I have same problem with XL series PSU as well, still pending diagnosis.

Looked good but still not comes up. I had PSU in pieces right now to make trouble shooting easier.

I'll let you know in the Compaq PSu thread I created.

Cheers,

Thanks! It's weird because I measured with a multimeter and the LEDs should have 3 volts DC, even tho they don't light up..

3.3V dc to dc converter (aka VRM) is internal to the PSU and if the processor card is not using 3.3V, will not light. Usually needed when using Pentium processor cards.

So the LEDs should only light up when it's connected to the motherboard?

UPDATE:
I've brought it to an electrician and he changed a few capacitors that were actually broken and now when I turn it on it seems to be in the same loop but the lights on the motherboard and PSU also turn on for a second.
It reacts the same as in this video: https://www.youtube.com/watch?v=uJxdE6gApCs at 20:00 .
The problem is that it requires a chassis fan to be connected and I don't have it. Does anyone have a pinout of the fan?
Thank you.

I have the 590XL version which works perfectly.

I can check that but it'll take a while. I have to take it out of storage.

The LEDs are not directly connected to other parts of the PSU components, that's unique for sure. The LEDs is wired directly out to the motherboard's extra pins in turn directly connected to the motherboard's power plane next to power pins on the connectors.

Think of this way as crude multi-meter in a power supply "measuring" direct at the motherboard's.

PS: this blink is exactly what mine does too except at the PSU's LEDs.

Cheers,

Yes, the electrician told me that current should pass thru the motherboard in order for the PSU leds to work. He also told me that I could wire the cables to an old PSU and it would work, but I'm trying to keep the original one.
Thank you for help.