Did I do something wrong? Sama Armor 650w

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Hey guys, new member here. I could use some help... 

About 2 years ago I've bought a prebuilt desktop ( I know...). Money was tight and I needed a computer for school.

Anyway, I saved some money and bought a used XFX RX580 8GB, wich means that the PSU must be upgraded too.

I've learned a lot from this forum about how important a good PSU is, so I decided to spend the rest of the money (about 80$) on it.

So I've bought Sama Armor 650w because it was the only Tier A PSU available in the stores in my country (Serbia) and within my budget.

I've pulled out the old no name PSU (still working btw), installed new one, instaled the GPU, checked all the cables, double-checked everything, 

but when I pressed the Power button absolutely nothing happened (like it's not even pluged in the wall).

The switch on the back of PSU turned on (even tried it on off position - no luck), I have reseated all the cables, tried without the GPU - nothing, reseated CMOS battery - nothing.

I didn't do the paperclip test because I was afraid I might damage the connector...

Put back the old PSU, the computer boot up without a problem (without the gpu because the psu doesn't even have 6pin/8pin).

So I RMA-ed the PSU, but after couple of days they replied me that the PSU is working and that they're sending it back....

Any information would be much appreciated.

My specs:

CPU: Ryzen 3 2200g

GPU: Integrated Vega 8 (currently)

MoBo: Biostar TB350-BTC

SSD 120GB

HDD 500GB

PSU: No name (I sneezed and almost blew it off the table)

RGB Keyboard an mouse ( aliexpress - no name )

 

Thanks and sorry for the long post...