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Can a computer not boot due to low power supply?

Posted on 2011-09-11
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Last Modified: 2013-12-11
Hi,
I have bought (second hand) a Abit N68SV with a AMD X2 7750 processor on it. I tried to start up my system using a old 300 W power supply. The result is that I don't get anything on the monitor, when I start the computer. So I am wondering whether the problem could be with the power supply just not being strong enough to start the system. However everything seems to be ok when I start, as the CPU fan is running all the time, while powered.

Cheers.
r
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Question by:rjorge
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by:Reece Dodds
ID: 36520226
What else are you running?  If it is just mainboard, CPU, ram... then 300W is fine.  Are you certain the power supply is known good (ie. works on a different system).  
With the X2 7750, the CPU fan should spin up full speed for a moment then slow down to the regulated speed during post. If it stays full speed when you get no pic, it indicates a M/B or PSU problem.  If you can get a system speaker attached to that M/B it will help determine any problems via the audible POST beep codes.
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by:Dr. Klahn
ID: 36520233
It's certainly possible, though if the fan is running normally a problem with the 12V is not likely.  The easiest way to find out is to use a DVM to check the actual voltages at the power supply connector on the motherboard.  This site has a pinout of the 24-pin ATX connector.

If you have a PCI POST 4 card, by all means install it and see if the system is POSTing.  If the POST 4 card is counting BIOS stages as the system is powered on, there is at least an indication that the machine is not entirely dead.
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by:nobus
ID: 36521245
you can calculate how much power you need  : http://www.antec.outervision.com/
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by:nobus
ID: 36521250
and did you connect the 4-pin aux CPU power cable?
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by:rjorge
ID: 36522036
@nobus. There is no 4-pin aux CPU pin in the board that I remember. Only the 24 in power supply pin. I think.
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by:asidu
ID: 36522103
You can try to remove connection for CD/DVD drive, remove off any other device and do a test.
The intiial bios flash should be seen on the screen at least. If you cannot see the bios flash
it could be the video out put to the screen need to be checked.

Check also that your RAM is connected correctly. If you have two or  more RAM sticks remove them all off and test the machine with just one. A faulty RAM stick could also give weird results.
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by:nobus
ID: 36523062
did you calculate the needed power yet?
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by:rjorge
ID: 36525228
@all
I've calculated the load. With the CPU+2Gb DDR2+SATA HD+ATI RADEON 5670 the calculation is 219w recommended wattage. I've even tried using it without the graphics card and it still doesn't boot.So indeed the 300w should be enough.
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by:Reece Dodds
ID: 36526478
are you sure you have the 4-pin +12v connector plugged in?

have you tested the PSU on another system or tested another PSU on this mainboard?

Are you doing all your testing with the mainboard outside of a PC chassis?  As to ensure nothing is shorting out.
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by:nobus
ID: 36527537
Are you sure the PS is OK?  test it on another system to be sure, or test with a KNOWN good one
>>  I've even tried using it without the graphics card   <<  difficult to see if it boots..
if you have a V-meter, you can check +5V and +12V on molex connectors

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by:rjorge
ID: 36527663
@all
I have noticed that I only had the main 24 pin PS cable connected. So there was a problem. The board has a 4 PIN ATX 12V slot but the PS only has a 2 PIN cable that fits in half the slots. Can that be the cause?
I've tested the PS on a older system, with a M811 motherboard and a Athlon 3000+ CPU. The system booted well, got image on the screen, although once or twice did not boot well due to the awkward position of the graphics card hanging in the air.
I've performed the tested with the MB on top of a bubble wrap, using my case's board connectors. The board was also out of the case.
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Assisted Solution

by:asidu
asidu earned 166 total points
ID: 36527841
>>4 PIN ATX 12V slot but the PS only has a 2 PIN cable that fits in half the slots. <<
You should get the right power supply connector.

You tested your PS okie with another system not the same one in question.
So what you reckon is correct.
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nobus earned 167 total points
ID: 36528630
you should have all 4 connected
you can use an adapter cable for that :  http://www.performance-pcs.com/catalog/index.php?main_page=product_info&products_id=196
or here :  http://www.ncix.com/products/?sku=15986
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Author Comment

by:rjorge
ID: 36533062
Hi all,
a small correction. The connector on the board is a 8 PIN, but the cable I had is just 4 pin. Still, I think what nobus said applies. I'll get teh adapter cable and if that doesn't work will simply change the PS.
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Assisted Solution

by:Reece Dodds
Reece Dodds earned 167 total points
ID: 36534226
I should've googled a pic of the mainboard before I commented... the 8-pin +12v connector is used for high end CPU's and overclocked rigs.  You will be fine to use the 4-pin molex and 24-pin molexes that standard PSU's have!
There is onboard video so you don't need an expansion graphics card.

So, have the mainboard out of the chassis.  

Reset the CMOS - use the CMOS RST headers on the mainboard, or, take out the battery, short the two metal parts of the battery socket together for 5 secs, put the battery back in.

Connect just the CPU, RAM (only 1 module) and PSU (which you've already tested OK on another rig and said is working ok)...

There is no system speaker on the mainboard so raid one from another PC temporarily.

Use a screw driver to short the two pins of the on/off header (where you normally plug in the power switch - on this board it is the RED ones) of the mainboard and if the mainboard is not faulty, you should have lift off!

If all you get is a spinning CPU fan and no picture, you have a dead mainboard, bad cpu (unlikely), or faulty RAM.  If you have a system speaker attached, an audible long beep will tell you if the ram is not detected properly.

A new PSU is only what, $50, so no biggy, but buying one is a waste of time if the mainboard is faulty.
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by:nobus
ID: 36534322
here you have one 8-pin :  http://www.directron.com/ad202.html
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by:rjorge
ID: 36569742
Final outcome. I've tested with a newer power supply and the MB did not boot. It was crooked.
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by:nobus
ID: 36571907
tx for feedback
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