ATX PS problem

Discussion in 'General Electronics Chat' started by snav, Feb 16, 2014.

  1. snav

    Thread Starter Active Member

    Aug 1, 2011
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    Not sure where to ask for help so here goes.

    I have two desktop ATX PSU which won't turn on the PC. One was the original and the other is a replacement. Both light the standby power light on MB when installed but won't start the machine when 'ON' button pushed.

    I checked all the connectors while manually turning the supply on using jumper and every voltage was in spec. I added hard drives and rechecked and still all voltages in spec. I am wondering if the circuit that says power is ok comes from the MB and maybe it's the culprit? Is there a test for the power OK that can be done with a DMM?

    I took another PSU, connected everything and the machine runs fine but the input receptacle/switch is wrong location for opening.

    I'm leary of buying another replacement but I would like to put the case back together
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    PSU 1: computer does not turn on.

    PSU 2: computer does turn on.

    Conclusion: PSU 1 is a problem. When reality meets opinion, reality always wins.
     
  3. snav

    Thread Starter Active Member

    Aug 1, 2011
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    PSU 1 & 2(oem parts) computer fails to start.

    PSU 3 works.

    PSU 1 voltages in spec.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Fact 1: Computer does not turn on when using PSU 1.

    Fact 2: PSU 1 voltages are in spec.

    Fact 3: NOBODY gives rat's ass that PSU 1 voltages are in spec. Why? See Fact 1.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Just because voltages are in spec when nothing is connected, it doesn't mean those voltages will be in spec when a load is attached.

    When load is not attached, the voltages generally run 2 or more volts high on the 12 volt rails. The 5 and 3.3 volts are questionable (may be slightly high and may be spot on).

    If you have a big video card, you may be drawing more amperage than the PS can support so the PS goes into a protection mode.

    So many things could be a problem it is hard to diagnose.

    Also, if you only have one accessory that draws a huge amount of power, you have to make sure that the each 12 volt rail can support the current draw of your accessory (usually a Graphics card). That is, some power supplies list huge wattages but the power is split over more than one 12 volt rail. Some supplies have only one big 12 volt rail. Look carefully at what you need and careful about what you use.

    Details of power supplies are usually right on the case. One rail or two is also listed. Post photos of your supply's label if you have trouble reading it.
     
  6. snav

    Thread Starter Active Member

    Aug 1, 2011
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    OK, So the cpu is 65w tdp(65nm Cedar Mill P4 3.4ghz), 3gig pc2700 mem, 1 250g sata hd and pci bus powered Graphics(gs8400) and sound cards(Audigy). Oem PS rated for 305w, voltage with loads from Drives (two for more load) causes 12 v to go from 11.72 to 11.85 5v goes from 4.934 to 4.925. Running from externally mounted 350w supply the total load is 100w after startup and peaks at about 134w during startup.

    I was actually hoping to find someone that knew of the power OK is a Motherboard signal to the PS. It looks like it is. But maybe it is supposed to come from PS to tell MB ready to start?

    Thanks in advance for any help.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Neither.

    PSU is too dumb to offer the service. No microprocessors.

    Mobo simply does not give a rat's a$$. It got the microprocessors, but it does not care, all it needs is power. Give it power, it turns on. Don't give it power, it does not turn on.
     
  8. snav

    Thread Starter Active Member

    Aug 1, 2011
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