[G4] G4 start up problems

[Larry Parsons]

I have a gigabit internet 450DP G4 that will not start up.  Powersupply has 
been replaced (new from Apple) as well as the motherboard (used from ebay).  
I cannot power up from the button on the front of the case but when I push 
the power button on the (old) keyboard the powersupply fan comes on.  There 
are no other signs of life.  I unplugged the power and pushed the reset 
button next to the pram battery, waited a few seconds then plugged in the 
power and tried to start up.  Still nothing.  Am I missing anything?  Would 
replacing the pram battery do any good?
Larry