Corsair RM850x PSU repair - over current protection?

[W1ngl3t]

Hi all,

I have a Corsair RM850x PSU that refuses to turn on.
The 5v standby rail is working fine, but when I try to turn it on it shuts itself off in a few milliseconds. There are no obvious shorts to be found so I started probing the supervisor IC and the switching controller to see if it was the over/under voltage protection or over current protection (OCP) being triggered and causing the shutdown.
With my oscilloscope I can see that the 12v, 5v and 3.3 rails are coming on for a short period of time. The 12v rail reaches 12v and almost immediately starts to drop voltage, while the other two remain constant. Furthermore, I can see spikes in excess of 1v on the current sense line and according to the documentation 1v is the threshold. What I don't know is if these spikes are enough to trigger OCP and this is what I would like to ask for help with.
I have attached pictures from the oscilloscope showing these spikes hoping someone can confirm or refute that this is indeed the smoking gun.

The "A" line on the oscilloscope represent 1v.

The OCP sensing IC is the Champion CM6901X. Datasheet can be found here: https://pdf1.alldatasheet.com/datasheet-pdf/view/311445/CHAMP/CM6901XIP.html

Any help would be appreciated!
thanks

[magic]

The 12v rail reaches 12v and almost immediately starts to drop voltage, while the other two remain constant.

FYI, on modern PSUs lower rails are often derived from 12V by buck converters, which is consistent with your observation.

Where is the noisy signal coming from?
Are you sure you are probing it right? Attach the ground clip close to the signal trace, or to the ground near the IC receiving the signal.
Do you see similar spikes when it's turned off?

[W1ngl3t]

Hi magic,

Thank you for your response.

I am aware that the 12v rail feeds 3.3v and 5v, but the 12v rail -in my opinion- isn't supposed to enter a steady decline right after reaching 12v. I'll show you what I mean

When the 3.3v and 5v rails are switched off the 12v rail momentarily jumps back up a bit, but this could be perfectly normal. Btw, the 3.3v and 5v rails are managed by a separate IC and seem to hold their intended voltages perfectly.
My other hypothesis is that the 12v rail is triggering under-voltage protection, but I have no way of proving it.

Regarding the probing, I've taken new measurements by moving the ground probe to the IC receiving the signal, but I don't see much improvement. Do you think all the spikes above 1v are just noise and are probably not triggering OCP?