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Old 2nd July 2010, 01:08 PM   #1
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Talking Old mini Atx Powersupply

Hi im kinda new at this so if you need more info just say...

I got an repair, i want to do on an OLD NEC pc, the Powersupply is faulty and its a mini atx so i really need to repair it and not replace.

the Powersupply was dead. i reaplaces one of the caps2200 uf 10v, now the powersupply starts and dies again.

can anyone help me with this?

Kind regards
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Old 2nd July 2010, 02:35 PM   #2
wwenze is offline wwenze  Singapore
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More detail regarding "starts and dies again" - did it destroy itself again or just shut down? After how long? If you short the green wire on the 20-pin connector to ground to start the PSU without a load does it "die"? While running what are the voltages?
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Old 2nd July 2010, 02:40 PM   #3
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by die i mean it starts and then needs to be unpluged to try and start again. dont have other words to explain it. The supply runs for about a half second without any load. "shorting the green wire with ground"
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Old 2nd July 2010, 09:36 PM   #4
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If it needs to be unplugged to restart then it's tripping a protection. Check the rectifiers in the secondary, it's very likely that one of them is blown.
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Old 2nd July 2010, 11:14 PM   #5
TechGuy is offline TechGuy  United States
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Most PC power supply need load to function. If you are trying to test the PS without a load, then it will do what you experience, shutdown after a second or less. If you don't want to connect it to a motherboard try connecting some 1-2Watt resistors in the 50, 100 ohms range on the 5V and 12V connections. Most of the time this works, Some of the more expensive PS actually have multiple regulators which prevents this method from working, unless you connect all many more resistors.
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Old 3rd July 2010, 06:17 AM   #6
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thanx, will check the tripping protection, dint think of that. *facepalm* i tested it with load aswell, both ways it does the same thing, if i test the voltage when it start it goes on and off, then i unplug and it does the same, really sounds like the tripping protection, i will check it out on monday. thanx for the advice!
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Old 3rd July 2010, 06:11 PM   #7
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Quote:
Originally Posted by TechGuy View Post
Most PC power supply need load to function.
On all the power supplies i've seen in the last 5 years their internal fan is enough load for them to start.
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Old 5th July 2010, 12:30 PM   #8
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i checked the whole supply, cant find any faulty rectifiers, do you maybe know how to test a CEP703AL mosfet transitor? i found 1 that test correct, but when it switches i get a dead short over the Drain and source.. should it be like that?

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Old 5th July 2010, 03:03 PM   #9
Eva is offline Eva  Spain
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Almost any power MOSFET (enhancement type) will measure as a drain to source short when the gate is charged above 3 to 5V. The short should disappear completely when the gate is discharged to 0V, leaving just the body diode.

Using lower gate voltages, like 2V from the diode test function of a multimeter, will only turn on most MOSFET partially (appearing as an high d-s resistance or a weak current sink), and won't be enough to turn on some at all.

The diode test function of most multimeters comes quite handy for semiconductor testing, it usually works just as 2mA current source flowing from one probe to the other, and the display shows the voltage drop produced on whatever you test by those 2mA, as long as it's below 2V.
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Last edited by Eva; 5th July 2010 at 03:07 PM.
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Old 5th July 2010, 03:19 PM   #10
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thanx, then there is something wrong somewhere on the line if that mosfet is working, i took it out and power supply works fine...its part of the protection.. hahah just hope it wont explode or something. thanx for telling me a bit more about Mosfets, i appreciate it im starting to learn myself more about switch mode supply's because most of the amps i work on nowadays got the switch mode's, and i hate not being able to fix it.
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