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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.
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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.

10 thoughts on “Diagnosing a faulty PSU”
  1. Avataaar/Circle Created with python_avatars Mark Rainbow says:

    HI Clive, I've been enjoying your videos for a few years now. I'm a heating engineer and fix a lot of boilers… I have to replace quite a lot of fans on Worcester Greenstar boilers. The "mechanicals" of the fan are invariably fine, but the fan control boards seem to fail quite frequently after a power cut. I'd love to send you a failed one (and one that works also if that would be helpful) for you to work your magic on. Ditching a ยฃ170 fan for what is probably a single component failure irks me!

  2. Avataaar/Circle Created with python_avatars Major Pygge-Phartt says:

    I generally find trying to repair computer PSU's a waste of time. All I ever get with them is totally confusing and vague readings and nothing that makes any sense, all too often for some unknown reason there's never a definite ok or knackered reading with them. And far too much electronics these days is built using micro surface mount and ball grid array components, some of which are just like little grains of sugar sweated on to the boards, and then there's those stupid multi-layer boards with copper tracks all hidden away inside between the layers so how can you service them when a track has failed inside somewhere, or if it's cracked, even slightly, then it's a total write off.

  3. Avataaar/Circle Created with python_avatars Major Pygge-Phartt says:

    Why not try using a desolder pump. And it's advisable to use a 50 watt soldering iron for that kind of repair as smaller ones lose heat to the large areas of copper on the board. And only using half the rectifier won't have enough current rating and it's already failed in one side so it needs replacing, but using the remaining half of the old rectifier is ok just to test the PSU to see if the rest of it still works or if it's beyond economic repair. And I've got a similar PSU from a laptop AC adapter which gives correct output voltage but it dies down instantly at the slightest little load on the output. Any ideas on that one? I suspect the transformer as just about everything else checks out ok.

  4. Avataaar/Circle Created with python_avatars Major Pygge-Phartt says:

    For this kind of fault, no output with no sign of any burn ups I would first check the output rectifier as in my experience with these types of power packs that's the usual culprit, and with this type of PSU a shorted output rectifier usually just shuts down the whole circuit so no burn ups, but of course it's a different story if the mains input rectifier blows as they sometimes do, that often makes quite a mess…And don't forget that the mains smoothing capacitor charges up to about 335 volts and can remain charged after unplugging the PSU, just in case any amateur repairers are watching who don't have that much experience…

  5. Avataaar/Circle Created with python_avatars Bruce Hislop says:

    I owned an commercial radio communications business for 40 years (think police/fire/taxi radios) and sold/serviced alot of power supplies over the years. We regularly repaired linear power supplies, but switching PS's are a different beast. We soon learned it was much better to replace a faulty SMPS than to repair it. Once you repair a piece of electronics the customer believes any further failures are your fault and should be covered by your repair warranty. They are also much cheaper to replace than repair these days… unless this is for your own equipment…. but still might not be cost effective to self repair!

  6. Avataaar/Circle Created with python_avatars Mick Diy says:

    this kind of repair is the god of all repairs, these days, electronic technicians are not the purest kind.. i remember a friend of mine who works with radar system in middle east, he travels even during wee hours for sometimes 1000 km just to fix a radar. i saw him fix my laptop by cutting some IC connections and jumping it from somewhere with several wires, insulate it and bingo, it works. people like clive and my friend are few among millions who does a work with electronics by just smelling it. salute to you clive. nice video and thank you for sharing it with all walks of life.

  7. Avataaar/Circle Created with python_avatars ash0787 says:

    good video, I have some familiarity with these things, I was trying to lower the output voltage on an ebike charger, because if you go all the way to the maximum it lowers the lifespan of the batteries apparently. I was thinking oh yeah I will just put a variable output voltage on it with a tiny digital voltmeter thing, not so easy !
    I managed to make it go up a bit but not down.

  8. Avataaar/Circle Created with python_avatars Jason VonIrsik says:

    So I just scored an AMETEK Powervar UPS it's rated at 1296w and after tearing it down everything looked in good condition. The guy didn't have a power cable for it so he didn't know if it worked or not. It's quite dusty inside and blowing it off did also me good but it's still dirty as shite. All the capacitors look good and nothing looks like it's been hot. I'm a novice and wanted to ask how I can give this thing a thorough cleaning? Do I need to disassemble the entire device and wash the board in some kind of chemical bath? The batteries had been replaced fairly recently by the looks. I just don't want to try and power it up and fry something. Any ideas?

  9. Avataaar/Circle Created with python_avatars LegoTekFan486 says:

    Yeah I remember seeing a similar arrangement in another disk player I'd taken apart, and the lack of anything on the secondary side that was either a voltage regulator with current protection, or anything else resembling a current sense circuit providing feed back to the controller. Therefore when I looked at it, I figured if there had been a short on the secondary side that the supply would have just been ruined. I didn't know about the feedback coil powering the chip.

  10. Avataaar/Circle Created with python_avatars Richard Kaz says:

    I just noticed that you didn't test that diode correctly. The diode pack has a common cathode on the centre pin. You placed your meter +ve on the anode and -ve on cathode first diode(left + centre pin) and said it was good the the next diode you placed +ve on the cathode and -ve on the anode (centre + right pin) and said it was faulty. Seems you got that wrong. Check the video closely.

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