No Power Up State

The computer is a Dell Dimension (300?). It uses the standard 24 pin supply. The computer will not power up. With the PSU disconnected or connected to the MB I can read approximately 5V on the Supply voltage pin and +3.85V on the Power On pin which I believe to be correct in the PSU off state. This computer uses a power switch/USB circuit card for power up. I have tested this card in another unit and it functioned normally. I measured +3.84V on input connector and across switch (switch open) as well as 0V (switch closed). The +5V supply was on in both states. In the unit in question the voltage across the power switch (switch open) is low at only +0.75V (even though the PSU input connector still measures 3.84). So I am dropping approximately 3V somewhere on the MB. This is the case whether the CPU is installed or removed. Where should I look for the voltage loss? Is there a component(s) sharing this signal line? Where on the MB does this Power On signal go...CPU...RAM...etc? Any help would be appreciated.