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.