Just thought I'd share a repair I did that I'm not honestly sure why the problem exist in the first place.
Got a two genesis, cd, 32x, and modded ps1 in a trade recently. Had a non-tmss that didn't power on.
Poor thing was in horrible shape. Missing the plastic bit of the power switch and volume rocker. Also rusted to hell.
After messing with and cleaning/resoldering all the contacts with rust the thing still didn't power on. I came to find there is some kind of fault in the pcb of the small power board. I don't really get how as the trace looks fine and I resoldered it too. Only works when I wired the trace like you'll see underneath.
Hope this helps anyone with power issues that just isnt making sense.