I was running into an issue with modchipped ps2s once a RePSU was installed and the top shell was screwed back on. Once the shell was fully tightened I would get no video. Loosening the screws would allow the console to boot up again. What I believe is going on is the extra space left with the lack of the PSU causes the metal shielding to compress much tighter on the mainboard once the console is fully screwed back together. 

I measured the thickness of the original psu, whipped up these spacers in tinkercad, printed, installed, and my issues went away. Even if you don't have a modchip in your console, it's probably best to print 4 of these out (2 if using a retrogem) and screw them to the original PSU screw holes.