PC turns on, but no video

[RedKaola]

My old PC is starting just fine, but the monitor doesn't read an output.

 

Specs:

Sapphire Radeon R9 280 Dual-X OC

FX 6300

T 30140 Biostar A960D+(new) / MSI 970A-G43(old)

Thermaltake 530W 

2x4 GB Crucial Ballistix Sport

 

I've tried:

-checked the GPU is working

-checked the CPU is working

-put in a different GPU

-put in a different CPU

-checked the harddrive is working and put a new OS on it

-tried a different PSU that is definetily working

-bought a new mainboard

-reset the BIOS

-tried a different memory stick that is working for sure

 

After all of this, I still haven't found the source of the problem.

Is there anything else I haven't tried yet that could solve the problem?