I think the better thing to do is to desolder everything you have soldered on the mobo. Clean everything as much as you can, check any possible short. The cube should start. If it starts then you haven't fried your cube and just take extreme care when soldering back everything.
If it doesn't start, check again for any short, but you probably killed something...
Is it pal or ntsc ?? If pal, open up the little box of the av cable and try to bypass the cap and resistor. This was needed on my cheap screen.