PC Power Supply with shorted 3.3V

[Askic]

Hello,

Today I discovered that PSU Chieftec BDF-750C PSu has died. Not sure when that happened since I didn't use PC for a long time. PSU is not that old, about three years, but just out of warranty.
I had some previous success in repairing different electronic devices including PSU. Until now, the problem was usually bad electrolytic capacitors after many years of operation, but this time is different. When trying to turn on the PC, fans move a little bit and then nothing. There is astable 5V SB (actually 5.13 V DC Standby voltage), but that's it.
I tried to jump start this PSU ( PS On with GND), but except for 5 V SB, no other voltage was present. I have opened the PSU and after waiting some time for the big cap to discharge made some measurements.
I have found out that there is a short on +3.3V (about 0.9 Ohm to ground, but that is mostly the leads resistance).

My plan is to remove the bigger PCB and try to desolder these orange (look at the attached picture) wires that go to the smaller PCB which lead to external connectors (PSU is modular). That way I will now whether the short is on the main PCB.

In your experience, what is the common cause of this type of failure?

Thank you!

[stj]

it's probably a rectifier diode on the heatsink shorted

[jonnyGURU]

The +3.3V is on a DC to DC board. That's where you want to start.

That said, the Chieftec Proton is a very not good PSU. Double forward, passive rectification... I personally wouldn't bother. It was e-waste when it was brand new.

Not that Chieftec doesn't offer some decent models (the Polaris comes to mind as a decent Chieftec model), but the one you have has a track record of being terrible.

[CapLeaker]

as always:

Upload some straight shot, high resolution pictures from top and bottom of the PSU using the attachment function. This particular PSU wasn't much good to begin with, but lets see some carnage!

[Askic]

Thank you guys for your replies.
Thing is very weird. I have disassembled everything and started chasing for short, could not find it, then again, I measured +3.3 V against the ground and I had 1K+, so short was gone. I assembled everything back and now it is working correctly. I will test it as much as I can.
I have carefully inspected everything looking for damaged insulation on wires, or some conductive parts that could be responsible for shorting, but none could be found (except for dust, but even that, not very much).

I do have pics with higher resolution, but right now, I don't think there is a point to upload them.

I guess I'll need to wait and see how it will behave.
Once again, thank you for your replies.

[jonnyGURU]

Thank you guys for your replies.
Thing is very weird. I have disassembled everything and started chasing for short, could not find it, then again, I measured +3.3 V against the ground and I had 1K+, so short was gone. I assembled everything back and now it is working correctly. I will test it as much as I can.
I have carefully inspected everything looking for damaged insulation on wires, or some conductive parts that could be responsible for shorting, but none could be found (except for dust, but even that, not very much).

I do have pics with higher resolution, but right now, I don't think there is a point to upload them.

I guess I'll need to wait and see how it will behave.
Once again, thank you for your replies.

Something simple? Pinched wire? One has to remember on PSUs like these is that everything is common. 95% of the time, SCP only works if the short is from + to ground. So it a +3.3V works one day and not the next, one has to look at the cables/connectors.

[stj]

i start with a brush, incase there is any conductive dust on the board or tiny solder-balls from shit manufacturing.

[Askic]

Just a small update. I have tested the PSU for about three days. It worked for about 5–6 hours each day, doing usual work. So far it works fine. We‘ll see, but so far it seems like everything is ok.

[momaka]

Maybe one of the output caps on the 3.3V rail had a short that got blasted away after a few powerups?
I know that seems highly unlikely... and even I don't think that's the case here.

However, I did have a few old pulled caps do exactly this. The most recent one was a Teapo SC 6.3V 1000 uF cap from an old P4 motherboard from 2005. I have a bag of these and reform them every few years when I remember to, along with some of my other pulled caps (I use these in breadboard projects quite often.) When I reformed them recently, it looked like all was well. Then I put one of them on my GM328 tester and... shows up as 0-Ohm resistor. Checked with my meter, and sure enough that was the case - cap had gone short-circuit (I suspect the short AC pulses from the GM328 tester is what triggers this, as I've had other caps do this as well.) So I was like 'f- it, let's give it the beans' and connected it to a bank of 2x 10000 uF caps charged to 9V. Did that, expecting a big spark. Instead, I got a small spark and the cap came back to life - holding up voltage and all (though only up to about 3.5V.) Reformed it once more and tested again with the GM328. This time, no more problems. But for how long, who knows?. In any case, quite a few of these had failed in the past on me, so it wasn't exactly a big surprise. But the (temporary) failure mode on this one was interesting and not like the others. Which is why I mention this here.

That said, I also think that O/P's issues was probably something else. Bad connector (SATA) somewhere indeed, as the others suggested? Or maybe a solder bridge / ball that has disappeared into the abyss when the PSU was taken apart. Another thing I have seen is thin strands of wires stick out where the wires are soldered for each rail onto the board and come very close to shorting to a nearby rail. Perhaps that could be what happened here as well, and somehow opening the PSU and moving the wires has taken care of this (for now anyways.)

Since the PSU is working now, I suppose O/P can just let it be. But for the future, consider adding more/better pictures and careful observations.

That said, the Chieftec Proton is a very not good PSU. Double forward, passive rectification... I personally wouldn't bother. It was e-waste when it was brand new.

What's wrong with 2TF and passive rectification?
Aside from lower efficiency numbers, I don't see how that makes a PSU e-waste. If done properly, it can be a pretty dependable setup.
Now, crap cap choices (especially for the APFC filter) and small/overloaded/overheating output toroid inductors - that's something definitely guaranteed to be e-waste down the road.

[jonnyGURU]

Aside from lower efficiency numbers, I don't see how that makes a PSU e-waste. If done properly, it can be a pretty dependable setup.

Lower efficiency, higher heat, higher ripple, worse regulation... and "if done properly" is not a statement used when a company like Chieftec is looking for the cheapest way to make a PSU.