If you don’t measure 3.5VDC at the power supply enable cable at the motherboard, then the power supply or motherboard may be bad. There is no easy way to test the motherboard, but the power supply standby voltage can be measured. If the standby voltage is betwee 4.5 to 5.5 VDC, the power supply is good.
To measure the standby voltage: Pull the computer out of its compartment behind the keyboard. There is a 2-wire cable that connects to the right rear corner of the motherboard and is secured with tape. Be sure you note the orientation of the connector before you unplug it. There is another 2-wire cable that connects to one of the power supply connectors on the left side of the computer. Once these two connectors are unplugged and the video and power cables are unplugged, you can slide the computer out of the control.
You'll need to plug an ordinary computer power cord into the power supply and plug it into an AC outlet. Once it's plugged in, you'll need to check the Standby 5 volt power supply. This is located on the 20-pin connector on the motherboard. There should be between 4.5 and 5.5 volts DC measured from the purple wire near one end of the connector to any of the black wires, with the connector plugged in to the motherboard.- If this voltage is between 4.5 and 5.5 volts , replace the motherboard.
- If this voltage NOT between 4.5 and 5.5 volts, replace the power supply.