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Forum Index : Electronics : PC microswitch, no turn on
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| domwild Guru  Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Sorry, off topic! An Intel desktop motherboard is connected to the ON/OFF microswitch. This switch does not switch the AC mains like on older PCs. The problem is that this PC does not switch on despite the fact that the microswitch is OK. A LED on the motherboard is lit indicating that the power supply is most likely OK. Help! Is there an easy way to fix this problem or does it mean the motherboard has gone West? Thanks. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Maybe the power supply itself. The motherboard led is fed via the tiny flyback transformer in the atx power supply. It provides sleep power only. When you press the micro, it effectively shorts the grey lead (from the bunch of leads from the atx pwr supply to motherboard) to ground and this wakes up the main power supply. To test, you can pull off the connector from the mother board and short it to any black lead in the same bundle. This should turn on the fan in the pwr supply. If it does, turn off, plug it back into the motherboard and do the same thing. If it does'nt this time, the filter capacitors in the power supply probably are low in capacitance and need replacing. If the computer has been a bit dicky lately to turn on then this is definately the problem. Thats how the effect can be noticed. If shorting the grey to ground while connected to the motherboard brings the fan on, but not the computer in general, start thinking cpu or motherboard. ........oztules Village idiot...or... just another hack out of his depth |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5019 |
Yeah off topic but I'll let you get away with it just this once. 
Try unplugging the microswitch at the motherboard and shorting the two pins with a screwdriver. I've seen those switches play up, even had a reset switch go short circuit once, took ages to track down why the PC wouldn't boot. If still no go, take the mother board and power supply out of the case, set them up on a table so you have a bare bones PC, ie powersupply, motherboard and if needed, a videocard. Apply power and then try shorting the pins. If it dont boot, your in trouble. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 418 |
It's the Green wire, not the grey one that should be shorted to ground on the PC power supply. Also, a 1A load on the 5V rail is required by the ATX guidelines for minimum regulation... Some PSUs don't fire up if this 1A load is not present. |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Thanks for the advice. And sorry once again for being off-topic; better to ask help from friends rather than strangers! Thanks. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Friends, Have shorted the grey wire to a black one, no start of fan on power supply. Have shorted the green wire to a black one and the fan started up twice but no more! Not that it matters that i have blown something, perhaps the fuse in the power supply; any more suggestions for magic smoke escape?? Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5019 |
Have you got a spare PS you can swap over? The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Dinges Senior Member  Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
Dom, The fact that the LED on the motherboard lights doesn't mean a lot: modern ATX PC powersupplies (PSUs) are in reality two powersupplies in one box: a small stand-by PSU (that lights the LED on the motherboard too when the PC is off) and the real/main PSU. If the PSU *did* turn on properly when shorting the green wire, as it did in your case twice, that means you must have gotten the right wire to turn it on after all. The fact that it worked intermittently would lead me to look into cold solder joints. Perhaps try whacking(*) the PSU a few times and then short the green wire to ground again - if the PSU turns on then, I'd *definitely* suspect cold joints. I'm not sure how handy you are with electronics. As PC PSUs are a dime-a-dozen it may not be worth it trying to repair it (though, I myself consider it an 'honours' thing - buying a new one is like surrendering :) ) As Oztules said, check the capacitors, also in the vicinity of where the green wire goes to the PC. When I was messing with PC PSUs half a year ago one of the capacitors in that area had swelled up (it was still working but the ESR was 150 ohm, so it was bound to fail soon). If you can't measure the ESR of the elcos you could simply replace them all. Bit of a lucky stab but it might work afterwards. But in all honesty, it's probably much easier and cheaper to buy or scrounge another PSU. Cold solder joints and dried up elcos would be my prime suspects. On the bright side, the problem will likely be just the PSU, not the motherboard. Peter ------ * 'whacking' (TM) is technical jargon for the application of precision shocks with an acceleration of 5 +/- .1 G, no more and no less. According to NEN/ISO 1337 the whacks (TM) should be applied at least 3 times but no more than 5 times and should only be administered by qualified service personnel properly trained in whacking. Attempts at whacking by unqualified personnel may damage the product or the whacking hand. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
there are no green wires on my atx power supplies. the grey one is the drive line Village idiot...or... just another hack out of his depth |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Thanks for the further advice. I may have damaged something when I shortened the two wires; perhaps I should have known that a microswitch only turns on momentarily. Thanks, Dinges, for this detailed explanation of "whacking" and checking for swelling of caps. Perhaps I will qualify sooner or later for at least a grey belt in electrickery with everybodys help! If I may ask another question: Just in case my "whacking" or caps changing is not sufficiently professional enough, are those modern ATX power supplies standardised as I may have to buy one, shame, shame! I can see the usual bunch of power connectors for disks, etc. and the motherboard supply BUT there is another one connected to the motherboard. One square connector with a 2x2 or four cable connections also connects to the motherboard and i am not familiar with that one. Two yellow and two black wires. It is an "Intel Desktop Board" and because of this may be non-standard as far as power supplies are concerned. Thanks. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
I'm lost. How come you people have green wires on your atx, and I dont? mine looks like this: 
The greenish looking wire is actually blue. On all of my atx ones the grey is the turn on wire. Is your green in the same pin location? (forth from the left)? ie purple, blue,black,grey....... mystified ........oztules Village idiot...or... just another hack out of his depth |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
Oztules, Different power supply manufacturers use different colour codes. When I was experimenting with modifying PC PSUs I quickly found out one can not rely on the colours of the wires. Clones, IBMs, Compaqs,... all seem to have slight differences to the colour coding. The safest way is to open up the PSU and look at the printed descriptions on the circuit board as to what each colour wire does. From memory, I think in your case the grey wire is on (or near) the location of the turn-on wire. It might however also be the white one. If in doubt google for the pinouts of the connector. The websites I've found in the past with the pinouts also gave colour-codings of the wire, but I quickly learned these were at best only applicable to some PSUs. Even the +5 and +12V wires have different standards (rather: no standard ?). Most clones use yellow=+12V, red=+5V. But I've seen white and blue used for them too. |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
Dom: I doubt you have damaged that PSU by shorting the green wire to ground. The turn-on wire needs to be constantly connected to ground for the PSU to remain on. When you press the momentary micro-switch to turn the PC on the signal goes to the BIOS; it's the BIOS that will actually turn the PSU on (and off) by pulling the green wire to ground; there's no direct link between the on/off button and the PSU. It's logical the motherboard actually signals to the PSU to turn itself on or off. Without it it would be impossible to turn the PC off by software. The 4-pin connector (square, not flat as for HDD/FDD/DVD power) has 2 black wires and 2 yellow wires. They're needed for P4 motherboards, where it supplies power the microprocessor. A P4-ready PSU can be used for a P3 motherboard (you simply leave the square power connector unconnected), but a P4 motherboard needs a P4-ready PSU with that square connector. PC PSUs are a dime a dozen and pretty standardized (ok, maybe not w.r.t. colour coding of the wires :) ) Some manufacturers (IBM, Compaq e.a.) use different PSUs but most generic AT/ATX supplies I've seen have identical physical dimensions. Just make sure the power rating of a new PSU is at least equal to the current power rating. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Thanks Dinges, I have no problem knowing what wires to use on the ones I have, but all 11 ATX I do have, all have the same wiring colours. not a green to be seen. I was gobsmacked when everyone else has green and I didn't. Thought the island had been cut loose from the rest of the world. Just to prove everyone else is correct and all my supplies are weird, http://en.wikipedia.org/wiki/Computer_power_supply clearly shows a green where my grey is. I have 4 violet, they have 1 etc etc. Mine are totally different. anyway, now I know..... no more advice on which lead does what from me any more. ......oztules Village idiot...or... just another hack out of his depth |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5019 |
There's a bit about hot wiring the ATX on my cnc page. http://www.thebackshed.com/cnc/RouterA2.asp Shameless web site plug intended. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Robb Senior Member Joined: 01/08/2007 Location: AustraliaPosts: 221 |
Classic gold! 
I had an ATX supply with faulty caps that had gone high ESR and consiquently damaged the motherboard caps. Replaced both faulty caps on mobo and PSU. Unit now "aok".  |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Thanks once again for the help. Will let you know what happened. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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