Corshair RM750 - doesn't allow for board to turn on

[kotel studios]

Hi,

While back I got this Corshair RM750 PSU from a friend which said it was broken. Plugged it into my bulb protection and turned it on. Was greeted with two 60w bulbs turning on (barely at max with PSU generating the voltages. I thought it was the APFC needing more current so I removed the bulb protection and plugged into my wall socket.
All the rails were fine, all generating the correct voltages when turned on.
Now, I have plugged in an known good board (intel i7 0th gen with iGPU, nothing else) and pressed the power button. Nothing happened besides the 5vsb LED onboard going dim for a split second. No sounds, beeps, fan spin and other LEDs. Measured the PS_ON and 5vsb rails when pressing the power button and there weren't any issues I could spot (5vsb is 5.08v all the time, PS_ON dips from 5v to 4.8v and then back to 5v). No obvious signs of burns/damage on the PCB.

This is my first time seeing such an issue. I have no idea what's the cause for it.

Any ideas?

[stj]

over current protection circuit probably false-tripping.
look for a big current-shunt and work back from there.

[kotel studios]

Okay, I have done reverse engineering on the parts which were exposed and I couldn't find any big resistors which represented current shunts.
Here are the pictures.

Any ideas?

[kotel studios]

Nobody is gonna belive this, but when I was going outside to take better pics of it, the corner of the PCB (near the primary MOSFETs) had shorted against the case and now I have 10ohms between positive DC and negative DC on the diode bridge rectifier in circuit, 31 ohms between Drain and Source on all of primary MOSFETs. There is also an burning smell inside.

[CapLeaker]

I guess you have to fix the shorted out components in this PSU now. What you should have done first place was checking the state of the supervisor IC in the secondary of the PSU when connected to a motherboard and didn’t work, vs not connected and forced on.

[CapLeaker]

Drain and source of the mosfet short could be because you turned the gate on. Do a proper test of the mosfet with your DMM in diode mode and try turning the gate off an on. There are you tube vids on how to do that,

[kotel studios]

Quick update:
The resistance dropped to 5ohms on both the positive and negative outputs of rectifier and on gate and drain of primary MOSFETs. My idea is to remove the rectifier and if that doesn't fix it remove those MOSFETs (that's where the burnt smell is). Hopefully only one went bad and not anything else...