Weird power issue

By UnseenGuardian ยท 9 replies
Jul 25, 2010
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  1. Hey Guys,

    Okay I'm new but I have done my homework, have read close to 100 threads now before posting.

    First off, here is my specs (yes I know it's old):

    MSI K8N Diamond Plus
    AMD Athlon 64 3700+ Socket 939
    OCS Gold Edition PC4000 Dual Channel DDR(500mhz)
    Silverstone 560Watt (but more importantly, 38Amps on 12v rail)
    2each Geforce 7600GT in SLI

    Okay, now as I said I have done much research and think I have a faulty mobo, not positive though. I need to pinpoint the problem and also figured out WHY this happened.

    My computer, when pressing the power button, starts up for 1 second then shuts off. All fans spin, etc, but no beeps (presumable because its only on for 1 sec). After this I see that the LED on the back of my PSU is red, when normally it is either Green or Orange. I have found out red might indicate circuit protection has been initiated.

    Okay, so maybe I have a short.

    I pull out the components to do a bench/breadbord test. I start with the bare essentials. I swap out ram sticks, one at a time to see if they might be bad. I try another socket 939 processor. Still same problem, starts up for 1 sec then immediately shuts down.

    I try jumping the pins for the power switch, thinking the switch might have been faulty, but it is not, same problem. I try jumping the 'green' wire and a ground on the 24 pin mobo connector, to test the PSU. It comes on and stays on until I disconnect these jumped pins. So it would appear the PSU might be okay (was not a load test, but still...)

    Now I am down to possibly the mobo, I inspected with a flashlight and magnifying glass, no bad capacitors, no burnt components that I can tell.

    Thinking my next step is to dive into PSU testing with my multimeter, but I will have to read up on that first.

    Here is the kicker: When I disconnect the 8pin CPU connector (the one that plugs into the mobo to give the CPU its power), when that is DISCONNECTED, then the computer will power up and stay powered on. Once plugged back in, however, and tried to start it does the '1sec on then dies' thing.

    So, i guess there might be short circuit in the cpu/mobo/psu wiring somewhere, just not sure where, (Sadly thinking the mobo is bad)

    Seriously guys, anyone have any ideas as to what could this be?
  2. dividebyzero

    dividebyzero trainee n00b Posts: 4,891   +1,264

    I'd say that your intuition is correct. Either the board has a failure in it's power delivery circuitry ( EPS 12v 8pin socket/burnt out traces/chokes etc.) or possibly the wires in the PSU's 8-pin plug aren't seated correctly. I would get the PSU power delivery checked with a voltmeter just to make sure it's A1 (assuming the 8-pin plug is ok) just to take that out of the equation.
  3. captaincranky

    captaincranky TechSpot Addict Posts: 13,012   +2,536

    I really think you should substitute the PSU for a known good unit, before you condemn the motherboard.

    The reason I say this, is that connecting the CPU causes quite a power drain, which may be enough to trip a failing power supply.

    A bad CPU is certainly possible, especially if this machine has been overclocked or continuously run hot, as in having the air intakes blocked, or failing to clean out the CPU heatsink fan once in a while.

    But, the normal likely hood of failure is in this order; 1. PSU. 2, motherboard, 3. CPU

    In any event, try and find a PSU with protection circuitry, in case it is actually the board.

    As I said, the PSU is the number one failure. Well, except in the case of older Emachines, which usually blow up the board and the PSU in one shot.

    Are there any bulging or leaking capacitors in your board? Check carefully, especially around the CPU, those are the caps for the voltage regulator circuit.
  4. UnseenGuardian

    UnseenGuardian TS Rookie Topic Starter

    No bad components as I mentioned previously after a thorough inspection (caps are all ok).

    I am worried if I buy a new PSU, and plug it in I don't want to harm it if there is a short in the mobo somewhere...

    How do you tell if a PSU has circuit protection? Is it a feature listed on the box as a selling point?
  5. captaincranky

    captaincranky TechSpot Addict Posts: 13,012   +2,536

    If the PSU shuts down when you plug in the CPU, then I suppose you could check for a short with an ohm meter, by sticking it across the black and yellow terminals of the CPU ATX plug.

    One problem, I have no idea what the resistance measurement should be. One supposes it varies in different board CPU combinations

    As to PSUs with protection circuitry, most decent brands should have it.

    There's always going to be an element of monetary risk when diagnosing a problem such as this.

    If the new PSU blows up, then you trash the machine. I dunno, maybe somebody has a better idea.
  6. dividebyzero

    dividebyzero trainee n00b Posts: 4,891   +1,264

    Most good PSU's have circuit protection. Example here - the Silverstone PSU linked has Over current protection (OCP), short circuit protection (SCP), Under voltage protection (UVP), Over voltage protection (OVP) and no (zero) load operation. Some PSU's will also feature Over power protection (OPP).
    Listing (and indeed, having) these protection features indicates a high quality PSU. If these features aren't listed by a manufacturer then rest assured that the features will not be available on the unit.
  7. UnseenGuardian

    UnseenGuardian TS Rookie Topic Starter

    Thanks man for that information. I will go searching for a new PSU with this in mind.
  8. UnseenGuardian

    UnseenGuardian TS Rookie Topic Starter

    Funny you mention testing the resistance. I actually did that today.

    I checked the resistance between different grounds and 12v power pins on the 8pin cpu power connector.

    What I found was that each one in all combinations (different power and gnds) had about 60-80ohms of resistance. Problem is, though, like you mentioned I also do not have any specs to know if that is good or bad. I somehow think that you don't want any continuity between power and grounds, but maybe that is only showing because it is disconnected from the mobo, or because the PSU is not on, I don't know...

    I am thinking of calling around some computer repair shops tomorrow to see if any techs have heard of this issue. If my PSU has circuit protection, as it seems like a decent unit, then maybe it is good and the mobo is faulty.
  9. captaincranky

    captaincranky TechSpot Addict Posts: 13,012   +2,536

    A CPU is a current based device, operating at low voltage. Hence one would not expect the resistance to be terribly high across its power terminals. In fact, a high or infinitive resistance measurement would be indicative of an open circuit.

    The Vcore of a CPU is about 1 volt, while the ATX connector is 12 volts, meanwhile the CPU may have a TDP of over 100 watts. If watts equals volts times amps, you should quickly see that the current (amps) would be quite high. As a necessity of Ohm's law, the resistance across the ATX terminal wouldn't be terribly high.

    In any event, DeeBeeZee has provided insight as to protected PSUs, so the only logical course of action is to replace the PSU, and fire the system up. No booty, bad boardy.
  10. UnseenGuardian

    UnseenGuardian TS Rookie Topic Starter

    Thanks for your help guys. I think I have done what I can and you guys have helped out with information quite a bit.

    Now I just have some decisions to make..

    Thanks again.

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