How To Diagnose Faults In Transistor Circuits - A Practical Example Samson TXM16 1000W Powered Mixer

hi guys welcome to Ryan electronics repair this is where I fix stuff and hopefully you learn how to do it yeah so I have here A powered mixer I've had a few of these on the channel before this one is a Samsung TX M16 stereo powered mixer 1000 watt so we have quite a large stereo amplifier in the bottom of this and quite a large mixer panel the problem with this one apparently is that the protect wise comes on and stays on so we have protect whites here there's two of them I don't know if it is on one channel or both but I guess now is the time to find out so I'll just get a power lead and let's see what happens I have this ready to power up I'm not going to put it on my light bulb current limited because obviously the owner has already powered this off anyway so it doesn't go bang I've noticed that the amplifier mode here is set to bridge so that might be something to do with why the protect wires is coming on let's just switch it on and see what we get well we don't seem to get anything just there's a switch on the back let me try again okay so both protect lights are on and do they stay on yes okay let's just set it into main right main left so that's in the stereo mode Let's see if the protect lights come on now yes and both of them are on which suggest there's a problem with both amplifier channels okay I've noticed this is Bent by the way can you see it huh when I'm turning it round you'll see uh maybe I'll just try straighten that up okay but I doubt that's anything to do with the problem this Channel's actually all turned down anyway so I'll open it up then I think I will disconnect the mixer from the amplifiers to see where the the problem is due to some voltage being sent in a DC offset voltage from the mixer because that would cause a protect fault whether it's a problem with the amplifiers themselves and as much as the output transistors or something as a problem or whether the protect circuit just thinks there's a problem like a phantom problem that there isn't actually there the last time I had something like this it was a broken track or burnt track hidden underneath the load of resistors at the back of the amplifier module I don't quite recall if it was the same brand but let's have a look I've removed plenty of screws and I've now got the chopped off this so let's have a look what we have okay we can actually balance this back here that's unusual normally the cables are not long enough so that's actually rather handy so we can see we have two cables coming down here from the mix it to the amplifier with quite a few connections on and we have another one coming down here which looks like it might go to a little voltage regulator so that's what we have between the two units so these will no doubt have some Supply voltage rails on and they will have the audio signals coming in from the mixer so I think the first thing I'll do before anything else is just to power this up again have a quick look at some voltages around it I'll just try and find something to wedge behind this so it doesn't just hit my monitor behind it once I've measured some voltages I'll probably end up just disconnecting this I've got just the amplifier units if that's where the problem is which it most likely is and then we can work on this amplifier I'm just having a quick look there's a views here which is intact there are two more fuses here which look like they are intact uh the two fuses this side I'm not sure if they are intact or not we can have a look they might be blowed actually and that's all I can see in the way it fuses in it okay I have it ready to go let's take some voltage measurements so the first thing we need to do is to find a zero volts points yeah the zero volts so this is a a ground pin down here in the middle let's just go to resistance range is that connected to chassis yeah it is so the chassis is probably oh is it exactly the shot so let's get a good connection yeah it is the chassis is probably ground there's a black wire here as well that also connected to it no so we can't say that wire is actually ground let's go back onto the chassis again other s point that's a naught volts so we can stick the probe in there okay it's a good place to leave that let's have a look then so we have some fuses I'll just check the fuses first so there's a fuse here which is good tighten that up a little bit Trimble fuses here that's good that's good these are marches uh two amp fuses this is a one amp fuse okay and then there were two six amp fuses here so these are probably powering the main amplifier because of the highest amperage views that's okay and that means okay so the fuse is really okay so the next thing there will be to measure some voltages just pop that into there we definitely have a good ground um I'll measure the voltage on the fuses first so normally the fuses will be connected from the AC of the Transformer going into the bridge rectifiers I can see the bridge rectifiers are actually under these two heat sinks so I've put the meter onto AC volts I'll just switch it on funds everybody I'll just check if it's still got the project fault yeah both project lights are still on so what have we got on these fuses 88 volts 88 volts 11 volts 18 volts 18 volts in this one this has nothing on it but this could be I might just try again this could be on the DC voltage rail so let's just go to DC volts see if we have anything on this one yes 90 volts DC okay I don't see another fuse right so we can say a few things already the two fuses with the 88 volts ac there with the powers I said to the main amplifier yeah the two with about 18 volts on here these will be generating a 15 volts Supply which will supply the mixer that's normal and it'll be plus and minus 16 and these will generate a plus minus 88 or 90 yeah the one in the middle which has the 12 volts on roughly will be 39 9 volts or 12 volt Supply could simply be for the fans this looks like a little Bridge rectifier in front of that fuse that was like another little Bridge reactiv diet possibly oh yeah it's just D byte for diode so probably are these are the large Bridge rectifiers there's two of them it's possible that they're both actually on the 90 volt rails maybe or it may just be that one is on the 15 and one's on the 90. I don't think so because there's one here down by these fuses so it might be that that's the evacuate other positive and negative rails um just seeing if these heat sinks are touching each other they don't look like they are quite I'm not sure this should be well there's no there's no connection between them anyway I'm sure we haven't got any short circuits because no fuses have blown otherwise you'd expect a fuse to buy them so the next thing to be will be to find the DC voltages the plus and minus 15. the 12 volts I guess okay so probably if you're going to be a plus of minus 15 it will show up on the cables going to the mixer section yeah somewhere in there we'd expect to find the person minus 15 plus a minus 12 maybe something like that the person minus 90 will appear on the main output transistors so let's have a look to see if we can find the voltage rails they're not marked on here okay the Box on here no not much on here either but this looks like it's probably supplying voltage valves up into here and then from here into here to the Nexus so I'm guessing we can we could look on the power supply here we can look here we can look here whichever is easiest to get to uh let's try it can we get yeah I can go to this one easily enough okay let's look for the voltage rails so weave the black within the ground a bit old again what we go well with each other volts range okay we've got looking for plus or minus 50 not thereabouts nothing on that one nothing on that one nothing and I'll make sure we can actually go out oh yeah the bad connection on the ground okay minus 14 there I think we can use the metal chassis ground actually get a connection onto it yeah we can't let's have a week again start from this end nothing five volts so there's a five volt Supply that sounds fine for logic circuitry I think it's a digital section of this mixer nothing minus almost nothing get that actually get at it that's the one's almost nothing plus 24 minus 14.6 nothing plus 14.95 that's important minus 15s and nothing so we've got plus or minus 15 we've got a 24 volt and we've got a five volt so they all sound reasonable now let's have a look on these transistors in the middle pins so these should be the collector and either one side is going to be the positive half and one the negative half or the front part of the positive and the back off the negative let's have a look 45 . so this is plus 45 there's a difference plus 45 towards the back so this half looks like it's a positive Supply it has a 45 volt Supply so you'd expect this side to all be the negative minus 90. well this tells us straight away there's something wrong yeah so on this side the main Supply is 90 volts but on this side it's 45 volts okay let's switch it off and let's think about that for a little bit it's likely and this is probably explains why there's two Bridge rectifiers down here now probably a class G or the class H amplifier and I'll explain to you what that is that also sort of explains why we have these big transistors which are different from the others you see that that one and this one and then these three are different yeah and then we have that one and this one and those sets of three are different yeah and then we have some sort of driver devices okay let me just explain to you what I think we have here and then we can check some more let me explain how we can have a minus 90 volts and plus 45 volt Supply on our amplifier because I think you'll agree that means something is wrong but let me explain to you quickly how these amplifiers work and then I'm sure it'll be a lot clearer I did a whole video by the way all you need to know about amplifiers to fix stuff and I've explained this in detail so I will link that video here and then if you want to know more you can watch it but basically most amplifiers and class bail Class A B amplifiers Class B doesn't mean they're not as good as Class A by the way these Class A and Class B and so on CD and more are different types of amplifier or if you like the principle the topology the layout of the amplifier so many amplifiers are class B and the way this works you have an npn transistor connected to a positive voltage Supply the base has been driven by the audio waveform and on the emitter we have a low value resistor these are power resistors several Watts five Watts 10 watts more depending on the the circuit and they're typically about 0.22 ohms I'll use all phones that's common so that's just though that you'll see it but we could put ohms yeah and then we have from here another resistor going to a p and p transistor and this is connected to a negative power available positive negative okay the speaker connects to the junction of these two resistors and this connects to ground so that is a typical amplifier circuit there will be some more resistors in here to set the bias of the transistors now just draw the next plane basically what they do so we have resistance like this and then we can have some capacitors here and then this point is where you send the signal into the amplifier this is and comes from a pre-driver stage so this is the output stage you'll have several stages in front of it to gradually increase the amplitude of the signal and the power but that's where it comes and the input signal is other side is connected to ground the reason for these capacities is to not affect the DC voltage on the transistor bases okay now for an npn transistor to switch on you need to put voltage on the base but transistors being transistors these are bipolar transistors you have to put about 0.6 volts on the base before it will switch on and this is positive yeah with respect to the emitter so if you were to measure from the emitter to the base these resistors are chosen so that you get 0.6 volts okay didn't leave much space did I for PMP trying to just switch on it's the opposite the bass needs to be negative with respect to the emitter so if you imagine from the emitter to the base on this side you will find 0.6 volts negative and those voltages are created by the ratio of these resistors let's put some voltage suppliers on here as well so let's say we've got a minus 45 volts yeah and we've got a plus 45 volts okay now the way the amplifier works is at the input you have an AC waveform this is your music okay this is the center point this is zero volts so the waveform is going positive and negative with respect to zero and this capacity to what I'm the best one for that matter will add the waveform voltage to the bias voltage this 0.6 volts is called the bias voltage okay so on the positive half cycle of the waveform the voltage on both bases will increase they'll both go up and the amount they go up will depend on the amplitude of the waveform on your npn transistor as I said you need to have more than 0.64 volt positive on the base compared to the emitter to make it conduct so as this waveform starts to increase the voltage here starts to go up and that turns the transistor on the transistor starts to conduct and the current flows into the speaker and down to ground so the speaker cone Moves In One Direction let's say it goes outwards okay the reason the current goes through the speaker into ground and not down through this transistor is because when the voltage on the base here increases this switches on but the voltage here also increases by the same amount so it becomes less than minus 0.6 yeah if you think about it adding any positive voltage to a minus will make it less minus yeah until it crosses zero then it also becomes positive so this transistor which was almost switched on switches off and the higher the voltage goes the more it switches off at the same time the more this switch is on so as it switches on more we get more current a few hours speaker might not be strawberry to go out yeah and that's why the current goes through the speaker gradually the waveform comes back down to zero at which point this has just 6.6 of a volts on the base it switches off the waveform now goes negative so on both bases this one goes minus and this one goes minus to go more negative so this one now switches more off it's off anyway it can't really switch more off but it's certainly off yeah it's more off if you like this one starts to switch on so what happens now because this is a negative Supply the the current flows through the ground which is more positive than the supply if you want to think about it's flowing from positive down through this transistor to the negative voltage value and that pulls the speaker currently all the way okay the more it switches on towards the peak of the waveform the more coverage flows this way yeah down into your speaker and the more the speaker moves this way and then grabbing the waveform comes back to zero again so the speaker comes back and as it crosses zero this transistor switch is on off so your speaker cone continually moves like that okay it's a couple things you need to realize about this circuit but if the circuit is working properly there's no Faults in the circuit and there's no input so basically this is naught volts there's nothing coming in to switch either if the transistors on the voltage here will be very close to naught volts but ideally it would be zero it'll actually be extremely close to zero okay because nothing's ever perfectly balanced but full Intense or purpose when there's no input voltage there's no output voltage the speaker cone is sitting in the middle position and it's it's not being driven either way there's no audio coming out okay at this point both transistors are switched off so one important thing to realize with an amplifier output like this is if you've got no input the output will be zero there'll be zero volts across your speaker because this ground is also zero okay so that's how it works now let's have a little think about a couple of issues with this not really issues but let's just think about how it works a bit more what's the maximum power we can get out of the amplifier well the maximum power depends on the maximum currents that will flow through the speaker and your speaker has an impedance let's say eight ohms yeah it's an eight ohm speaker okay so if we're given voltage and what's the maximum voltage that could ever appear on this end of the speaker well it's either plus 45 if this transistor is fully on or it's minus 45 when this transistor is fully on so the highest voltage that can ever appear because our speaker is 45 volts at the peak yeah in fact because the waveform is continually moving the average or RMS root means Square we can consider it to be the average it's going to be less than that okay well we could have a maximum of 45 volts across here and the resistance of our speaker is eight ohms so how much current can flow in it well we can use ohms or it's quite easy huh V look at the triangle i r yeah okay we want to know the comment current equals V over R cover it with your finger I equals V over R so the maximum current that can flow through our speaker is 45 divided by 8 which equals quick calculator 45 divided by eight five point six two five amps yeah that's the maximum current that can ever throw through our speaker what's the wattage that we can get out of our speaker well what equals V times I so 45 times the current just clear that 45 times 5.25 236 Watts 236 Watts that is the maximum wattage we can get out of our speaker what if we want more wattage out of the speaker well there's two ways and there's only two ways you can do that really with this circuit one is to decrease the resistance of the speaker so we can put a 4 ohm speaker okay this formula that was 45 over 4. huh which let's just call it your weapons shall we run the 11 point something it's the 11 amps yeah and the wattage is 45 times 11. 495 I can do that let me add so by decreasing the resistance of the speaker we can get more wattage out of it because you're getting more common through it well if we want even more currents well we can't keep reducing this indefinitely but what we can do is increase the voltage so if we put More Voltage in that in this formula V is more so we get more wattage in fact in this formula V is more so we get more current yeah let's say we put 90 volts in I put some minus 90. we'll go back to this formula we'll just use this one obviously they both apply yeah so now the current is 90 over 4. uh 90 divided by four it's 22 and a half amps yeah so the coverage is 22.5 so the Watts is 90. times 22.5 yeah shall we do this 90. times 22.5 two kilowatts yeah that's what I watch isn't it but words consider something else okay these transistors who you think the transistor self is dissipating the most power when it is not conducting so effectively this end is 45 volts and this end is zero yeah it's not conducted is it dissipating the most wattage there or was it dissipating the most wattage whether it's fully turned on so it's passing the maximum current of that example we used 45 volts yet every 11 amps so is it dissipating the most wattage when it's passing no coverage or 11 amps well the answer is basically it's dissipating the same wattage at both points and the reason there's this what's as we know equals voltage times amps yeah when the transistor is off we've got 45 volts on one end and zero on the other so the voltage across this transistor is 45. what's the current passing through which zero basically uh his name is damage so what's the wattage 45 times 0 equals zero what's there's no comment for me there's no wattage yeah well obviously it's switched for me on well now the transistor is basically like a short circuit yeah so the voltage across the transistor is near as damage is zero and the current now flowing through I transistor is the 11 amps [Music] zero times eleven what's that equal well it still equals zero yeah so our transistor when it's fully conducting or fully Switched Off is not dissipating any power but what happens when it's half on and half off yeah so effectively you've got 45 volts on this end you've got about 22 and a half volts on this end yeah what happens now let's have a look at it well the speaker now that's 22 and a half volts acrosses okay so we need again to find out the current V over R so we now have 22.5 yeah these Essence is still the same four equals well it equals half of that yeah which is about 5.5 amps yeah what's the wattage now well what's equals volts times current we know there's 22.5 volts we know the coverage is 5.5 so 22 and a half times five and a half is the answer 22.5 times 5.5 the speaker now is dissipated 123 Watts where's the rest of the wattage well it's been dissipated by the transistor because the transistor also has 22 and a half volts across it it's also passing five and a half amps it's the same columns so the transistor now it was also dissipated 123 Watts so the transistor gets hot and that's why this type of amplifier in these big heat sinks what happens now if we increase the voltage to 90 to get more power well this suddenly becomes a lot more than it was yeah it now becomes much more than it was because we know the total current was about two kilowatts this becomes about a thousand Watts you know so by increasing the voltage get more power we need to put massive heat sinks on the transistors we need to put bigger transistors yeah if you like them more of them but we still need to get more wattage so there's a way around this problem and that's this we can take here another transistor or a mosfet these could also be mosfets yeah very similar and we connected to a 90 volt Supply Plus huh and we do the same on the negative end so here we take another transistor to remind this 90 volt Supply so we've now got the plus or minus 90. yeah these two transistors then via some circuitry they're controlled by the input signal such that when the output waveform well that's just Droid volts 45 volts 90 volts yeah I just drill it okay 45 volts 90 volts okay when the output waveform is going to go above 45 because we can detect that we can set it's almost at 45 and it's going to go up above that we can turn this transistor on increase the voltage to 90. so it can continue to go past yeah and as it drops back down again below the 45 we could switch this back off and we can reduce the supply voltage so the waveform comes down through here and then again on the other side once it goes above minus 45 we can switch the transistors on in fact we switch them on both together you don't switch one on the other you'll switch them on yeah so again as it goes above that this higher voltage is available yeah maybe we can switch it back off again so that's what's happening thank you without amplifier that's why our amplifier has both the 45 volt and the 90 volt plus a minus Supply so that is what is happening with our amplifier our amplifier is 500 watts per Channel we need more than the 236 Watts with the plus and minus 45 volts Supply because instead of having a plus of minus 90 volts Supply permanently which generates a lot of heat it uses a switching method okay and this is called a class G amplifier okay and that's what we have plus G amplifier now there's something wrong with ours on the positive Supply we have 45 volts but on the negative Supply we have minus 90. so there's something clearly wrong if the problem was a short circuit transistor here huh then you would expect probably that only one channel of the stereo amplifier would be faulty because this is one channel we have exactly the same for left and right channels and ours has a full-time both channels so unless we've got short circuit transistors here on both amplifier channels which you can't have it's unlikely that's the problem there could be something wrong in this drive circuitry so that this transistor has been switched on permanently it's on and stays on but again you'd have the same problem on both channels and we have a problem that appears to fact both channels yeah and having two thoughts the same on each channel is a little bit odd yeah unless our by chance circuitry effectively switches the power on both channels between 45 and 90. but that seems a bit strange to me why would you do that if one maybe one channel hasn't been driven yeah it was only got the quiet bits on I don't know but it seems wrong to me so he might be we've got something wrong with the power supply or there's something wrong with this switching but that basically is why I can see a push 45 and a minus 90. and this is why the protection lights are on because the protection lights basically the protection circuit monitors the voltage here and the voltage here should be around about zero yeah it shouldn't be off One Direction or another because if it's airs it puts a DC voltage across the speaker which will burn out the speaker coil the speaker is an inductor it's a coil and when you're passing an AC signal through which it does reactants it has inductance yeah but when you're passing DC through it is much more like a short it will still read a resistance but basically a DC voltage here will burn the speaker out so your protection circuit basically monitors these voltages to make sure they balance the monitors the voltage here to make sure it's around zero when there's no input so that is the theory and I think there's enough there for you to understand roughly where we need to have Amplified to see what the problem is so the next thing I'm going to do is try to identify all the voltages Plus 90 plus 45 minus 45 minus 90. to measure the voltage here and obviously check these transistors and see if I can see where the problem is okay let's test resistances first let's have a look around this so it looks like or they haven't checked that these three transistors are probably the output positive side for one of the amplifier channels and these are the negative we know this is all positive voltage so this is almost certainly one amplifier channel that we can see here and then we have another set of three another set of three three of these three of these it looks kind of symmetrical yeah so this I would say is the other amplifier Channel there were two of these larger transistors almost fits here and there are also two of them there so I'm guessing these are the switching between the two voltage supplies and then we have some sort of driver transistors there to there's three on this side actually it's slightly different okay and then we have two there we have three there okay so this is all kind of symmetrical if you like let's have a look what we've got so first of all these are probably doing the switch I'm doing it for shorts there's no short I'm on continuity no short here this might be a mosfet I'll just get the part number one moment it's irfp 9140 and that sounds like a mosfet let's have a quick look yes it is a mosfet so we can just say that our diagram when I drew it was as I said towards B transistors or mosfet so I'll just show you that here we have it so in our case this is actually not a transistor it's actually a mosfet but the principle is the same i r f P nine one four zero n and I've just checked for the ones on the other side because you'd expect these to be p-channel mosfets because these are PMP Transit isn't in fact they are this one is i r f p one five zero n and that is a p Channel mosfet so that's it these are transistors by the way um these are quite common types these are C sorry these are c5240 and the other ones will be the VMP ones a1962 okay so we basically know what we have there on our amplifier we have one of these for the positive and one for the negative on each channel so there's four in total and these is actually three in parallel yeah to give more comments so we've got three four watts of three in parallel and we can see now yeah three and three one of each yeah three is three one of each so that's what we have there's no shorts around here let's have a look a bit further because this is the side that was really strange [Music] move my phone one moment no shorts there meters working no shorts there so there's no problem around those probe's a little bit loose um okay so there's no shorts now let's look on the actual output now so these this is the base okay there's no shorts I don't want to diode mode okay there's no short there so there appears to be no shorts on the output stages okay but the POS this side seems to be the ones working correctly yeah I would say what's that voice yeah they're reading the same so this side MPN no short okay now that's the uh switching transistor okay keep going so there's no short circuits But there again you would expect that because that would be a difference no it was the same 2000 2000 I'm just going uh from Collective three meta ah something reads odd on this one yeah so I'm going negative to The Collector to compare with this I'll have to go to positive to The Collector okay so I'm reading something unusual around that set so on this side here they all basically if we check collected we made it reach quite High especially with the Positive The Collector which is the way it would normally be in circuit because this is the positive Supply and these all read sorry that one that one basically the same on both channels yeah they'll be all basically the same okay on this side this is a negative end so the negative Blackpool will go to The Collector and that means like a diode yeah that means like a diode same on that one and on this lot these read High the same as those do so there's a problem on this Channel somewhere that's causing these readings and it's not as short it means like a diode let me just say very minded complimentary so I'm having to reverse the priority of the polls to make a comparison okay they will be the same so the problem seems to be around here we'll have to take the amplifier board out and have a look to see where the difference is just before I do take this apart I'll just show you because we know it does effect and we switch on without anything drastic happen I just want to check and show you that all the voltage rails we've just been discussed are actually present on this so we'll just switch it on and this should all come in on here so this is the power coming into the main amplifier so if we just go on to the chassis which is ground we should have what we expect so that's 91 volts positive okay that's a 90 volt Supply that's a 45 volt positive so we have both of those that's ground basically that's ground basically that's the 45 volt negative and that's the 91 volt negative so we know that the way we were looking at this circuit is basically correct and we have the two sets of positive and negative 45 and 91 volts it's sometime later when these have arrived in the post so we can probably fix this with a better look I've had a something of a cold the L for four or five days so my voice isn't backed properly yet so you can just have a laugh at it or you could even get a comment and say hey let's get well soon yeah I must respect most other just be laughing huh that's the way it is so I've ordered some transistors these came from France I could get them from AliExpress but they were going to take a long time and these actually came as a pair so you get the the C for three seven zero which is the npn and the a1659 and there's two pairs now on the board here that's what we actually have so these are the ones I remove the faulty ones and these the C four three seven zero which is what I have here so it might be and it probably should be really that these transistors are like matched pairs if they've been sold like this so what's up I'm by matched I mean there should be with similar again and what I think I will do is remove the npm transistors as well the c2sc's and I'll change them I'll fit the pairs just good practice really I think um but I'm interested to see whether or not these transistors are basically the same sort of gain and I can also compare them with the ones I took out I can still find them what's one four five that's one of the npn most everything still find them I mean I can't actually because it was short yeah we catching up with where I was so I can compare them with the c to SC transistors at any so this is the other one the npn they're obviously better transistor analyzers than this if you're doing a lot of Audio Repair is probably worth getting one but I think for what I do they matter to this is probably okay unless somebody tells me otherwise yeah that's very similar one four four one four five they match the uh PMP I'm thinking probably should but I mean I don't know if that is actually going to be the case let's have a look PMP foreign [Music] [Music] actually no these are probably the drivers for the output stage actually and because the shows effective that's why the voltage is wrong because they circuit that detects where they we did higher voltage you know for the size of the waveform the short effect we just drilled that circuitry so that's how we press the wrong button I'm not 100 guys I'm only varying on three out of four cylinders yeah 173. so we can see the two pmps are basically the same as each other the two ampns are basically the same as each other so I will replace the two shorted ones I'll replace the other two as well as a matter of course unless if this actually now works I've replaced the transistor pairs then just out of Interest by the legs I've just used this I bought a row of this mechanic solder from AliExpress 0.6 millimeter this is a 150 gram roll and this is another lead-free solder so this one is in fact 37 lead 63 and 10 so it's not quite a 60 40 but it seems to work nice yeah it was nice clean soldering on there so these the two I took out the uh npns which are actually good I'm just interested to see how the gain of these compares to the two that I fitted which I think were only about 147 yeah it's a bit higher actually funny if that's closer to the uh PMP possibly a better match okay yeah they're actually much better to the PMP ones than the ones he sent with them I've changed them now we can always put them back in so this is the result of the soldering nice joints quite happy with that so if you're looking for some uh solder yeah this is best I've tried so far from AliExpress I've just put the screws in just a loose just to make sure there's all the transistors will actually fit because if you've unsolder them and resolded them or you've changed some it it may well be that where the legs have bent you've got the length of them wrong when they don't line up so I've got them all in as I say Loosely and they do fit around this because there's a couple of adjustments one here and one here and they may be something to do with balancing the channels where we have to we're not really sure or how we would do so I don't have any information on this but if anything I think I'd probably adjust this so that the output voltage when there's no signal is here I mean somebody else probably knows better than me or not but first of all let's see what happens the other obvious thing is this capacitor is gone and if we look on the all the channel the same capacity was gone whether this is related to the blown transistors I don't know but what I have noted is that there's a pair here and the capacity is between I'm gonna repair here in the capacity between them and this may again be something to do with the driver or pre-driver stage so clearly we have to change these two capacitors as well all the others look visibly okay doesn't necessarily mean they are okay but for now I think I will just change the two that are bad and then let's see if the actual protect mode has gone off I've just put the screws in Loosely for now but they all just in place so there's a couple of precautions we should take with this before we Power It Up really one is that these transistors with the metal tabs which are connected to the center pin so the thing on bleep yeah you can't even bleeping yeah this shouldn't be connected to the heatsinkle always only one side should be if you see what that means so I just want to make sure that you know this is isolated this is isolated I do have a connection to the heatsink you can need the bleep so I just want to make sure that they're not short intuits do have a insulation washers under this or maybe it's just the design of these they're not yeah yeah the tab is collected connected so you know there's no short there yeah we need to make sure all of them just to make sure that we're not connecting these are just Plastics I don't think they would connect anyway but no I've been trying okay same on this side because if you have a short for many of these to the heatsink it's gonna go bang or it would go Bank if we weren't taking the other precaution which we're going to take so the other one is that we're going to power this up by the light bulb okay so we can test it now at least we can test it safely in the case with this amplifier the heatsink is actually bolted directly to the chassis so effectively the heat zinc is ground zero volts all the metal tabs on these transistors will be at minus 45 or 90 volts depending on how it's switching is and all the metal tabs here will be on plus 45 all 90 volts so this is why to be sure we don't have any short circuits from the tab to the heatsink and that'd be because the little marker washer underneath Has Come away or it's not messing usually it's like a little see-through washer don't lose them let's say wash it they kind of like square with a hole in yeah I'll show you one yeah these sort of things the varying size these are probably for this type of transistor so usually embedded in the heatsink compound is one of these don't lose them yeah and make sure they haven't got a stray sometimes you may find this sort of thing where the actual heat sink is insulated from the actual metal okay so it doesn't touch the metal case anywhere and it has like some sort of plastic mounts to hold it away from it so it's insulated and in that case sometimes you may find that there's only Mica washers on either the positive or the negative side to insulate the transistors from the heatsink in that case the metal chassis ground the heat sink will be at plus or minus 45 or 90 but it's insulated from the case and depending on which set of transistors has no Mica washer under it heatsink impossible miners whatever the voltage is so that is actually quite a normal sometimes for one side to be connected but not both okay so just bear that in mind it varies a little bit on the design of the amplifier so this is the white ball that switched to limit so the power's coming through the light bulb just double check that yeah it is so I'll switch this on now I've got no input connected to it no speakers connecting to it I just want to see what it does when I power it up these are the protect lights the two of them here and the Power white so let's see what it actually does well it went on bright they went damn I'm not happy with that I seems to be drawing quite a bit of power rather it may just be going to dimmer and Jimmy yeah it's on but protect light is on okay obviously the capacitors have charged up or something like that so it's not got a short in it but the protect lights are still on it may just be the current limit of making it do that but let's have another go at sea yeah bright and then dim protect lights are on okay it didn't go bang or anything like that previously with the protect light so let's just try and see what he does protect lights are on so that's without the light bulb in the circuit so once again I think we need to have a look at the voltages around this amplifier because there's obviously something else still wrong with it well that's looked at the main voltages we have in this so I'll just Power It Off fans of money what have we got now so this is plus 45 and this is minus 45 so that's correct now 45 for this channel well let's see if we can figure out what's wrong with this still so we'll switch it back on again directly on the mains because nothing bad happens basically it's just untangled my meter wiser moment okay so from ground let's check the power rails so we have this should be the Porsche 45 yeah oops slept don't go sleeping plus 45 that's okay what's on here plus 45 let's look at the negative mail is this already 90. no it's reading minus 45 how about the other channel minus 45 so the power rails are balanced now so the other reason a chat white would come on mainly unless there's some problem the protection circuits is if the output is wrong so with no audio input coming into it no input coming in yeah the output should be basically zero so the output is coming through these low value resistors off the emitters here and here so let's have a look so that's right yeah point was it should all be the same because you're all connected together 0.165 165 so that's near to zero if we measure the other end they reached practically the same so there's basically no current flowing through there or very well what about the other channel so this should be the same oh no this is 21 volts and that'd be on both sides okay so you decide to resist it 21.4 21.4 so there's basically no comment flowing that's because it's in protect mode so the relays are open and there's no speakers they are oid of the protect mode there's a stop that voltage going straight through your age ohm speaker to grounds cause number of current to flow possibly destroying the speaker damaging the amplifier more so some things making this side turn on we can see this on the emitters so the emitter is connected to the resistor yeah we can go the emitter we can go to the he met it it's on the base eight let's look at the other channel zero so you can see clearly that something's putting a voltage on the base here eight volts limit is 21. so the base is actually below the emitter okay it's one of these ones 20 volts zero volts so zero volts on the base here zero volts on the base here twenty point one something emissive 21.3 20.4 so those are turned on I would say because the bass is a little bit away with the new meter and these are PMP transistors I think so remember um BC is seven point oh I've got a bad area let me get a good connection like eight volts 21. so the base is below the emitter and these are npn transistors so they are off this side 20. Bass 21.3 on the emitter so the base is below the emit well that's the negative rail there's something wrong anyway in this channel somewhere there's something wrong here he's out it's like it's not balanced the transistor is being switched on by something further back obviously a schematic would help but we know there's a problem in there now basically I'll just switch it off a little while we can talk about it we have two effectively identical channels and yet there's something wrong with this amplifier yeah compared to this amplifier so obviously schematic would be useful but I don't have one maybe I can get one let me switch the other camera off yeah get rid of my drop or go and get them over there let's see if anything's getting warm maybe a little bit of warmth somewhere in here I think food ease of repair Maybe the first thing I'll do is try the thermal camera so let's get the thermal camera and let's have a look to see if we can spot something on this amplifier Channel that's getting hot that isn't on this one and hope we can I'll show you what I do see then so we have this group of components here with like two power transistors and the whole bunch of them and the pattern of these is basically the same as this pattern the same number of transistors the same number of power transistors so if we look here you can probably see there's like a whole bunch of components there and they're all pretty warm and you can see the transistors there's two to this side which is a bit cooler than the others but that's basically what you can see there yeah and then one of the power transistors is effectively hot and the other one is cold by the way zombies yeah what it is but it's uh one hot one another one's there it's like purple he's not lighting up at all okay it's kind of here if you look at this channel we have a definite pattern so the transistors if you look at them you see what's warm on that one compared to what's wore on this one the pattern is very different she has that little cluster that's that little cluster and then on this one the two large power transistors both of them are cold yeah I think you can see one's actually here and one's actually here so there's something different in that little clump of components so they're strange thing gear really is this channel appears to have the wrong voltages on it yeah it does have the wrong voltages on this but they're both those power transistors the effects we hear they're probably a bit hard to see but here on the screen here and here cold yeah if we go to this one this channel appears to be working but one of those power transistors is like 68 degrees and the other one there's about 31. compared with 29. compared to theirs 27 so in actual fact this appears to have a problem look at 70 degrees now that that can't be no yeah 71 degrees so let's take this out and have a look this is the transit that was getting hot let's just see if we can remove it we might have to add some leaded sold it actually yeah put up a little bit get a little headed solder on it but I wouldn't be surprised to find this transistor actually okay and there's something around you or something else and what also it does puzzle me guys is why this one is getting really looks like too hot to get this channel the output's really okay well they're not okay but the there's no offset DC offset a little Club here okay yeah there's no DC offset on the output and the other channel has a big DC offset but those transistors are not getting hot which makes me think I may have different faults on each channel here in actual facts so let's have a look this is another one of these a 1659 transistors I actually have one more that's all I have but we can always use the analyzer uh let's just have a quick week I wouldn't be surprised this is okay so it's a PNP so negative to the base that doesn't surprise me we've got the analyzer to be sure but that looks good to me okay PMP so you can transistor gains as good as the other ones that's fine and I did say I thought that would actually be working so that isn't faulty these things were not exactly symmetrical if you look at it there are some differences it's not like there's two symmetrical channels here exactly I think now I really will try to find a schematic for this to see if that will help us to fix it well guys obviously that transistor getting hot isn't actually faulty but there's something else wrong with this but there's a few things about this is don't make sense to me I'm going to measure another voltage in particular just to be sure that I'm correct and then let's put this on a piece of paper but this whole problem is saying two different things to me at least let's just make a few more measurements follow with me and then we'll have a good chat about what's going on or what might be going on there's something that really doesn't make sense about this to me I'm going to show you I've put this on paper as well so I can show you the image on the paper but just bear with me right okay so we'll go on this channel first okay base is zero collector plus 45 emitter well yeah almost zero base zero collector minus 45 emitter well yeah almost zero this amplifier channel was to protect whites because both lights are on which tells me there's a problem on both channels unless there's some problem in the detect circuitry itself and it is Phantom thoughts but that's what I'm saying yeah bear with me on an amplifier and it's quiescent like this you shouldn't have zero on the base you should have like a little bit less than plus 0.6 with comparison to the e-meter on here and more or less close to minus 0.6 on the base with respect to the emitter here because the transistor is supposed to be almost switched on that's the first thing so there's something wrong with this we know this transistor here is getting hot but it's not faulty there's definitely something wrong on this channel now look at the other one unless he's even stranger npn transistor base is eight volts 45 volt Supply positive emitter is 21. now the bass at eight is lower than the emitter which is 21. it's like 13 volts lower than the emitter so this transistor should be switched off and yet for a positive voltage to get through to the emitter it says it's not just a switched on but it can't be switched on yeah look at the PMP ones base let's go into a good connection 20.3 yeah emitter 21.25 that has more than 0.7 volts that's more than one volt I think about one volt across the base emitter and negative it shouldn't be able to support one volt that would say the base emitter is open circuit but they're not because I looked at them yeah they're not right this is the weird thing if I've seen 7.8 980 and I'm seeing 21 volts here what will be the voltage if I measure from the base to the emitter that was 21 minus eight yeah 13. do you want to know what's here look you met it bass nothing there is no voltage difference between the base emitter ah but if we go to ground that was on ground I mean there's lots of grounds on here I can go to a ground on here another ground if I measure them eight 21 so it's reading different so there's a voltage difference but isn't a voltage difference well there isn't is there you just proved it no so that transistor's turned off so where's the 21 volts coming from look on the PMP side yeah same sort of thing go to ground it's the eighth pinup on the axis ground one two three four five at you out hmm what's happening and down to the way the meter go on this one 21 20.4 yeah what's the voltage across the base emitter well the base is about one volts less than the emitter yeah base to the wires out the fans 0.2 so that doesn't make any sense I mean how can we have completely different voltages with reference to zero volts but when you measure across the two points the you know the same voltage so that's why there's something wrong with my test mate well we can soon prove that can't we here's the uh little an end meter with a different set of leads yeah let's have a look volts let's see what it says please just put it somewhere where we can see it uh you can see I think you can see that okay definitely meter different test leads power on I can use the same ground I can use the chassis grounds use the same one I can prove this is the same ground because if I measure the voltage from ground to here it's it's minus 45 and if I measure from shutty to the same place it's minus 45 yeah I'm looking for the chassis this time that we don't mind so what have we got on the base eight seven point eight eight Metro it's 21. what's the voltage between the base and the emitter zero that's a paradox yeah that's a paradox not again not fix it I can't even figure out how the voltages can measure like that but well the only real reason to that actually is that the emitter there's nowhere near to actually being on ground which it won't be because the railways are open so that immediately effectively is floating and I think that's what's causing the strange readings but it doesn't quite explain why the bass this is something to do with floating voltages I think which is not helping me to fix it yeah I did say I'd put this onto a piece of paper but I think it was clearly enough so the markings in the blue pen note on the npm base notes on the PMP base 0.165 point of that channel is surely those transitions should be almost biased on two these voltages well they don't make sense do they they don't make sense this transaction power is more on than it could be but when you measure across the emitter base it isn't same with that one something really seriously weird about this but I've got to fix it yeah well I've got a next step we can take guys I have a schematic and this is the next step so this is thanks to one of the guys on bad caps Forum I was asking if anybody's on the schematic and somebody posted me this one up it's under a different file name but I think it is actually the correct schematic it just says txm amp uh BD that's all it says so it's not exactly the same well the full model number but I think this is actually the correct amplifier because there was some interesting traits that make people correct first one is these connections the 14 way in the nine way which is what we have I was using uh this point to pin eight is ground when I was actually making those voltages that's how I actually knew where the ground was and what we're seeing basically if we look at the schematic it's quite similar to what I thought it was so this is your main amplifier output this is the npn side yeah npn transistors and these the PMP these are all those emitter resistors and there's three output devices in parallel one two three okay this bit of circuitry this is what switching between the 45 volts Rail and the 90 volt there okay pretty much as we described it's been switched by these transistors and they're picking up some sort of drive from the output so without me really going into exact way this Arrangement is kind of like a some sort of a comparator type circuit with transistors and it's effectively monitoring the voltage on here can you see that so this is the junction of all the emitter resistors and doctor this is your relay okay then this connects to your speaker output there and this other child to there okay and it's wide that you can have both speakers connected to one connector or you can have one on one and one on the other and that matches the way the speaker connectors actually wired up the speaker on sockets okay so this bit of circuit we can see is monitoring the voltage on here and it's switching this on in such a way that when the voltage on the output is getting towards the 45 volts and that's what I think it's doing is comparing 45 here with whatever voltage it sees here yeah and this effectively switches it's a bit like an up amp if you like made of transistors but I'm pretty sure that's what it does this circuit's got a name and I can't remember what it's called because I'm not a circuit Builder but I do basically know how it works and we have the same on the other side this is monitoring via a resistor Network the minus 45 is also monitoring the voltage here and when it goes negative gets close to 45 it switches this one and this puts the 90 volt rail here the transistors we had short I'll have a work but I'm guessing they're in this circuit let me have a quick look okay actually I'm wrong the transistors ran short with this one okay and this one and these are actually driving the output trousers on the negative side there okay and there that's where the fall was let's have a quick look how this thing actually works now I'm not really good for the whole thing so at this point you probably don't need to and we'll just look at one channel because other channel is the same apart from the fact it's got a different folds apparently so we have input somewhere here signal uh we have signal signal one sg1 not Stargate by the way so signal comes in Viber capacitor and it comes into this circus and this circuit yeah these two here and here base of both these transistors one's npm one's PMP so effectively as it goes positive waveform it turns this one more runs it goes negative turns this one more on I think the other pair is against some sort of comparative to do with feedback I'm not too bothered about them let's just follow the signal path in yeah and let's just follow the positive half cycle first so signal comes in positive off cycle so it is this transistor on which drives this transistor okay that then drives this transistor which is the pre-drive and these your output stages okay so the amplifier itself effectively positive off is that one that one this one and these four stages yeah of amplification all the rest of them are to do with compensation and feedback and stuff like that yeah same for the negative off cycle this one which drives this one which drives this one which is the driver the one that was short and the output status so again four yeah and exactly the same under the channel they're identical channels okay so that's the path of the amplification that's a signal path coming through and we can use that to trace so we could put a sine wave into the input and we can see where it goes okay all the rest of the transistors we see here as I say are either to do with feedback all day to do with monitoring the output okay and to do with switching the voltage rails it's a little bit of a circuit up here it's quite obvious because it says Phantom which is very Phantom this is the 90 volt positive and this is where we saw 90 volts DC on the fuse and if you look the 90 volt comes in here and this effectively is a voltage regulator so it's dropping the voltage through here and that voltage actually goes to the plus 48 that's the Phantom voltage for microphones condenser microphones so that's what that circuit's doing and what the fuse with the 90 volts DC is actually part of the Supply the Phantom 48 volts this little thing I think is just not there and this sort of thing is just not there there seems to be like a whole block yes and do not use them txm so those things are just not there whatever they do I don't care because they ain't there looks like some sort of clip sensing or something um so that's how it works guys what I think is that I mean this is just more supposition for me because all you probably just shout out is going to tell me what's wrong with this but if there's some sort of problem with this base Drive circuit here I mean we've changed that transistor because that was short but this is the complementary the positive one I actually changed I changed the pairs I changed it but I'm wondering if there's something wrong here so that this voltage rail here the base is actually floating there's floating all those are tired that shall hold it no it doesn't hold it to ground it goes down there if this is somehow floating that might expect well I get these weird voltage on the bass that don't really exist okay and the same on the negative half exactly the same on the negative off but I do think it's those transistors have changed the transistors the one that's getting hot on the other amplifier I'll just tell you which one this is I know the vampire Channel q118 where's that well it's here so the one that's getting hot is effectively in the drive circuit uh into here that also that feeds well I was another transistor down here if I wonder if some of these transistors might be short although I did effectively have a look around a long time ago I think what we should do with this now guys we could put a signal in and see how it goes but I think it might make sense to kind of start at the beginning and just measure a few voltages just following through the four stages of the amplifier and both half and see if we can see anything with the voltmeter otherwise that doesn't give us anything conclusive then we're going to start using the oscilloscope let's have a look at this floating rail that we seem to have so I'll just go on Resistance range that's from this emitter rail of this amplifier the one has zero volts on and we'll go on to this one do we see any difference well yeah that appears to be open and that has some resistance yeah so this is why it's floating and this is really you know tools of a MAG but it's actually dropping okay so there's a difference there so let's have a look on the schematic to see where something might be open circles effectively there seems to have an open circuit from the emitter resistors to ground and this is high resistant she was expected to be a high resistance okay but you expect to find something there so this well and effectively is the output from the amplifier all the emitter resistors that goes through the relay to the speaker and the relay is open at the moment this is the protection circuit basically so there's any paths to ground we can see well there isn't one here because there's a capacitor so from here to ground that capacity would really open not that way the switch on the relay is off so it can't be going that way so I'm going down here oh there he goes to a 23k oh and that goes to ground so we have two 222k resistors here from the output of this amplifier we have two more here from the output from the other amplifier which connect to this transistor and we have 22k to ground so yeah through there we should read 66k basically so it could be I'm reading through here and maybe the resistors on the other childhood actually open circuits as anywhere else well I'm using resistance modes I shouldn't be switching any semiconductor Junctions on so I don't see any path via these ways diodes transistors yeah I don't really see any obvious path to ground there so let's have a look at these resistors and see if maybe we've got an open circuit resistor either two eight four two eight five or one eight four one eight five and then we have resistor 22 that's common to both sorry resistor three that's coming to both so I can't see that being open it's going to be either this one or this one two eight four two eight five or it's going to be one eight four one eight five let's check that out I might be getting somewhere now guys so these are resistors 184 and 185 they're here and that's the transistor Q5 and the emitter is the one that goes to 20K resistor basically there's just really now he's just Auto ranging I'm sure it's going to go somewhere well obviously something else is in parallel where he's reading lower but you can check you can check so this is the emitter of Q5 even though we have the the resistors what eight four one eight five yeah the transistor connects to one of them which is 22k which reads all right which goes to the other ones I think is here yeah and then the other end is over here somewhere yeah so they read basically okay so I know that isn't the problem now I was trying to find the all the resistors two eight four and two eight five and I couldn't find them anywhere I should have here somewhere but I got distracted while I was looking so I couldn't see this I bet you could see this on the overhead camera because the angle I'm setting at I'll show you this because you'll have to believe me the angle I'm sitting at in my line of sides like that yeah what I can't see is what's behind this and for that matter I can't see what's behind these resistors yeah maybe I could have seen that problem and this is where obviously I failed in my visual examination because I couldn't see behind let's move these wires there yeah I bet you could see it before new event some of you probably even strapped things out but hey I found it yeah so this is um a resistor and it's open circuit yeah I think that's a bit open circuit that one guys what do you reckon foreign I could use the multimeter but I think that has open circuit that one there's r267 there's another markings let's have a look see what this resistor is a bit of a glue off here and this is the amplifier Channel that's got the weird thing going on with the voltages and something floating but it's definitely open now okay left this door what's it say very glued on that is guys it is a resistor marking or something let's get some more of it off well I've got the chewing gum so I put these set tubing up far enough out of the way to see this is r244 now you can see it that's r244 that's uh open circuit yeah then the transistorial that's connected to that wouldn't surprise we can see it there with the wind missing chunk missing q221 so let's have a look for r224 and q221 and our schematic and see if he's in any way related to that voltage that was floating okay so I'll just zoom in a little bit this is the resistor that has actually blown this one so this goes between the driver or pre-driver these it we'll call this the driver this is the driver these are the output devices so this is the npn driver via a diode via the resistor which is blown open circuit and then via the PMP driver so this end connects to the negative voltage rail like we've got nine zones original if you fix one problem this goes to the positive voltage Rail and the current is obviously flowing through this transistor through the diode through this resistor and blowing it and through this transistor now I've already changed the two transistors I just although the npm was tested okay on both on the channel I've changed them yeah I've changed them both so that I have done I'll need to test this diode 104148 I've got some I'm not sure I have a 68 ohm two watt resistor so that's that damage the other thing that blue is the transistor and the transistor is blown is this one okay so well this is the one that went short oh yeah go back to base was yours straight through the diode straight through the transistor to ground so you can pretty much say that going short blue this one that's fairly obvious okay I need to test that in this diode because the code went through the diode I needed to test the other diode so let me test those two diodes and let me see what this transistor is and do I actually have any I have the meter on diode mode okay so the two diodes D two one zero is here and that means okay actually yeah and two one two is here ah that one's short okay so that one went short two one two so we need to change that diode and this transistor well I think it's rather open circus is the best description of that uh but it has a part number on it so that's if I've ever got those haven't got the resistor I know I've got the diodes actually I made a slight error in that but it didn't really affect what I was saying or make it wrong it's the all the transistors that went short it wasn't this one it was this one so this is the npn driver transistor this is the PMP and on one of the channels this went completely short circuits all three terminals this is a transistor that blew the lid off on the same channel as when completely short this diode is actually reading short this diode reads okay this is blown up so short blown until it went down through this one to another diode and enter the transistor that was short so what I'm going to do now is I'm going to test this transistor the PMP so that was q221 this is 222 I'm going to test the diodes two one three and two one one as you don't see any damage around here and after that I'm going to go on to the other channel in the same area and do the same thing although this failed in a different way I think this one just went short bass emitter okay so on this side this is the transistor now this is a PNP so it's going to be black onto the base so that's a diode Junction that's a diode Junction it's open collector emitters that probably looks all right this is one of the two diodes on this side and that means okay and this is the other one so that part reads okay what I'm going to do now is look at the same two transistors on the other channel and I'll let you know if I find any problems there here's transistor queue one two one now this is an npn this is the base pretty sure so that reads okay Junction okay collaps you meter the Army's open and then we have the diode just to be in the area she's one of them this is d114 that was d110 this is d112 and that all reads okay and then we just look at the PMP side of this amplifier channel so that's over here this is the PMP so this should be negative on the base let's get you know it's not in the way it's okay negative I think it might be the other end actually all the way up let's have a look yeah that means okay if he's okay so that looks all right and then the diodes that one and the other one is here one month three it's a slightly off the shot with the ones here so they all been okay so we're really down to this blown up transistor the 68 Ohm resistor and some diodes well the good news is I have some of the transistors the poles but the read okay that we can replace let's have a look I have four above them so I have a few chances what nice game that's a good one I'm fine with that I have a few more so I have transistors I have the diodes what I don't have is the 68 ohm two watt resistors or anything it's similar I could put together so obviously I need to order this resistor and the transistors I have but that problem although it's obviously a problem it doesn't really explain why we had this floating voltage on the base and emitters of these transistors so I think there's probably still something else wrong with this so let's have another look at the schematic and see if we can figure that out again this is the actual schematic I've printed out because it's a little bit easier to show you because I'll show you the PCB at the same time but basically to recap I actually made an area and this is before I had the schematic so we knew this transistor was short on both shells I changed them and I said at the time well I might as well change the npn ones which I assumed with these ones next to it but I didn't think it through properly because this is the heatsink it says a heat sink heatsink and this is the MP and the positive half of the output stage the driver the negative off and the drive and it makes sense that that transistor is on the other side of the amplifier the npn half the positive half what I actually changed was this one so this transistor is the same type as this one but it's sitting next to that one and both of them for both channels are sitting on the same side of the heatsink that's why there's extra transistors on one side I said I've got three on one side and two on the other and I couldn't really figure out why that's why so what happens checked is this transistor okay also this transistor is the one that's getting hot on the other channel what they both called on this one so there's a bit of a tie-in as well with what's going on with the other channel but let's stick with this one for now let's have a look at this transistor there's another reason I'm interested in this we've got something going on with floating Bass voltages on this amplifier as well possibly on the positive half because with a positive voltage on the emitter and that's also the base here also connected to this transistor so there's a lot of reasons to look at this and make sure this transistor is good on both channels but we'll do this one first I have my meter on diode mode and the transistor is here so this is the npn side effects so this is a complement of the shorted PMP on the other side so is it good so base coaxer well I should read like a diode Junction it's not basic meta not correctly emitter this is short this is next to the other transition that blew up on the 68 Ohm resistor I think they're all in this part of the circuit so that is short while we actually do the same transit to only all the shallow okay so this is the same transistor on the other channel bass collector that means like you'd expect to diode Junction basic meta and this one isn't short okay so we have a short circuit on collects we miss it on this one now this raises it huh sorry guys the voice is getting worse but we've got to fix this so this raises an interesting point now this transistor is short collectively met it this end is connected to the post 90 volts the other end is connected via some resistance this 4.7 ohms it says to the base of the output transistors but we know on here we're only seeing about eight volts and if that's short then sure you would see 90 volts on here unless trackable these resistors are open circuits so we better check these and I'll check them on all the channels and while I'm at it on the PMP side we've got the same sort of setup and we know this transistor was faulty so I'm now going to check the continuity from the emitters of these driver transistors to the base and if I don't see continuity I'm going to check these resistors so guys this is the transistor that was short okay the image should go to the base of all the transistors and it doesn't as I suspected the complementary of this one this is the npn the PMP is actually here and that's the transistor that I thought that was this one and I changed okay let's look at this one so we went to the emitter well that means okay uh we have a connection to the base here because I'm on the other channel so on this one this is the transistor that reads okay and yeah and this is the base of all the transistors so we're going to have a continuity and then on this side this is the PMP that is the world I've changed thinking it was this one on the before I had the schematic obviously it looks like this channel has got all the bass resistors on the output stage open on the positive side and this as it's on the negative side so the base resistors are actually down here you can see one two three four five the six of them okay now there's only three base resistors well not schematically we were just looking at which is now here are the smaller there was two watts of resistors so there was effectively the bass resistor another one connected into the emitter circuit yeah hence the six and they're probably not all 4.7 though but let's check them so let's go on a good Channel first we know this one looks like it's good so resistor it's close enough to 4.7 this one what does it really 33 ohm resistors and facts this would be another 4.7 I would think yeah it's good enough 32A uh just with another 4.7 yeah I suppose you know this will be another 33. yeah that's a good channel so this is the bad negative side on this open 33. open 33 open where's your 33 oh is it this one so open resistors here I'm sure we're gonna find the same on that one Savoy open as he must be because the voltages we were seeing on that short circuit driver would be 90. 33 open take away oh another 32 inches there is really what how many open ones are there right one that one should be reading like 3.1 k or something that's what it is this one yeah so those resistors are all gone and then this channel we think is good or this side yeah let's have a look yeah that should be the 4.7 so this is the other gold side uh foreign 4.7 the other ones down there I think yeah so David okay on that as well so we need some resistors that's the next thing we need doesn't explain exactly why this one was getting hot I think the next thing to do with this now is probably gonna be just to be sure because I need to order parts I think these might be one one resistors you look a bit unusually not like these these could be cool so like these could be half Watts I can have a look but I think what I'd like to do with this is actually I'm gonna take all the transistors off the small ones and this off and see if I can see any problem with them and probably the shaming that off as well because the damage has obviously gone quite a way into the amplifier so I'll now check the other various devices it's time consuming but I think it's probably the best way at this point to see what else is wrong with it and another good reason for doing it this way and I'm sure some of you will sympathize with this it gives my voice a rest a little bit Yeah rather than trying to talk it all the way through okay let me see if I find any more damaged semiconductors on this circuit board in silence so guys I've replaced the shorted transistor I've replaced the shorted diode I still need the resistor I don't have anything like those I've been through all the other transistors on the board in the end I didn't unsold them all but what I decided to do is check for shorts anywhere now while I was doing that I noticed something quite nice about this it's been staring me in the face but I'll mention it so you'll see that the transistors the symbols on the stilt screen are either like a a shape showing where the flat side is the orientation and some other side wide and some of them aren't and I thought well why is that and the most obvious reason being that the ones that solid white will be PMP and they are and the ones that are also npn but the other thing you'll notice these white dots yeah that's the base so if we go to one of these we know straight away it's going to be an npn transistor and we know where the bass is so we can go straight away red to basically because we know it's npm we should go Junction huh Junction and then there shouldn't be a short between the other two which are the reasons yeah so quickly we can just test it and then these are PMP so if we do the same thing with the red on the base we won't find the junction between both legs okay if we go with the black one onto the base because it's p and p we get the junction ah and native to the collector again uh not a junction just a climbing resistance so you can very quickly go through knowing whether or not you have all the semiconductor diodes that you should have because you know the priority of the transistor and the better bases yeah I've also tested all the diodes so I haven't found any faulty diodes basically I know there are no short circuits around any of the semiconductors on both channels the other thing I just quit mentioned is these resistors so these are the base resistors there's one here I can weigh it down a bit okay so you can see it's a four point seven Ohm resistor I don't know the rating of these according to the schematic there's nothing special but they're different from these so these can be core to what these could be half watt um also is it possible these are like whoa inductance or zero inductance resistors so a resistor internally has like a metal Film Track like a little spiral inside it that if actually gives a resistance why why would I have wire wowing inside them but if actually that means the resistor has some inductance attacks like an inductor because of the coil of either the tracks on these and the metal oxide or the wire and I know there are carbon resistors that are non-inductive so they have effectively a solid block of carbon and the composition of the carbon with various impurities in each sets of resistance so does anybody happen to know if there's anything special about these resistors because I need to order some or can I just like well I haven't got any half water anyways I'd have to buy some off what so I need to buy some resistors and the bit of advice guys yeah please I'd be uh much appreciated as well as all the diodes and transistors another important thing to test to these little trimmer pots these can't and often do fail and what usually happens is the little wipe it the metal wiper loses contacts with the horseshoe-shaped carbon truck so defensively goes open circuit now quite often with these you'll find that one end is actually connected via the PCB truck to the wipe it effectively so if you're Luke you'll see that the wipe and one end on the PCB tracker connected together okay you'll Defenders the resistance now this one's reading 372. and if I just adjust it it should change and we can put it back to how it was you'll see it's changing yeah so that's that one so it is working yeah let's look at this one so there it is again one end on the track is actually connected to the wiper directly j0 ohms and the other end I'll just get onto his and see if I can wedge the Probe on there we go okay did before those is reading five three two uh let's make sure it's actually worth is actually adjusts well the advantage of reading over 470 ohms and this is actually a 470 ohm pot makes me think there's something wrong with this one yeah this one's got a problem can you see that yeah those two pins are connected together on the PCB anyway yeah you see it's reading higher it's really higher than the value of the pot hmm okay so that's something else that we need to change as well it's quite possible that darts caused the original failure I have some of these in stock so I can replace both of those and don't need to wait for parts but it's something else that we need to do I have to wait now for the power resistors I can get them on Friday so probably Monday I'll be back onto this the Tremor pots I can just change that now I know what the problem is I'll change them both and set them back to about the middle and then hopefully find out how to balance this I know someone's going to chug on here how to do it and of course the other resistors as well the open circuit base resistors but I'm confident now that I can actually plug this in using the current limiting light bulb of course and it's not going to go bang nothing drastic it's going to happen and I can probably then inject a signal into it I think all the voltages will be balanced always and we can actually see if it's working properly so there'll be a part two okay another video completed that was a long one again guys but this work takes time I'm hoping you appreciate that I can't imagine any way to make this shorter and still teach you everything that I've taught in this video to explain things yeah but hey somebody go through one of my videos if you really want to waste some time and that's amazes everyone too if you want to spend some time but it might be a waste of time go through and you tell me all the bits that should be or could be edited out without removing any of the important content yeah and then the reason it's a waste of time because I might ignore that anyway but there you go sorry about the loss of voice on this one it's not getting a lot better but I have to keep on working and I guess I have to keep you guys entertained a little bit as well okay hope you learned quite a lot on that one I learned a few things again or I came across a few things that my previous knowledge helped me with like the floating voltage thing very strange as it looked but now if you guys see such things hopefully you will know how to determine what is really happening yeah when it comes to transistor circuits the voltage between the bass and the emitter is King yeah that's the thing that's the thing to measure if you're not sure and measure it directly not like comparing one voltage to the other and subtracting one from the other one yeah as you've just seen I'll see you all soon guys on another word electronics repair video got quite a few things to do things that we've got a couple of interesting things to review that I think you guys would be interested in as well so I'll see you all very soon ciao for now

Channel: Learn Electronics Repair

Views: 96,402

Keywords: learn electronics repair, learn electronic repair, electronic repair, school, lessons, course, training, free, fault diagnosis, trouble shooting, troubleshooting, pcb repair, component level repair, speaker repair, amplifier repair, powered speaker repair, active speaker repair, mixer repair, transistor fault finding, transistor faults, transistor circuits

Id: JokusaT7eok

Length: 117min 43sec (7063 seconds)

Published: Thu Nov 24 2022