It is not turning on

This machine is straight from the capacitor plague era :-(. It's usually the motherboard caps, or the nonstandard power supply. That said, you can sometimes fix it by disconnecting the 20 pin and 12V "P4" connector and that sometimes helps. If it doesn't and the motherboard caps look fine, the nonstandard PSU is bad so you will need an exact match for his unit.

DO NOT USE A STANDARD 24 PIN TESTER ON THIS SUPPLY! YOU WILL KILL IT! You need to jump the green and black and test with a DMM. You will need to confirm the pinout to be sure, but this is the pinout Dell used on the nonstandard P4 machines: https://www.smps.us/power-connectors.htm...