If your ps3 does not power on you've to solder the tristate point to a 3.3V power source
There are many models of ps3 motherboards, you can identify the tristate point of your motherboard by looking here on ps3devwiki:
http://www.ps3devwiki.com/wiki/Hardw...Pinout_Gallery
You can use some scotch tapes to hold a cable on the right position, you can avoid unnecessary solders
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If you correctly link the tristate to a 3.3V point ps3 won't turn of and you'll be able to flash your dump (you still need a valid dump)