800W PC PSU by Corsair with fluctuating 5V

[jackster]

So I have an old 800W ATX PSU which works otherwise but the 5V rail fluctuates between 4.5V and 5.1V even with no load. Under load the fluctuation seems to be the same, at least with one mechanical hard drive connected.

3.3V and 12V are fine.

All electrolytic caps look ok visually, but should one still start the repair/diagnosis by replacing all relevant caps?

[temyth]

is it DC-DC? i would check the cap's

[jackster]

is it DC-DC? i would check the cap's

Each rail is independently regulated. The two 16V 2200uF electrolytic caps in the 5V rail look good on the top, but it's very hard/almost impossible to see the rest of them due to so many cables and other components cramped right next to them.

But just in case I have already ordered replacement caps.

[temyth]

Each rail is independently regulated. The two 16V 2200uF electrolytic caps in the 5V rail look good on the top, but it's very hard/almost impossible to see the rest of them due to so many cables and other components cramped right next to them.

But just in case I have already ordered replacement caps.

there are regulation modules for each reails that you can put apart of the main PCB? also, would you please share the specific model of the PSU?

[jackster]

there are regulation modules for each reails that you can put apart of the main PCB? also, would you please share the specific model of the PSU?

No there are not separate regulation modules AFAIK but there are separate chokes for each rail (3 in total). And oh yes sorry the exact model is Corsair Gaming Series GS800, techpowerup has good pictures of it: https://www.techpowerup.com/review/corsair-gs800/4.html

[temyth]

No there are not separate regulation modules AFAIK but there are separate chokes for each rail (3 in total). And oh yes sorry the exact model is Corsair Gaming Series GS800, techpowerup has good pictures of it: https://www.techpowerup.com/review/corsair-gs800/4.html

alright, so the PSU it's not DC-DC and the voltage is controlled by switching the primary mosfets. Try checking the voltage comparator in the line that its unstable, they get bad pretty often. They are commonly TL431A or other kind of comparator. In this PSU it may be in a SMD version.

[jackster]

alright, so the PSU it's not DC-DC and the voltage is controlled by switching the primary mosfets. Try checking the voltage comparator in the line that its unstable, they get bad pretty often. They are commonly TL431A or other kind of comparator. In this PSU it may be in a SMD version.

I replaced the two 16V 2200uF electrolytic caps in the 5V rail today but it didn't help so the problem is elsewhere.

So I followed your suggestion and went searching for the voltage comparator of +5V and I think I found it, it's a Texas Instruments TL431AC. It seems to be connected to the +5V rail via these two resistors:



Now I just need to wait another month for the replacement comparators to arrive from aliexpress 😅

[stj]

3.3v and 5v are usually on the same transformer,
so if it is and the 3.3v is fine i would look at the 5v output rectifier

[jackster]

3.3v and 5v are usually on the same transformer,
so if it is and the 3.3v is fine i would look at the 5v output rectifier

Thank you for the suggestion, I think it looks like 3.3V and 5V are on the same transformer. And the 5V output rectifier seems to be this one (STPS3045CW, circled with red colour):


Might as well order some of those rectifiers just in case, the voltage comparators were dirt cheap.

[прямо]

That could be for the 5v standby.

You still have 3 pairs of those dual rectifiers, which I believe one pair is for 3.3v, another pair is for 5v and last pair is for 12v.