Diagnosing a faulty PSU

let's take a look at a faulty power supply and see if we can work out what's wrong with it so i'm going to zoom down this it is mitsumi electro limited that's either electronic or electric coal and the message that came with it is hello clive i'm sending you the 12 volt switching regulator for my from my blu-ray player for a postmortem analysis the player had been left on standby for ages but failed sometime between christmas and my friend come over to watch a film the 12-volt output appears to be zero volt and almost dead short no fuses are obvious explosion skid marks i can see these are all good things actually uh keith lambo so it seems quite well made i'm going to zoom out a bit here because i've already taken some pictures of the front of the back of the circuit board we'll take a look a little look at this first and then we'll start testing it and diagnose it so we have the incoming supply here the mains here and there is a fuse there's a little sort of strange it's it's mark almost like it's a zener but it's a transient suppressor metal oxide varistor there's a class uh x2 suppression capacitor with discharge resistors across the back of it a con mode suppression choke the point of the car mode suppression choke is if current is flowing in through the circuit and out again it poses low er sort of resistance the path of current if common mode noise is coming out in the same direction it has quite a high impedance it sort of pushes back against that it's just uh for it's a nice way of creating a very efficient interference suppressor there's a couple of capacitors going class y capacitors going to the ground tab here and then it's going to the redirect fire the bridge rectifier then charge rectifies and charges this capacitor 450 volt 82 microfarad which is quite a beefy one and that then powers this rather unmemorably named chip an fsg m300n which is a very standard chip which is a switch mode power supply all in one package it's got the driver transistor as well which drives this transformer the output pretty much just has a diode here uh big smooth capacitor 25 volt 3 on mgfird the diode is completely unmemorable the name it's an mbrf 10 100 ct it's dual shortcut diode three pins both the diodes pointing in towards the middle uh and then the output is just a single voltage uh two ground connections two m 12 volt and one d 12 volt they're both come together but one just goes via a link and one goes via an inductor then doctor is to the hold on the inductors to the m connections for maybe motor perhaps let's start probing about the factors dead short circuit is good because uh that suggests well the first suspect most people would think of would be the capacitor from the output it is not domed it doesn't mean it's not the one that fault but we really are getting across the back of this on the diode continuity test across that we're getting a dead short but my first suspect is actually the diode package here one of the diodes in that package may have gone dead short and normally you'd expect to read the diode drop plus a sort of like a slight resistance of this winding but i'm not getting that so tell you what let's remove this diode and test it and see if the fault goes with the diode being removed or at least uh if the fault is alleviated with the diode removed i'm just looking for the solder i'm not seeing the soda there is soda here somewhere but i've just not been very well organized right now just grab some i have some soda so let's reflow i'll zoom down this there's a diode there with this of loads of a heat sink in the vicinity basically just that bare tracks in the circuit board material to take a bit of solder and expose it to airflow movement it seems quite generous because it's two five amp diodes you'd think uh it would take a lot to blow that but to be honest sometimes if it is this it's faulty sometimes uh they just blow so i'm going to try a couple of things here i'm going to try heating all the pads alternately until it just randomly pops out which might be possible it is wiggly it is want to come out but it's not coming out hour now it's very very hot hold on i'll just make a complete mess of this is more or less you know what should be popping out right now is popping out right now it's out now has one of those diodes failed so i'm going to test from either side to the middle pin that's what i'd call a normal diode that is a dead short circuit one of the diodes has failed in that now does that mean that if i was to temporarily correct the duff diode off and solder that back in that incidentally will have stretched the capacitor because this is the output of this power supply let me just zoom out a bit uh this is the schematic of that chip power supply and that is minimalized minimalists they've not shown all the components but this is the output that's this diode here and if it's going short circuit that uh winding will just have been that capacitor will kind of seen a bit ac-ish which isn't ideal but also because the uh if it shorted out this uh coupled winding here the feedback winding that actually powers this chip will have seen there's basically a short circuit on the other side and it will have been doing that thing that it just pulses out but ever see the led lights pulsing uh it will be doing that because uh it's detecting effects as short circuited output and it's uh its own power supply needs that uh load to be normal on the other side right tell you what where is my desoldering braid yeah i'm not organized at all here's some random desoldering braid i'll use that oh this is a new rule i shall do what i normally do i shall add a bit of flux to it because that does help so much with desoldering with the braid way too much flux i completely forgot this is the uh wide needle flux bottle not to worry doesn't really matter there's no harm in having too much flux as demonstrated by the mega poles of flux bouton by lewis rossman and co right where am i here this is where i'm wanting to soak up some soda here ah that hissing pop of flux can i remember which pad it is i'll just do both of them i just pressed unfeasibly harder the soda in there and felt the thread of the bit that screws in just go there excellent that's called tool abuse this is not being helped much but the fact that this is on a heat sinking copper plane but it's done it's fine red i'm going to stick this butcher diode back in it's just going to be a single diode of the two but they are just effectively in parallel and i'm going to re-solder that if one of the diodes does fail really just replace the whole diode let's see if this actually works a bit i don't think it's damaged the primary side i should have actually made sure that was standing up straight before no it's not it's fine i'll just squish it a little bit yeah it's good enough okay i'll just reflow these sort of joints and then test again and now when i put this across the uh capacitor you're probably going to get uh because it's not going to shunt across it it's probably going to see the usual thing that happens when you probe across capacitor you're going to see the voltage wavering up slowly like that yep that's a diode junction that is looking very good and its charging capacitor that is fine right tell you what i'm going to plug this into the mains and we'll see if the capacitor survived or not one moment plays the leads have been connected to it let's power it up so this is my cliff quick test i'm just going to bring in here i haven't a clue if this is going to do anything undesirable i guess we'll find out when i power up it may make noises it may just hiss it may do nothing or it may just put out 12 volts the capacitors don't always die when this happens but sometimes they do right tell you what i shall power up we'll see what happens powering this is good silence is good we do like silence the capacitor is not bulging up excitedly so i'm going to nudge that circuit board which is live over to the side i'm going to set this round to the 20 volt setting i'm going to put the negative on the chassis connection here and this one there we have 12 volts uh the unit is kind of it's proven is what i'll say i wouldn't say it's repaired because i wouldn't leave it like that so um in this instance the full repair to this would be to change this capacitor and this joule diode and that would pretty much give it a new lease of life it's not uncommon for these diodes to fail now do i give you a walk-through of the circuitry i could give i keep going to reach that and i keep going down to let's measure the voltage of course that big capacitor because that is quite a big capacitor that looks like it really hurt if i touched it let's put this up to a thousand volts and stick it across that capacitor it may have discharged it's still at 270 volts that would hurt like shed that would be quite a spicy moment yeah that's annoying that's because the circuit's not going to load and if you know that is annoying that it's not good discharge resistors that's uh a memorable device right maybe i won't explore that then or maybe i'll discharge it uh but the gist of it is i'll just pull this over like this i can show you in this let's uh tame this down a little bit because it's quite ferocious maybe not that much maybe about there um yeah so it's got the the main supply comes in goes through bridge right for capacitor there it's interesting this chip has the built-in mosfet for driving the coil uh it has the startup resistor the startup resistor charges this capacitor which is used to actually power i could zoom down this can i which is used to power the chip but only at the beginning as soon as it powers up this resistor is actually disconnected from that and it relies purely on the current coming from this uh bootstrap coil coming through this diode and that resistor and charging this capacitor and that's how it's powered that's how it can detect that that short circuit was happening the other side what would have been happening there is this resistor would have charged that off it would have started up but it wouldn't have got its power supply from here so the capacitor would have discharged that uh i'm just looking here that's this capacitor here and then it would have started the cycle to charge it up again so it'd be going d dip just waiting for that fault to clear the other side um i could go into more detail i could say that this is a very minimalist approach it's got all the extra interference suppression circuitry handling that boarded caution at the side uh which uh let me just uh brighten this up so the it's got a extra components in the back for instance it's this is the the sensing on the secondary side to actually provide the feedback the voltage is there is done by this little chip here with some support components that drives this up to isolation which signals back to the chip the chip itself because it's switching an inductive load it's not showing the drawing under here but they're up here between the drain and the positive rail is actually a diode here and then uh capacitor the couple of resistors across it's designed to take it's like a little mini surge tank it takes the spike off when that turns off but it without actually clamping the transformer itself because that that's needed when that energy gets transferred across and that's more or less it it's a very straightforward design looking up that chip the uh fsgm 300n will give you all the information need about that it's basically just a very minimalist solution to that but there we go but before i before i do go i'm going to measure the votes across that again see if it's holding up if it's still capable of delivering a zap has it tamed down yet it has uh that will have uh just been the general power that circuitry has drained that away that's good but it's useful to know that the circuit board stays hot for a fair amount of time after you've unplugged it yeah that is very useful to know

Channel: bigclivedotcom

Views: 65,845

Rating: 4.9783196 out of 5

Keywords: psu, repair, diode, short, circuit, blueray, mitsumi

Id: MZDX-1Arg7o

Length: 14min 5sec (845 seconds)

Published: Wed Aug 04 2021

Reddit Comments

[u/nemom]

From the description:

"""

A very common PSU fault on a fairly nice power supply from a media player.

Don't be fooled by the cheap SRBP (Synthetic Resin Bonded Paper) style PCB. It's been designed with common sense and safety in mind to comply with UK standards.

The sizing of the diode array is probably mainly for the increased passive thermal dissipation.

"""