Troubleshooting Seasonic M12II Evo 750W

[purajga]

I got this broken power supply at an auction a few months ago and I've been making occasional attempts to fix it since.
The lightbulb test revealed that the primary side was charging the main cap properly (one brief flash when turning the PSU on). None of the main rails on the secondary were shorted to ground.
The problem - when turning the main PSU switch on it would drop the 5Vsb line to 0 (it goes from 5 to 0V when measured). The power supply wouldn't start when bridging PSON to ground. Every now and then when turning the switch on it would actually have the 5Vsb rail on but it still wouldn't start. When connecting the PSU to a motherboard that has a standby LED, the LED would immediately turn off in first case (when the PSU loses 5VSB) and it would flicker in second case (when the 5VSB line didn't drop to 0).
After and unsuccessful repair attempt by replacing the 5Vsb IC the problem remained the same.

Fast forward to yesterday, I had some free time to poke it with a multimeter and I noticed a broken diode. The solder mask under it was cracked and separated from the PCB as well.

I don't know the specs of the diode because it doesn't have any markings on it, so I need some help finding a replacement. An electronics store in my country sells this one:

I'll include some pictures of the PCB and the problem area. The diode in question is D103 on the PCB (it was already removed in some of the pictures).

Any info is appreciated.

[redwire]

Re: Troubleshooting Seasonic M12II Evo 750W

The diode D103 is an ordinary 1N4148 but in a small SMT package. Notice the PCB pads are actually for a larger diode, which is what I would put in.
Something in a SMA or SMB package or even MELF, but fast-recovery only.
I would use ES1B or LL4148 MELF. Schottky parts like SS16 60V are too low voltage.
No good (too slow) is S1D, LL4002, M7, RS1 etc.

The diode seems to be for supplying power to the 5VSB IC.

[purajga]

Re: Troubleshooting Seasonic M12II Evo 750W

Thanks for posting. As per your advice, I soldered an LL4148 MELF diode and - good news! The power supply turns on now. All that's left now is to test it with higher load.

Interestingly enough, when I first hooked it onto a motherboard, the 5vsb LED turned on and then immediately off, making me think that the repair was unsuccessful. After I disconnected the PSU from the motherboard and jumped PSON to ground the PSU turned on. After that it started working normally with the motherboard.

[purajga]

Re: Troubleshooting Seasonic M12II Evo 750W

Unfortunately, my repair was short-lived. At first the PSU would lose 5vsb when switched on, but it would resume working normally after switching it off, waiting around a minute and switching it back on (I'm assuming some kind of protection kicks in here).
After a couple of days it just kept losing 5vsb every time, just like before.
The replacement diode is still good, though.
I'll try replacing the startup caps, but overall I'm out of ideas for now. There's nothing else physically damaged like that diode I missed before.