My store bought computer (emachines, 2004) wont start. There is no LED light when you hit the start button on the front, no noise, no nothing. The external power protector is working fine, the monitor comes on and gives the check your PC message... but nothing else. I bought another internal power supply unit and put it in the machine.... it has a toggle power switch on the back and when I hit that the power supply and fan came to life but nothing from the computer ....no light at the front power button.... no booting sounds, nada. What do I do next? Motherboard, cpu? (I will look up what they are later) . I am crazy without my computer! Thanks.
So you've tried a new power supply - fan came on but little else? Hmmm, could well be a power surge. Check all the cables going to the Motherboard though - if it has no power it won't do anything! lol
If you bought this machine within a warrantee period, send it back!
If all was well past that point, and this is a new malfunction, be very sure that motherboard connectors are seated FULLY, but without you putting so much pressure that you risk cracking the substrate (the board itself). Connectors should "lock" into their respective sockets, and feel firm as they do so.
Be sure that the power supply meets or exceeds the power demand of the entire machine, including peripherals such as hard-drives, cd-drives, etc. As a *very* rough estimate, expect at least 400Watts for a 1.3GHz (gigahertz) processor alone, and add up wattage for all your periphery. Many power supplies have "impedance protection", meaning that if they detect too high a load, they simply will refuse to energize, as a protection to the unit.
To calculate watts, multiply voltage (V, or volts), by current (A, or amps), for all your drives, including your floppy drive. Include both sides; the 5-volt and the 12-volt, and add the watts together for the total wattage demand of the drive.
You can never have a power supply that is "too powerful" for your machine....ever.....it is like saying $200 million is too much to buy a car. You can buy any car you want, with plenty left over, and that is desireable, moreso than not having enough. Always opt for the most powerful unit that can fit in your machine. The least you will benefit is longer life of the unit, properly taken-care of. Units with ball-bearing fans are most desireable, as they are 90% less likely to "gum-up", or fail as a result of the fan getting gummed-up bearings and stalling, leading to it's failure, and the transitive failure of the power-supply itself due to lack of cooling.
I'm assuming you have not moved any jumpers on the motherboard, if you don't know what I mean, then you prolly haven't, and can ignore this possibility.
When you say the fan came to life, you mean the power-supply fan? If you can, check the CPU fan to see if it runs, but judging by your query, it will not. It seems that your motherboard is still not getting power, or slightly less-likely, it is doing nothing with it. If you open the case, some motherboards have a light on them to verify that it is receiving power for the CPU. An LED looks like a 3mm-wide cylinder with a rounded top, made of clear acrylic (LED info) click me, so if you locate one on the board, but it does not light, it is receiving no useable power. Most motherboards have this feature, but don't assume too easily.
If the CPU fan is not running (the "small" one that is on the motherboard itself), it is likely you are not getting power, and almost always, the power-supply is at fault, if not the connection to the motherboard itself.
Sometimes, a motherboard will have bold letters and numbers printed on them to indicate their manufacturer and model number, that will be the largest characters printed on the board. If you can get these, it might help to determine your actual equipment, as opposed to the "brand-name", which is often different.
In almost all cases that I am thinking of, turning on the "mains switch" (the one on the power-supply itself), should not turn anything on except standby power. This switch should remain ON at all times, except when servicing the computer, or protecting it from an expected power surge (a weak but often effective protection against lightning strikes). Mainly, this switch is used so that you can remove all power to the motherboard for service instead of having to pull the connector free. It is a "global shutdown" for the power supply, just like unplugging it from the wall. That switch is not to be used to turn the computer on or off. If the power-supply fan runs when this switch is turned on, before the computer is actually turned on, the power-supply may be at fault.
Most modern computers use an ATX power-supply, which only turns on when the power switch on the front of the cabinet is activated. Otherwise, only standby power is used, to prevent draining of the motherboard's CMOS battery (the watch-battery "button-cell" mounted on the motherboard, to maintain BIOS [Basic Internal Operating System] settings).
Mainly, it seems your motherboard is not getting power, so that is your first step in diagnosis. Be sure that your computer is free of dust, as dust is the enemy of any and all electronics, just as much as heat is. Dust traps heat and moisture, making it the ultimate nemesis of any electronic component, especially those as sensitive as the internals of computers. Dust traops heat, and moisture conducts electricity, creating unintended paths across terminals that should not be crossed. Always keep a clean computer. Dust on the outside means much more dust on the inside.
Failing all the above, keep posting, someone will be more familiar with your issue than I am, I'm just going on past experience.