Opening up a switching power supply is dangerous. Honestly for an ATX supply I personally just replace them. http://repairfaq.org/sam/smpsfaq.htm is a good resource if you want to take a look.

deanodley wrote on 2024-04-15, 23:47:

Hi all, so today I switched on my Windows 98 machine and the PSU cuts out after about 1-2 seconds. I removed the PSU from the PC and tested it on the bench but it does the same thing. I think this is some sort of short circuit protection, possibly caused by a faulty component. With hardware this old, the first thing I would be looking at would be the capacitors but I'm wondering if there are any other pointers someone could give me to things I could test?
Thanks.

If it's ATX, then they're inexpensive enough to make just buying another one a better option. Depending on how old your system is, look for one that has strong +5v and +3.3v rails, something like an Antec Basiq Power can be had very cheap on the used market.

But really, I wouldn't try to repair it unless you're specifically qualified; could be anything from a failed capacitor to funky VRMS or just bad solder joints. Don't do it, just replace.

Thanks for the advice, and I would agree you shouldn't mess with these PSUs unless you know what you're doing. Fortunately I have a background in electronics and am well aware of the dangers of handling mains-powered devices. If the fault is something simple I will fix it, but replacement ATX supplies that can deliver 30A on the 5v rail are now not that common where I live.

You can search for the controller IC data sheet and use the example schematic to get some hints.

I'm getting a short across all three legs of an SLB1040CT diode in the output stage which could explain why the sc protection is being triggered.

deanodley wrote on 2024-04-16, 18:30:

I'm getting a short across all three legs of an SLB1040CT diode in the output stage which could explain why the sc protection is being triggered.

That should be SBL1040ct 😉
And yes, if that rectifier is short-circuited, that could indeed be the problem.

BUT!!!
SBL1040ct is a 10 Amp only rectifier... i.e. a very weak part. Given the 40v rating, it's either for the 3.3V rail or the 5V rail. I'm betting on the former, as even the cheapest gutless PSUs always use at least a 16 Amp or 20 Amp part for the 5V rail. Still, a 10 Amp rectifier for the 3.3V rail is very very weak, and if I had to guess further based on what I have seen over the years with dealing with (and repairing) cheap PSUs, the PSU you have is probably a cheapie too. That said, I'm still not discouraging you from trying to repair it. However, just beware that it may not be that great of a unit. Posting some pictures of it here would be good way to tell.

Moving on... before you conclude that rectifier is shorted, note the following:
Many cheap / gutless ATX PSUs have minimum "dummy" load resistors on the output rails (3.3V, 5V, 12V.) Some of these PSUs use really low resistance values. Not at all uncommon to see 10-15 Ohm resistors on the 3.3V rail or 20-47 Ohm resistors on the 5V rail. The reason I mention this is because if you used the continuity (audible beeping) function on your multimeter to look for a "short-circuit", beware that many multimeters will beep to indicate "good" continuity on up to 50 Ohms (some even beep on as high as 100 Ohms.) Thus, if you used the continuity setting to test for a short-circuit, know that those low-resistance dummy load resistors can trick you to conclude that there is a short-circuit on the output of a PSU / across a rectifier, when there really isn't. I've seen people run into this mistake when diagnosing PSUs many times.

As to the original problem of the PSU (not wanting to turn on and only getting the fan to twitch), there could indeed be a multitude of things that could cause this - bad caps, shorted output rectifier(s), a bad solder joint, shorted output toroid, bad voltage reference IC (typically on the 3.3V rail), and etc. Again, would be best to post some pictures of the PSU so we can see what you're seeing.

deanodley wrote on 2024-04-16, 13:44:

If the fault is something simple I will fix it, but replacement ATX supplies that can deliver 30A on the 5v rail are now not that common where I live.

Agreed - if the problem is not too complex, it could indeed be worthwhile to save some of these older ATX PSUs with stronger 5V rail.
However, beware that given the 10 Amp rectifier (likely) on the 3.3V rail, I am leaning towards the PSU being a cheapie / gutless wonder and thus not having a really powerful rectifier on the 5V rail either... in which case, the 30 Amp rating on the label for the 5V rail could be fake.

chs486why wrote on 2024-04-16, 00:17:

Opening up a switching power supply is dangerous.

Not really.

Nothing personal against your post, but I keep seeing general statements (misconceptions) like this posted everywhere online, and almost always its by people who know nothing about electronics repair. There's literally 100's of things that people do on a daily basis in their life that are way more dangerous than this (e.g. driving... not to mention how many people text and drive or are distracted with other crap on their phone and think NOTHING of it.) Yet, the misconception about "the dangers" of opening various electronic devices, as if some daemon will jump out from inside and instantly take your soul with it - doesn't work like that! Granted there are exceptions, like MWO's (microwave ovens) for example, that I'll agree are extremely dangerous. But ATX PSUs? Opening one is about as dangerous as boiling an egg... so long as the PSU is unplugged from the wall, that is. Then again, it should be "common sense" that it's dangerous to work on anything while it's energized / plugged into household AC power, just the same way one knows not to stick their hand/fingers or metal objects into a live AC wall receptacle.

But with AC power removed, most electronic devices (and not just ATX PSUs) are pretty tame to work on... again, with some exceptions noted, like MWO's.

momaka wrote on 2024-04-17, 09:10:

Not really. […]
Show full quote

chs486why wrote on 2024-04-16, 00:17:

Opening up a switching power supply is dangerous.

Not really.

Nothing personal against your post, but I keep seeing general statements (misconceptions) like this posted everywhere online, and almost always its by people who know nothing about electronics repair. There's literally 100's of things that people do on a daily basis in their life that are way more dangerous than this (e.g. driving... not to mention how many people text and drive or are distracted with other crap on their phone and think NOTHING of it.) Yet, the misconception about "the dangers" of opening various electronic devices, as if some daemon will jump out from inside and instantly take your soul with it - doesn't work like that! Granted there are exceptions, like MWO's (microwave ovens) for example, that I'll agree are extremely dangerous. But ATX PSUs? Opening one is about as dangerous as boiling an egg... so long as the PSU is unplugged from the wall, that is. Then again, it should be "common sense" that it's dangerous to work on anything while it's energized / plugged into household AC power, just the same way one knows not to stick their hand/fingers or metal objects into a live AC wall receptacle.

But with AC power removed, most electronic devices (and not just ATX PSUs) are pretty tame to work on... again, with some exceptions noted, like MWO's.

I somewhat agree, but in defence of the person who said this, it’s not terrible boiler plate advice to discourage it when you don’t know the skill level of the person asking. It should be common sense not to open a PSU if you’re a casual yes, but people still do it. It’s not like a guaranteed death sentence, but most of us have had a nasty kick from a capacitor at least once and though rare, I do recall hearing of at least one person actually dying.

That said, it sound like OP knows what they’re doing, as do you!

Dan386DX wrote on 2024-04-17, 14:29:

I somewhat agree, but in defence of the person who said this, it’s not terrible boiler plate advice to discourage it when you don’t know the skill level of the person asking. It should be common sense not to open a PSU if you’re a casual yes, but people still do it.

Looked at from this angle, I do see a point to what you're saying.

Still, even in such cases, wouldn't it be better to probe / try to generally determine the person's skill level rather than outright discourage them?

Actually, one thing I can tell you from teaching people electronics repairs over the years is that the skill level / knowledge of the person is not that important. What's important is if the person is willing to learn, has good attention to details, and can closely follow directions. If / whenever I run into individuals that disregard / skip given directions, only these are the ones that I try to discourage right away, since they are the most likely to goof up a detail and do something silly. Sadly, these are also typically the individuals that never really ask for help or that never listen to the advice given anyways... so it's almost moot what we tell them. The best I can do in such cases is tell the person that whatever they are repairing is not worthwhile doing (to them).

Dan386DX wrote on 2024-04-17, 14:29:

It’s not like a guaranteed death sentence, but most of us have had a nasty kick from a capacitor at least once and though rare, I do recall hearing of at least one person actually dying.

ATX PSUs 600-700 Watts and under don't have large enough caps that hold enough energy to cause cardiac arrest. In fact, it's usually not until you get to the 1+ kW units that the input capacitors become large enough to start posing *some* danger.

While the shocks from the primary caps can be painful (I've had more than a few myself - some of them intentional to show the dangers to people), they are not deadly.

As for most older ATX PSUs, especially those without APFC (pretty much just about every PSU from the mid-2000's and earlier), the Joule rating of the primary caps is really too small when fully charged. At worst you'll get a good sting out of them... that's if you're fast enough to get to them before they discharge. The non-APFC PSUs in particular have bleeder resistors across each primary cap, draining them to safe levels in under half a minute. Couple that with early 5VSB circuits, and you get primary caps that discharge almost within seconds of the power being removed. So the danger is pretty much null. In that regard, the gutless wonders are the best 😁 - not only do they have really small input caps, but they also discharge super fast typically due to such PSUs using really crappy and inefficient 5VSB circuits.