1st power cord: – If the computer is plugged into an AC 220 Volt power supply through a power cord. For Testing purposes you can connect by plugging the computer power cord directly into a wall outlet one side & other side connect a bulb or multi-meter to make sure that its working or not . Also you can check power cord through multi-meter by setting buzzer mode.
Volt switch: – Check the voltage setting switch in rear side of computer its correct position or not there are 2 option one 110volt Ac / 220v AC. Generally it’s selected into 220 volt AC.
Monitor: Before the dissemble computer you most sure that your monitor ok or its working. How to conform its working or not, connect monitor supply directly to power outlet & remove the VGA cable rear side of monitor which is connected to cpu rear side vga port . Now power on the monitor supply, then after few second you will see the massage (no signal found or check VGA cable) that means your monitor working fine & its ok.
Power LED: every computer have 2 led one for indication main power line & another one for help to indicate hard drive operation status.If both LED not glowing that means missing power from SMPS.(SWITCH MODE POWER SUPPLY) The primary function of smps to convert the alternating current (AC) to direct current (DC) .For a computer or CPU SMPS work as a same, a computer smps different output follow as below tableA Good ATX SMPS wires gives the following voltage as the output. And work for different purpose.
Note:- Its mandatory that working or repairing should be ESD safe, because the computer motherboards made by ESD sensitive components, any type static current will damage your device.