Element smart 750w atx psu repair

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I have an atx psu from High Power Element Smart 750w and yesterday it just died. The psu does nothing now and I also tried the jumper between green and black and nothing at all. Inside, there is no sighn of any kind of fizical damage, the figh voltage side looks fine and the caps are all ok. The rectifier is also working and so I actually don't know how to get it working again.

The standby voltage from the purple wire is fine and it stays at 5v even if the psu is connected with the motherboard but the ps_on wire is not pulled high as it should be. The green wire it has only 2.3v and that is a residual voltage as it will go to 0.8v when the psu is connected to the motherboard.

I tried to pull the green figh using the standby voltage with a jumper and then imidiatly pulling it low with a groung jumper but nothing worked.

Any idea on what could go wrong in this kind ok psu?

[ambrosia heart]

(1)The psu does nothing now and I also tried the jumper between green and black and nothing at all. Does fan of this psu rotate? If not, the psu has gone.  Normally, this
fan will rotate once green wire is shorted to ground. 
(2)The standby voltage from the purple wire is fine and it stays at 5v.  This means 5V supplied to chips controlling psu. If your mouse has led, it lights up.
(3)The green wire it has only 2.3v and that is a residual voltage as it will go to 0.8v. I encountered once, may be bad contact. Put the red probe deep into the connector
hole with green wire and move red probe forwards and backwards,  black probe clamped to the psu chasis/casing,  then switch on the computer to see if the voltage drops to
zero. If voltage drops to zero, computer switches on.

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The psu does nothing and so, I know that it is gone but I really want to repair it.

Yep, the standby voltage is there and the mouse will power on and also the green led on the motherboard.

The green wire is, as I said, not doing the thing it is suposed to do as I tried to mesdure the voltage dirrectly on the pcb and it was the same. 2.something volts and dropped to 0.8-0.5 when pluged in any motherboard.

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Vould it be a mosfet or a transistor? I was thonking that if the standby voltage is good than maybe there is something with the ps_on circuit.

Also, I forgot to mention, the first time this happened, I just unpluged everything from the back of the pc, waited for 10 minutes and also reseted the cmos battery and after that, by continiously (rage) pressing on the power button, the psu finally started but that was the last time I saw it doing anything.

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Also, I just seen thst the main cap from the 240v rail is a bit balkey but it does not have the same metal top so i can not tell exactly. It has some kind of a plastic top.

Could it be this cap? I've seen a lot of Xbox power supplies that are not working corectly just because of the main cap that does not even look boosted.

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What is the posibility to fix this power supply if I find the bad part, would it be just one chip or it would be more than that?

[coromonadalix]

without schematics  youll be guessing this and that ...   sure the simplest thing is to search for a fuse

check if you have the constant 5 volt voltage present on the plug   see atx psu specifications / pinouts

and you surely know  some psu are very cramped in space very hard to work in them

[ambrosia heart]

What is the posibility to fix this power supply if I find the bad part, would it be just one chip or it would be more than that?

Do your ATX power supply have this typical structure?  open its casing and take a photo, posting it.

[mariush]

5v standby is a separate power supply from the rest of the power supply. It typically has its own controller chip, diode, transformer, maybe mosfet if not integrated in the controller.
So it's perfectly normal for the power supply to output 5v standby yet not function.

My first check would be to see if the fuse is broken or not.
Then I'd check for obvious swollen or leaking capacitors
The primary capacitor (the high voltage one) rarely fails and if usually there's a round plastic or paper disc glued to the top which may make the capacitor look like it's swollen.

I'd test the diodes and mosfets attached to the heatsink on the secondary side (the one closest to the cables going out the power supply) and check if they're shorted, or broken in some way. You would have to desolder the components and use a multimeter to check them... you can search youtube for "how to test mosfet using multimeter"
 
If everything looks good, you're getting into the dangerous territory... high voltage, risk of death if you're not careful.

I'd continue measuring the voltage across the primary capacitor to see if there's voltage reaching that capacitor... maybe the bridge rectifier shorted or the active pfc circuit fails and doesn't provide power to the main power supply (the 5v standby being separate power supply, may not need the active pfc circuit to function, it may be connected directly after the fuse and bridge rectifier.

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I checked every component that came to my mind and all of them ware parfectly fine. No short circuit across diods or ceramic caps and also the voltage across the main cap was 360v DC. I remembered that some times the board may also have caps with a bit of voltage in them and so, I took the multimeter and tlested across caps and across the main transformer. Still nothing strange. I just took the pcb and put a pice of tin foil over it and than just shorted everything to the groung that I have in my house.

That just did the job perfectly fine as the psu worked with the green wire jumped to groung.

What may cause this situation and how I can osolate this so it would not happen again.