Steps to diagnose and repair switching power supplies

hello so we are back wednesday yeah wednesday and i'm back from holiday but two days monday and tuesday was quite busy you know a lot of jobs so let's fix something yeah the first i just want to update you remember i made like few videos about the samsung t585 you remember okay that's my phone you remember these tablets with a very no fold yeah with no picture on the screen and always is the cable the connect the cable is coming out from this connector here but not this time yeah so i just want to update this just don't do same things what we did so what we did so the connector was fine and we said you know what is the screen so even if i plug this replug the connector back was still no picture and obviously we said you know is the screen so we bought a new screen yeah okay but you know i did found before once yeah so this is the second time when i found this fold actually the cable from this connector is coming out you know because the cable is flashing here and the battery is flexing the cable is coming out but most of the time from here i found one from here and i said you know what let's check first to be sure this connector is fine and after that or replace the screen how to replace the screen you have to take out the digitizer and after you take out the screen now the second problem yeah the second problem you don't have access on this connector no not from this side not from this side so the only access on this connector you have it from the other side you have to take out the screen the digitizer i know it's very stupid yeah so here you'll have something like that you have a piece of plastic piece of plastic which is part of the body here so you don't have access on that connector what i'm doing i just want to give you an idea i'm melting this i have a solar iron special for plastic this one is special for plastics and i'm melting things yeah so all what you have to do you have to melt here a space and you can access the connector and obviously i meld this i took out this piece of plastic and i did have access on the connector and yeah that was the problem okay if we check now it's working fine so keep in mind yeah keep in mind just to don't buy the screen you know like us so it's displaying is working fine okay but like any normal person he will well he will see something like that he'll try to take out the screen to access this connector it's more easy just to melt the plastic and you have access from this side without you know it's a high risk to take out the digitizer and screen also the screen is stuck on the frame with some glue on the side see it it's not it's not very easy not very easy okay let's put this back now let's start something else i don't know how it's on the other countries but these here are very these are very [Music] you know a very common tablet very common everyone has one of these okay good let's learn something today okay let's learn about fixing uh power supplies monitors i think it's a power supply issue here we don't know yet this yeah okay what is this if i'll show you from the back you'll know straight away this is some kind of monitor and it is a monitor it's a lcd one we have here the high voltage change but yeah these are cheap on this day we are selling like monitors like old ones we like 15 1 5. but this one is from some cash machine this this monitor and cannot be replaced because of the size so this going inside of the machine so we have to fix this and find the problem let's have a look what he's doing why i'm saying i believe it's a power supply issue is because i plug the cable power cable and all what is doing is this you can see that this light here okay it's flickering i don't think that's normal i don't think what do we have on this side we have like power cable and vga connector okay so let's open this and fix it you can have capacitors yes you can you can have capacitors you can have many issue like partial shorted power rail you can have like uh you can have a few things yeah even the the microcontroller from the vga board which is starting the power supply even that one can be faulty many issue let's try and fix it i believe the man already tried and it goes somewhere somewhere else try to fix it i'm not sure about that uh okay i think i have to take out these connectors from the screen from the lcd high voltage connectors like normally it's not worth to fix something like that this is a lcd monitor they're cheap it's not worth to be repaired but this one must be repaired because it's from some i told you some cash machine i don't know i don't know okay so the connects are out this is out and that's all let's put this on one side let's get concentrated on this one yeah so what do we have here we have the power supply and the backlight yeah and the control board in the video here that's what we have here so what components we can recognize here yeah like here i can see we have the high voltage area based on the fact on the fact is near to the power connector we have four pins here plus and minus which is mean here is the rectifier bridge yeah here this square probably from the main switching mosfet yeah we have the opto device here you can see here to here to here so this is the secondary the low voltage and here where are the [Music] high voltage outputs for the lights for the backlight so this array for the backlight here this is the low voltage this is the high voltage and this is the video board here but the video board can also start the power supply it can be an interesting repair hopefully the picture is better now although we should check first we should check what he's doing let's break the plug the cable the cable is plugged let's take the multimeter volts volts okay minus and what we can see here the output of the transformer here you have three pins doesn't mean it's a double diode and this is the output and the output is 15 volts 13 15 13 11 10 okay that's not normal this is one output do we have another output here no these things is doing the same so we have a power supply with only one output here yeah that's all yeah this high voltage range okay so also i suppose this is 15 volts 14 13 11 12 the voltage is changing let's try something let's see exactly how it's changing let's see where is my microscope microscope microscope is not on screen oh it is on screen sorry it is on screen facing offline available use usb driver font because it's not plugged in let's plug the microscope maybe we can learn something useful who knows my mouse is not working okay starting it's on let's switch on dc channel dc this is on good let's see [Music] we have ground and the main voltage okay what's that so here is supposed to be a line it is suppose you can see the signal let's change the let's change the on higher okay that's better yeah so that's what we have on the output where you're supposed to have like 12 or 14 i don't know but i should post 12 because it's a standard thing so i suppose here she'll be 12 volts so what the power supply is doing is going up the voltage goes down yeah let me show you so you can see the mouse here okay what do we have more here because we should have a low voltage for the video we do have a switching power supply here a second one and that's the output and the same thing nearly the same things nearly but our main problem is the output which should be a straight line yeah because it's dc should be a straight line i have something like that now a good question is why do i have something like that this is a very low frequency spikes here i don't think our capacitor capacitor are not i'm not doing something like that so let's say if you have low capacity you will see the switching frequency from the high voltage area you can finally find it on the secondary here but not this this is low frequency channel one is showing like 2.7 kilohertz does mean 2007 hertz here that's the one 2.7 kilohertz yeah which is very very low this is audio there is no way this signal can be from a switching uh you know like a normal switching power circuit the working frequency sorry so what do you think is the problem it can be a protection yes this kind of oscillation here can be from a protection let's say it's protecting is going up something is wrong it's we have a shorted power rail and it's dropping the voltage but i don't think so and let me explain why let's check again the signal does the output signal and actually the switching the only part where the power supply is switching when it's going up is going up and after that is coming slowly down that's the capacitor getting discharged if you have a short it will go straight down yeah will not go like like like that like slowly down and after going slowly down at some point it's starting again so i don't think we have a shirt i don't think it can be a capacitor maybe let's check the board there it's very simple to check if we have like any blow up capacitor okay this area is very dangerous you have around 300 volts there i feel let's see and we have 18 okay it's getting discharged already that's good this is a simple basic power supply you can find it on the monitors you can find it on [Music] tvs like simple tvs it's a basic switching power supply you can see the input here the input we have a few as you can see the sign feels here you have uh i don't know the english name of this thing after that you have the coil that this transformer which is for uh to keep away whatever is coming from the like different noises you know line noises and also to keep inside all those uh the the frequency from here you know all the noises after that you have the rectifier bridge you have the high voltage capacitor and this is the minus probably yes the minus plus l so it's coming here to this coil it's coming here plus that plus 300 is going inside the transformer here it's coming outside from here and from here another coil here is going to the transistor the switching mosfet it's a classic one very simple very simple also the here probably have a cheaper module we'll all uh will be driver for this mosfet and the voltage for this one is taken from the transformer you see that this is the second coil of the transformer you have a diode and you get dc here from here you have a resistor you have a capacitor this is important this capacitor from here yeah this can be gone it can be and after that probably here you have like around 12 volts and it's going here good it's so simple this is this is basic it's it's a beauty to see like a basic power supply because you know you can fix it whatever is there check the caps okay exactly what i told you one more time the high voltage thing we have the output we have the double diode here here we have a you know this is a switching power supply we have this coil and this chip here probably is making five volts for the logic board they have the high voltage area for the backlight yeah but the capacitors are fine that's bad okay i'll not check the capacitor capacitor are fine so we don't have like any blow up capacitor around here the next step let's provide a voltage let's provide the 12 volts with the power supply and let's see what happened now let's supply the power this is simple you have only one power supply at 12 volts i think let's get our power supply 12 volts 12 volts ground and plus taking one amp one hand no 180 milliamps 180 milliamps but look here yeah look here exactly what i told you remember i told you it's not that short because it's going down slowly the wave wave shape is going down slowly that's when the capacitor gets discharged if you have like a shorter something is getting too much power the capacitor will get discharged instead instantly you know so look what will happen if we supply 12 volts the output of this you can see the led the led is on you can see that so the flickering thing yeah sorry i'm not touching here the flickering thing is not from the logic board it is from the power supply okay so we we don't have nothing to do with the pole with the logic board it's fine we don't care about that we don't care about the backlight here we don't care about the five volts this small switching power supply here and our problem is here on a high voltage and low voltage it's not even on the low voltage can be okay let me explain you you remember we spoke about the power supply how this high voltage you know this mosfet how he knows you know how he can knows the voltage from output how how he knows to output exactly 12 volts how this switching power supply knows how to change the what is the name you have the the wave shape obviously a small wave shape here yeah here will be low voltage on the output yeah a longer one will be high voltage on the output so this part knows yeah it knows what will happen here because of this optical device octal coupler i think it's on the english name this one yeah here you have a led inside it's a it's a black thing here you have a led just keep in mind you have a led here and here you have a photo transistor just to separate the ground so you will not get electrocuted here okay so that's how this knows what voltage is here yeah so this driver of the mosfet he will check whatever the signal is coming here and it will switch more or less the mosfet and then that way the voltage here will get changed [Music] this is a very precise thing yeah and if this is reading only this uh if if the driver of the mosfet is reading the [Music] this opto device here opto coupler i think that's the name the one who it's output into the led power it's a chip it's a chip you will believe it's a transistor but it's not let me open and show you maybe i should saw a show under the microscope one second i think will be will be more easy to understand what's going on here yeah so what do we have i wish i can have like a higher working distance so here is the mosfet yeah and here is the driver okay and here is the opto coupler here we have the photo transistor here we have the led yeah but the led is getting powered yeah by this guy okay this one you will believe it's a transistor but it's not actually it's a chip with three pins and you can have a look together i'm trying to make this longer you know for you to understand but on a normal situation i'll go straight there and replace that thing because most of the time that's the fault when you have something like that probably first i'll replace the the tl431 that's the that's the one anything is a transistor but it's not i believe it's a 4-3-1 yeah this is a as431 yeah it can have different names can have different names let's see what he's doing here let me find google pl or v1 let's see how it's working aha i found it one second yeah so we have here look that's that's the chip but we have a application yeah give me one second okay so if we get like an application and the nearest application from here with our schematic is is i try to find exactly the same what we have here yeah the nearest one is this okay so that's what what we have on this tv yeah so one more time the our opto coupler here the led you see so this is uh it's reading the voltage this is the output this is the double diode which i told you we have a double died so it's it's reading the voltage 12 volts out you can see uh is reading the voltage and based on whatever is reading the voltage goes up and down yeah it's outputting here the voltage to the led actually it's not outputting it's it's it's shorting to ground so on that point the circuit will get close like that yeah plus led minus okay so this is a very important very important thing you'll find this tl 431 and like most of the power supplies okay now i know the theory it's it's it's it's boring let's try see practical how we can check this thing yeah well let me think first how we can check okay give me a second check out uh okay how we can check because that's a cheap you cannot check that chip that's no way how you can check that chip okay someone is ringing me one second okay it's a cheap you cannot test it with a multimeter yeah let's try to test it on our way or how you'll check yeah just tell me how you will check how you will check no um how we can check how we can check very simple yeah we have the 12 volts let me connect the ground as ground we have the oscilloscope and we can read the signal from the optocoupler here from this opto device we can read the led yeah led and i will use 12 volts on the output okay what do i want to do now i want to do a lot but i need a wire one second give me one second i need a wire we cannot find a wire huh don't worry that's the easy way so you have to be to pay attention because that's the way how you can check if the reading of the voltage and the output into the high voltage area is good yeah so you have to pay attention that's very important wait for the soldering iron he's not hot yet just wait one second so this is the 12 volts this is the output of the double diode here the output of the switching transformer you have double that and it's changing from the ac to dc yeah and here is supposed to have like uh 12 volts dc and like how i said i will call it the power supply for the 12 volts dc yeah yeah okay and uh let's try with oscilloscope to read the output of this led okay so we have some voltage there 10 11 12. 13 is going down okay this is good you see 12. and it's getting changed 11.9 exchange so on 11.9 it's open yeah it's 11.9 the voltage is high as soon as the voltage is reaching 12 volts 12 for something the voltage goes down okay so my tl431 is working okay that's bad why the voltage is going up to 16 okay let's check the photo transistor from the other side here ohms the multimeter and this here we have this is the high voltage area we have 26 kilo ohms i'm still 26 let's spin this and we have 73 kiloohms and we're going up on the voltage 12 now it's not changing let's check that area 11 volts going up up to 30 and we have same value 73 kilo ohms nothing change let's pin this 26 kilo ohms with 13 volts going up on the voltage 11 volts same thing 26 kilo ohms going up 14 26 kilo ohms so here is changing for here on the input of the driver is not changing so it's a miscommunication between this part and the high voltage area the communication which is made by this photo finger let me check straight on the pins maybe you have a track issue 72 26 okay going down 26 370 that's weird i want to check something else yeah i'll switch on volts i'll go here on the led yeah not with oscilloscope and i'll check the voltage yeah so let's see how this is changing we have 10 volts and it's zero there up 12 is not changing it is changing 1.1 more and the voltage is constant okay so how do you explain that yeah this is the led what i'm checking here you have the opto coupler this is the led 1.1 x do you think is still low 1.1 for led can be and if we are going with the voltage down it's 12 volts the area 12 volts going down 11 and the voltage just goes you can see on the on the power supply here i know on the power supply on the multimeter the voltage is gone so here is changing here from one point one goes to zero but on the other side it's nothing changing so we can have like a dead optocoupler here or or my tl is not good enough it's working but like partial working that's a weird fold i can swear probably never i found out octocopter let's try something else here let's try something else let's try yeah so i'll take my power supply i'll go with the current down yeah let's say on the power supply let's say 12 volts two volts yeah let's apply voltage to the led of the opto of to the optocoupler and we can read the output on the photo transistor here so on the photo transistor let me switch on ohms we have we have 25 kilometers i don't know which is plus or minus he's not doing anything he's not taking current y minus yeah of course not okay let's try let's try this is plus this is minus of the led from the opto coupler and it's not taking anything now this is possible plus no plus minus is on the other side so on the other if i try to put power on the led my limit is 1.1 you can see there that's the leaving 1.1 1.1 volts 1.2 okay let's check the output one more time minus plus 1.2 there's no even point right here we have our opto device dead let's take it out because with 1.1 volts should be you know the photo transistor should be open but it's not it's close and it's not driving the current that's the one the black one maybe i'm wrong like how i told you you can find that on any power supply let's say this one here you can see it here here i told you so let's replace it nice replace yeah that's the one on one side you have the led on the other side you have the photo transistor okay let's get this one out this will be hard to be solder let me find another one from another power supply board yeah here you have three you have one two three you can see let's take one out all are the same it's not it's nothing to worry about okay let's replace it here that's the one you have a dot so you know which the position is oh okay everything is solder on place nice holders let's check now let's check now maybe i'm wrong okay so again 12 volts let's check the [Music] left check with the multimeter the output of this which now is same 25 kilo ohms okay that's good and plus here okay 10 volts 11 12 and it's changing but not too much i don't know that's normal can be normal 25 kilo ohms when i'm reaching 12 volts 12.5 is changing you can see it's 12 kilo ohms yeah so what do you think that's that's mean i'm i'm checking on the photo resistor trucks yeah hopefully i'm checking right no no yeah one more time now it's right so we have 10 volts what do we have there 74 kilo ohms you can see 74 kilo ohms we're raising the voltage raising the voltage on the secondary rise in the voltage 12. 12.5 and it's going down to 276 ohms yeah here on the on the photo transistor on the other side because it's a semiconductor 274 ohms yeah so that's mean everything is working how it shield now you can take the power supply out and this is something more more important because that's the only way how you can find how many volts has a power rail so in the moment when these get changed yeah like on our case from 74 kilo ohms to 200 ohms that's the point of the voltage of that power air so you don't know what voltage has this power in bone is getting change you know that's the point where the voltage should be you know what i mean hey that's weird i was sweating is the tl431 is faulty i was nearly to swear you know i was nearly too bad that's a weird case yeah let's check now yeah let's plug the power let's plug the power come on goes there it's not going there let's plug the power let's see what happened power huh what do you think look at the led here and the voltage ones volts the voltage minus plus 12.1 is stable yeah this is repair probably now if you connect the screen we can have a picture but it's not you see it's like solid green it's not switching anymore but again this is a weird things you know because i don't remember i don't remember it that's more fair to say like that i don't remember to find uh optocoupler faulty like this one this opto coupler i think that's the name i'm not sure on my language i know i know what's the name but not in english but i think that's the name on english octocopter copper yeah let me check yeah that's the name yeah that's the name yeah yeah yeah you know what i mean that's the that's what we replace you see the opto things with the led and phototransistor ok and now we can test it with the screen but again yeah whatever we are diagnosed in here we have no schematics yeah so again this can be interesting because with this basic knowledge you can fix any power supply just using these basic things here checking out checking the switching on the opto coupler even the laptop power supply are the same the phone's power supply is the same all are the same i think we spoke before about the power supplies okay okay everything looks fine looks fine let me use a screw to keep this in place because you know it's easy when it's dead it's easy when it's dead it's more easy to find a fault the problem is when you have this weird kind of things so you all what you can see a meaning when you take the joke what you can see you can see the leds flickering this led from here yeah and you think probably you know probably the the the the logic board yeah the logic board is faulty because the led is coming straight from that chip maybe that maybe that's faulty or if it's the power supply what can be on the power supply plug this and we have the dell sign and picture okay yeah it's fine it's okay you understand so one more time let me repeat myself because this is very important just using these basic things like basically checking checking the power rail checking the the output of that tl 431 checking the input or to the high voltage thing you can you can like separate the issue yeah that's that's the first step to find exactly what is your problem you have on the high voltage area yeah because look like on this case you can have the voltage of that output of that phototransistor good there is it's not putting the voltage sorry it's it's it's uh it's changing the resistance only but maybe the driver is not reading that photo transistor yeah you have a driver issue that's that's why it's important to know exactly what is your fault if that driver is gone or the copter coupler or the tl-431 or okay so i'll stop now the power supplies are nice yeah it's interesting to be repaired if if i told you from where i come on the power supplies like 20 years ago or 2020 20 years ago when the the color tv comes out and was a lot which switching power supplies are faulty was on that moment when they switch for like power supplies linear power supplies with the resistors and they switch to switching power supplies on that moment and i was was no no way on that time to find to find you know like to find the parts of the power supply and it was changing or removing the power supply and modifying and make a another power supply and i work a lot because you have the on that point you have the the high voltage was like 140 150 volts and after that you have the low voltage swings and i was 12 volts i learned a lot on that on that moment even now i remember that telefunken tvs yeah you haven't you open the tv from the back and you have a power supply here on the metal case that power supply was with the uh the the main high voltage transistor was uh buu 208 i remember now you can find that's something what you can find but one that mosfet was opened by a driver which was a a double t restore anyway that part you couldn't find it so it on that that time you cannot repair the power supply just because that's that small thing now some teresa are fast there is a fast switching there's confined and we have to remove that power supply and put another one and make a modification yeah okay but thank you for watching like and subscribe if you like the video and see you on the next one yeah bye

Channel: Electronics Repair School

Views: 57,475

Keywords: repair switching power supplies, diagnose and repair switching power supplies, power supply guide tutorial, power supply repair

Id: fy89K50Jdzc

Length: 63min 9sec (3789 seconds)

Published: Wed Jun 05 2019