Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

[parakeet]

Hello.

I'm trying to fix a Corsair CX500 ATX Power supply. It works fine with no load or with a couple of devices connected but turns on for only 0.5s in the motherboard. I've tested the motherboard with a different, lower wattage rating PSU and it works fine, so I know that isn't the issue.

3 Different ways I've tested it:

1. Switch on with no load: All voltages good except -12v (-10.8)
2. Switch on with CD drive and HDD connected: All voltages good, peripherals working
3. Switch on with 24 Pin connected to motherboard: Turns on, then off after 0.5s

In the first 2 scenarios, the supervisor IC gives the Power Good signal, but in scenario 3 no Power Good signal is given.

Resistances of voltage rails to ground:
12V: 350Ω
5V: 465Ω
3.3V: 3.3kΩ

Output Schottky diodes test good in circuit, all SMD resistors and diodes also test good in circuit.

Any help would be greatly appreciated.

[Per Hansson]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

Try to load down the 5VSB circuit in your test no 2.
For example if it is rated for 2.5A give it a load of 2A and see if it shuts down from that?

[parakeet]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

Hi, thanks very much for your reply. I don't currently have anything to use as a load on the 5VSB but I'll update when I do.

[CapLeaker]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

Which supervisor ic is it? Sounds like the supervisor doesn’t like something and goes into shutdown. There can be many reasons for this starting from bad caps in the main filter capacitor in the primary, over to bad caps in the secondary, or a bad supervisor IC. Even if you defeat the supervisor, it still will be hard to see what the actual problem is, until you start loading the individual output rails and measure ESR on each cap, including the main filter capacitor (specifically those snap in caps). Been there and done that.

[parakeet]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

The supervisor ic is an ST9S429-PG14. Didn't know that even the bulk filter cap could mess things up once it's running. Once I find a suitable load, I'll test each of the rails again, especially the 5VSB then test each cap like you said.

Thanks for the suggestion.

[CapLeaker]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

If you want you can read up on a PowerMan IP PSU that I repaired a while back.
https://www.badcaps.net/forum/showthread.php?t=106011

[parakeet]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

If you want you can read up on a PowerMan IP PSU that I repaired a while back.
https://www.badcaps.net/forum/showthread.php?t=106011

Thanks for the link. I went ahead and tested the main filter capacitor which tested good. Then tested every capacitor on the output side, and all of them were good(<0.5 ESR).

Try to load down the 5VSB circuit in your test no 2.
For example if it is rated for 2.5A give it a load of 2A and see if it shuts down from that?

It's rated for 3A. Best I could do was to load the 5VSB with ~3 Ohms(2 Ohm resistor + crocodile clips), it continued to run fine.

Starting to think the issue is just a bad supervisor IC, though I have no idea what the probability of that is. What are the chances this IC just fails on its own?

[CapLeaker]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

Happened before that the supervisor is bad. This supervisor IC does not monitor only voltages on the different rails, but it also monitors current. The FPO pin goes high, it sensed a fault and that pin goes back to an optocoupler to the Pwm on the primary. You can force that pin low by grounding it to secondary gnd, thus disabling the safety features of this chip for testing only.

Did you try this psu on a different motherboard? Is this psu a modular one? If so you might not have used the cable it actually came with. They look identical, but there are some wires reversed in that plug. I put the cables from a Thermaltake to an EVGA PSU and my computer was instant dead and shorted a protection diode on the hdd. Like I said, the cables looked identical, just the wiring was different. You’ve got to do some thorough testing

[parakeet]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

Did you try this psu on a different motherboard? Is this psu a modular one? If so you might not have used the cable it actually came with. They look identical, but there are some wires reversed in that plug. I put the cables from a Thermaltake to an EVGA PSU and my computer was instant dead and shorted a protection diode on the hdd. Like I said, the cables looked identical, just the wiring was different. You’ve got to do some thorough testing

I haven't tried it in another motherboard, unfortunately. I cannibalized the only other one I have at the moment. Also, the previous owner said it was faulty, so I assume it didn't work in their motherboard either. I'll put this on the todo list though. Just in case.
The PSU is not modular. That being said, I'll check the cables anyway. I think I'll just have to try to test each component on the board(to a certain point) and pray. Also, disable the supervisor like you said.

Thanks for your help. I'll continue to crack at it, but if you or anyone else has any more suggestions I'd be grateful to hear them. I'll be sure to update if I make a breakthrough myself.

[anikin73]

Re: Corsair CX500 ATX PSU｜Works under some/no load, but not in motherboard

For psu that cannot take load, u can try these 2 easy tricks,
1 big capacitor is bad. The biggest one . It cannot store charge. U power up and measure how fast it drain... if it drain ultra fast.. spoilt.
2 secondary capacitor spoilt , bloated