Is the input fuse blown (the round thing that says T2A 250V)? If so, you also have a short somewhere on the primary, making the whole situation even more problematic.
Personally, I would try to fix it out of curiosity and to test my skills, but it certainly isn't worth it for £30+. The caps should not be that expensive...

I would start by diagnosing the short on +5VSB. There is no point in replacing anything before that is fixed.
After that, I would replace all the electrolytic caps with good brands (except maybe the big one the primary) and hope for the best.
Since this PSU obviously has no PFC, it is relatively safe to work on it with the classic light bulb in series trick.

watson wrote on 2024-05-18, 16:26:

Is the input fuse blown (the round thing that says T2A 250V)? If so, you also have a short somewhere on the primary, making the […]
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Is the input fuse blown (the round thing that says T2A 250V)? If so, you also have a short somewhere on the primary, making the whole situation even more problematic.
Personally, I would try to fix it out of curiosity and to test my skills, but it certainly isn't worth it for £30+. The caps should not be that expensive...

I would start by diagnosing the short on +5VSB. There is no point in replacing anything before that is fixed.
After that, I would replace all the electrolytic caps with good brands (except maybe the big one the primary) and hope for the best.
Since this PSU obviously has no PFC, it is relatively safe to work on it with the classic light bulb in series trick.

Thanks for this - the fuse didn't look damaged so I hadn't actually tested it - there is no connection across it - so it must be blown!

Any tips for tracing the fault on primary? The cracked thermistor next to the fuse is reading about 9R so that seems OK at least? The bridge rectifier does test as short between two of the pins (I understand each combination of pins should either read closed or diode drop of ~0.5 ) - could that be the problem?

The cart cost was creeping up toward £30 because I'd included a new large capacitor (120uf 400v, £5.20 each) and other components come in minimum 5 or 10, or I've added at least one spare of each etc. I'd kind of rather bulk out the order to get to the minimum of £40, than pay a £9.99 handling charge on an order less than that! (might chuck in another SSD or something ...)

justin1985 wrote on 2024-05-18, 23:13:

The bridge rectifier does test as short between two of the pins (I understand each combination of pins should either read closed or diode drop of ~0.5 ) - could that be the problem?

Shorted rectifier is usually effect and not a cause, that is to say you should also check (desolder if necessary) the switching transistor(s). When the secondary caps vented it might have caused a spike that upset (or even killed) the PWM IC, which in turn overloaded the transistor. But it could be just the bridge, I've seen those go bad but that's rare.

The stuff vented from electrolytic caps is both corrosive and conductive (even the residue) so wash the PCB well. And dry, obviously, take your time with it. And be careful if you haven't repaired a few of these before, it can be dangerous due to line connection and charged primary cap. I suggest you use 100W bulb as input current limiter, this should be enough to let the PSU turn on without (or minimal) load but will protect any possible replacement bridge and transistor from failing again if you missed something. Also is safer for you (except touching the primary cap).

Deunan wrote on 2024-05-18, 23:54:

Shorted rectifier is usually effect and not a cause, that is to say you should also check (desolder if necessary) the switching transistor(s). When the secondary caps vented it might have caused a spike that upset (or even killed) the PWM IC, which in turn overloaded the transistor. But it could be just the bridge, I've seen those go bad but that's rare.

The stuff vented from electrolytic caps is both corrosive and conductive (even the residue) so wash the PCB well. And dry, obviously, take your time with it. And be careful if you haven't repaired a few of these before, it can be dangerous due to line connection and charged primary cap. I suggest you use 100W bulb as input current limiter, this should be enough to let the PSU turn on without (or minimal) load but will protect any possible replacement bridge and transistor from failing again if you missed something. Also is safer for you (except touching the primary cap).

Really useful, thank you! I'll try desoldering and testing those today, before I commit to ordering parts. (I did, of course, carefully check there was no charge using a meter before touching any parts directly!)

Thinking about alternatives, I was looking at PicoPSUs and the handy looking 'vintage computer' breakout boards designed for them (I like the fuses!), but when the board only takes 12v and standby 5v, that seems like unnecessary complexity ...

USB-C PD with a trigger board set to 12v, plus a buck converter for the 5VSB seems like a possibility? (I noticed the board wouldn't boot unless 5VSB was connected when I was lashing it up to an ATX supply)

The old PSU is only rated for 100w total anyway, so comfortably within the rating of USB-C PD.

Or perhaps a 12v+5v open frame Meanwell type power supply board?

Either way, is there any simple way to replicate the ATX soft power on?

Well I figured the easiest and cheapest way forward is a generic PicoPSU and hacking up a 24pin ATX extension cable. Figure I can cut off the other end of the cable and solder on only the connectors I need, and probably still be able to hide the whole thing within the original PSU case.

It feels like powering the drives direct from the PicoPSU, rather than via the step down circuitry on the motherboard might help be a bit kinder to the old board too?

Pick whatever suits you best. I would try to fix the original PSU - but I do have some (if limited) experience fixing these. Pretty much everything can be replaced except the transformers, and sometimes you can try to swap those as well but fo custom designs like this one it probably wouldn't work.

Deunan wrote on 2024-05-19, 23:35:

Pick whatever suits you best. I would try to fix the original PSU - but I do have some (if limited) experience fixing these. Pretty much everything can be replaced except the transformers, and sometimes you can try to swap those as well but fo custom designs like this one it probably wouldn't work.

Thanks for the encouragement! I will keep it, and keep replacement caps etc in the Farnell basket for when I next have other things to order. But I am put off by the prospect of ordering one set of parts only to find there is a problem with something else as well.

It's annoying there doesn't seem to be a UK electronics component supplier that's both comprehensive and doesn't penalise small orders ...

I guess that least it is most likely the 5vSB short is caused by one or more of the failed capacitors? But that wouldn't necessarily explain the fuse being blown?

justin1985 wrote on 2024-05-20, 07:09:

I guess that least it is most likely the 5vSB short is caused by one or more of the failed capacitors? But that wouldn't necessarily explain the fuse being blown?

IMO any dead short on the output rails should be detected as overcurrent by the PWM IC, and should trip the PSU into shutdown. Now the standby rail is bit different but either it's limited pulse width anyway or a separate free-running circuit. Neither should have the ability to blow the fuse.

My guess is the caps on the secondary main line (5V usually) went bad and this caused very unstable operation, which might just upset the IC enough to kill the switching transistor. These usually go short, blow the fuse and possibly short the rectifier as well. Then they go open by exploding. If there is no PCB damage I'd replace the fuse, rectifier, check and possibly replace the transistor(s), and any caps in the base drive circuits - as well as the obviously dead ones and anything else tired or marginal I can find. Primary side caps are usually OK and the ESR isn't really critical here. Bring the PSU up with isolated variac with 100W lightbulb in series. If it fails look at it again (replace the IC if cheap and available), if it works remove the 100W bulb and try some 12V car lightbulbs as load. Verify voltages when loaded.