PC Turns On Automatically Once Power Supply is Plugged in.

What make/model of PC ??  

What make/model of power supply?

What was the problem that caused the PC to need repair?

... that data will make it a lot easier to suggest what might be happening :-)

EMachines T2778
Bestec 250Watt PSU
The problem that caused it to be needed it be repaired is that the pc had no video and it turned on by itself.

logical guessing here..

CPU
Memory
Video

stuck power switch on the case

System Info
http://www.emachines.com/support/product_support.html?cat=Desktops&subcat=T-Series&model=T2778

CPU:  AMD Athlon™ XP 2700+ Processor (2.167GHz) with QuantiSpeed™ architecture
Operating System:  Genuine Microsoft® Windows® XP Home2  
Monitor Bundle:  eMachines 17" eView™ 17f Monitor
(16" Viewable, 0.27mm dot pitch)
Chipset:  NVIDIA® nForce™2
Memory:  256MB DDR (PC 2700)
Hard Drive:  80GB HDD
Optical Drive:  48x Max. CD-RW Drive; 16x Max. DVD Drive; 3.5" 1.44MB FDD
Video:  NVIDIA® GeForce4™ MX graphics (1 AGP 8x slot available)
Sound:  nForce™ 6-channel Audio
Network:  10/100Mbps built-in Ethernet
Modem:  56K ITU v.92-ready Fax/Modem
Peripherals:  Premium Multi-media Keyboard, 2-Button Wheel Mouse, Amplified Stereo Speakers
Ports/Other:  6 USB 2.0 ports (2 on front), 1 Serial, 1 Parallel, 2 PS/2, Audio-In & Out
Dimensions:  7.25" W x 14.125" H x 16" D

I'm thinking that 250watt PSU is not doing the system justice.  so try this...

disconnect your drives (floppy, hardrive, cd/dvd).. power your system up.. got picture???? no?  reseat your CPU and RAM.. try again?  nothing?

Reseat your video card? working??? No???

Replace that power supply...

Ok this is the news, the 250 watt PSU went kaboom, so I replaced it with a 300W i had lying around, now when I plug it in nothing happens, i push the power button the cpu fan spins up for about 1 seconds and thats it, the only thing hooked up is the motherboard...

E-machines doesn't provide much technical data.   A irwinpks posted above, they simply list the basic specs of the system, and have a user guide available for download(http://downloads.emachines.com/userguides/Desktop_UG_Nexgen2_3_en.pdf), but no service manual or other more detailed specifications.

I agree a 250w supply is marginal -- but, like virtually all low-cost vendors, it's adequate for the system AS DESIGNED.   Does this system have any add-in cards (e.g. an add-in AGP card instead of the on-board video)?   If so, try it without those cards.

If the CPU was working, I'd expect at least a "beep" code as the BIOS began executing the POST code.   Since you've tried a new motherboard with no change in symptoms, it seems likely the POST is simply not executing ==> thus a bad CPU.

There ARE other possibilities, but that would be my 1st guess.

kaboom went your motherboard too???

do you have at least RAM & CPU installed?

"... a 300w I had lying around ..." =>  I presume you connected not only the ATX power connector but also the auxiliary power connector ... correct??

this isnt a p4...

... and it WAS a standard ATX power supply ==> not a Dell unit ... correct again ??

switched out the ram to a new stick, and yes the cpu is installed.

yep, a rosewill psu got it off newegg not too long ago.

blown board.......also make sure that the PSU is truly an ATX

heres the psu Im now using: http://www.newegg.com/Product/Product.asp?Item=N82E16817182001
and i doubt its a blown board, the boards brand new... it doesnt smell burnt, and the fan does spin up for a second or two.

irwinpks, the mb didnt go kaboom just the psu.

Are you using the onboard video?   ... if so, do you get any video if you use an AGP card?

... if neither, I'd still say CPU => since there are no signs of the POST code executing, and you've used two different motherboards with the same symptoms.

nope onboard video. I think your right must be the cpu.

just plucked the cpu, still same thing fan spins up for a sec or two.

If the PSU is shot, the likelihood that it would trash other connecting components is high... Considering garycase's most recent comment....it could be and/or problem with the CPU

i would pluck my 150 dollar antec phantom, but its a pain in the rear to take out lol. i was wondering if it could be something with the front panel connectors, i.e. the power button.

like a stuck switch?.. just attach your power connector.. nothing else  test that.

no other cpu for this socket.

i dunno i need to see if this new mobo is fried...

it's going to be hard to tell, if you don't have spare parts avaiable..

as garycase mentioned, audio beeps during POST are a good sign of the CPU functioning... if a replacement doesn't correct it...and the PSU is known to be good... and you don't have a stuck switch..

motherboard.

It could, of course, be a problem with the power switch -- but those are very rare.  A power switch is a simple momentary-contact switch that carries almost no current.   What does the system do if you do NOT connect the front panel power switch to the motherboard?   ... and is the front panel connector a multi-pin header or individual connections?   If individual connections, try just (very carefully) shorting the pins for the power switch with a screwdriver and see if the system behaves the same way.   If so, the problem is definitely NOT the front panel switch.

i have two motherboard to work with, two sticks of ram, two psu's, im thinking it must be the cpu, but what would cause the fan to only spin for about a second? even with the cpu plucked?

nature's calling, be back in 10min...gary is good to help ya' for now.

Consider the normal sequence of events:

Motherboard powers up -> resets CPU -> CPU begins executing BIOS POST code -> system boots if no errors

It would appear that what's happening here is the CPU never starts execution ... so:

Motherboard powers up -> resets CPU -> CPU doesn't do anything

Not sure why the system shuts down at that point;  but it's not a surprising sequence.   There MAY be some other cause;  but with the same symptoms on two motherboards;  with two different sets of RAM; and two power supplies;  it sure sounds like the CPU to me.

$14 PSU? Yikes. To avoid further damage, get a QUALITY POWER SUPPLY & surge protector. (Details in my post at  https://www.experts-exchange.com/questions/21856241/computer-restarts-on-startup.html)

irpwinks i messed up i was trying to split the points between you and gary, if you want points ill make another post.

Just post a note in Community Support asking for the question to be re-opened => then you can close it again as you see fit :-)

... also, why did you close the question?   Is it resolved?  ... or are you simply confident it's the CPU ??

No problem...accidents happen...

go here.
https://www.experts-exchange.com/Community_Support/

post in the title... "Please Reopen to Split points"

then put in the body.
https://www.experts-exchange.com/questions/21877323/PC-Turns-On-Automatically-Once-Power-Supply-is-Plugged-in.html#16849509

usually in an hour a moderator reopens the question. Then locate ABOVE the comment box  SPLIT POINTS.

------
BTW

Thank you!!

Aloha from Hawaii,
Irwin

I closed it because theirs not much else we can do, is their?

I have a quick question, I want to test a motherboard without putting it in the case, Can i just sit it on a cardboard box and power it up or do i need to sit spacers in it and then sit it somewhere?

You can sit it on a cardboard box and power it up.

you can...but carefully... make sure there is no bare metal lying around the board.  I usually stand the board on its side after everything is plugged in.

watch out for smoke.. ;-)

thanks

ok i gotta run but i will post something most likely tommorow once i try some new things on it. Thanks for all the help gary and ir.

You'll be a lot more helped, in future at least, if you get a better power supply :)
I've seen too many computers fail because people don't consider the psu important enough to not buy a guaranteed unit. But it supplies power to ALL components, so clean, stable power is critical.

Will system power back on using power switch or do you have to unplu and replug power cord?

Usually, a PC will power on and off normally with a failed CPU provided is electrically connected.  There are always anomalies, but having to plug and unplug to power on momentarily normally denotes a failed capacitor.

willcomp - I had the same thought initially => but since he tried a new motherboard I dismissed it (odds of having a bad cap on two different boards just seem too small).

I agree odds are small and nForce2 boards should be past the "bad cap" era.  However, symptoms are those of a bad cap.  Only way to tell if CPU is cause is to swap CPU.  A PC that partially powers can be difficult to troubleshoot as we all know.

Please forgive typos in previous response.  Got interrupted several times and lost my way.

regardsless what the root of the problem is strongly recommend the owner to change the power supply.

I've had issues with bad or mis-cabled IDE cables doing this, too. Try pulling everything off the board but the video, ram, and power and see how it behaves. And pull power from the drives.

i assume this person still needs help?

I don't think I caught anywhere about checking to see if the LED wires are plugged in right or not.  Have you checked the mobo manual to see if their all attached correctly on the motherboard?  I had this exact problem trying to plug in led wires to power on my pc and I didn't know the motherboard model, ect.  trial and error was my solution :)

The problem seems to be caused by either a bad cpu, or a bad motherboard. Im out of supplies to trial an error.

1.) The LED Wires were not the problem, The were set correctly as shown through the motherboards manual.

2.) The IDE Cables were not the problem, I set the system up barebone, CPU, Memory, and Power. Still The Problem occurred.

3.) The chances its the motherboard are slim considering I tried 2 Different motherboard, One being brand new.

Conclusion:
  My best guess would be the CPU is faulty. To replace a CPU of Socket A for 60 bucks, would not be worth it for the price you can get a pc now. I am going to close this thread with the conclusion that The CPU is the root of the problems and replacing it would fix the problem considering everything else has been replaced at least once already Including the RAM, PSU, Motherboard, and even swapped it into a different case. I will be awarding the points to the two people who gave their all and were most supportive on this thread. I thank everyone for their help and wish you guys the best of luck.
       
         Yours Thankfully,
                Mechy

cool. thank you!