiter in case there's some problem effectively assured i mean we didn't see any shorts but once the speakers are attached if some current can flow through the speakers so let's see what happens ah if you heard it was a little bump on the speakers yeah a little bump with a switch off so that's a good sign so let's connect an audio input now let's see if the amplifier is working okay [Music] okay guys [Music] copyright on this one just a demonstration any of the sugary fines [Music] i hope you guys enjoyed that video um i don't know how this is going to work out when i start making the video and in the end it was just that visual problem you know with the uh the resistor that the wire fell off however regardless of what the problem was i do hope you've learned quite a bit about amplifier repair and now feel confident to get started if you've not done this sort of work before or a little bit more confident if you had a little bit of experience and maybe felt a little bit overwhelmed sometimes by amplifying repair so that's it for this one um i could ask you all to hit subscribe if you like it but hey that's the maybe you can think begging people to subscribe isn't it it seems to me so i'm not gonna do that i'm just gonna say have fun and see you all soon guys on another working electronics repair video ciao for now

Channel: Learn Electronics Repair

Views: 278,258

Keywords: electronic repair, school, lessons, course, training, free, fault diagnosis, trouble shooting, troubleshooting, pcb repair, amplifier repair, dead amplifier, amplifier no power

Id: 0oWBcTTeMNQ

Length: 104min 26sec (6266 seconds)

Published: Fri May 06 2022