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HP Compaq: DOA - Obvious Things To Check?
Family members HP Compaq/Older Athlon X2 just went belly-up.
Only sign of life is a single green led glowing on the mobo: no power supply fan, no CPU fan, no response to DVD drive's eject button... First thought was power supply, but when I swapped in a known good power supply, no change. Then I figured "Maybe the on/off switch"... so I pulled out the switch and used a knife to jumper the two leads coming into it - figuring that would duplicate the action of the switch... but no luck there either. Unless somebody has a better idea, next thing I am going to do is to revisit the power supply and verify correct voltage on each output lead. -- Pete Cresswell |
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HP Compaq: DOA - Obvious Things To Check?
(PeteCresswell) wrote:
Family members HP Compaq/Older Athlon X2 just went belly-up. Only sign of life is a single green led glowing on the mobo: no power supply fan, no CPU fan, no response to DVD drive's eject button... First thought was power supply, but when I swapped in a known good power supply, no change. Then I figured "Maybe the on/off switch"... so I pulled out the switch and used a knife to jumper the two leads coming into it - figuring that would duplicate the action of the switch... but no luck there either. Unless somebody has a better idea, next thing I am going to do is to revisit the power supply and verify correct voltage on each output lead. The PS_ON# signal has lost its ability to pull to a logic low. Just a guess. Process starts with +5VSB. There must be +5VSB for the system to even think about starting. The power supply has two parts. A section with just +5VSB on it. A section with the major power rails (3.3V/5V/12V etc). We know the +5VSB is working, because you have a green motherboard LED. The LED also tells me the motherboard is an Asus OEM. As Asus is one of the few manufacturers who provides a monitoring LED (green in color) for +5VSB. The +5VSB feeds the LED directly. If the LED glitches, doesn't glow at full brilliance, these are cheap indicators of trouble with the "supervisor" half of the ATX power supply. So we suspect your machine is getting +5VSB. You also did the right thing, by checking for power switch. But there is an additional step. If the Power button is "jammed on", the machine can't come out of reset. What you really want, is to disconnect the twisted pair for the power button, and do a momentary touch with your "shorting wand" on the two terminals. If you own a separate push button (I have one here for this test), you can slide your "known good" switch and cable in place of the one provided by the computer case. Once you do the momentary contact thing, PS_ON# is an open collector signal on the main cable. It pulls down a pullup on the PS_ON# signal on the PSU. If you had a multimeter, you'd check the voltage on the line. A typical failure, is (for some reason) the transistor on the motherboard, can no longer pull the signal down. Maybe the signal drops from +5V (PS_ON# not asserted) to around +2V. This is not sufficiently low to be recognized as a logic 0. You might want a voltage in the 0.4V to 0.7V region as a good solid trigger. The user can short PS_ON# to ground, and force the PSU on. And that test then is a workaround for a defective motherboard PS_ON# driver. The computer will likely start. OK, so what's the downside of operating this way: 1) You've done an override on THERMTRIP. The computer cannot now shut itself off in the event the heatsink on the CPU falls off. 2) You've disabled the ability of the computer to do "Soft Off". When you select "Shutdown" in the OS, the Soft Off shuts down the main power rails. With the shorting jumper in place on PS_ON#, now you need to remove the shorting jumper so the computer can go off. But for the purposes of a test, jumpering PS_ON# to GND will help work around a defective PS_ON# response. In fault analysis, you can either isolate to the nearest subassembly, or the nearest three components. The three components in this case might be motherboard, PSU, cabling. You know that something is wrong with PS_ON#. Motherboard end might be "too weak". PSU end might be "too strong" (that happens sometimes). Since you've swapped in power supply, that helps eliminate the "too strong" theory. Leaving motherboard or some cabling flaw. So you're getting pretty close to blaming the motherboard. Not a lot wrong with it, but still a nuisance. And this failure is *entirely* too common. This circuit should not fail, yet there are way more failures than should be happening. And I don't have any convenient whizzo theories as to why. When I was a new grad, an engineer at work called me over to review something he was doing. He had premature failures in the very kind of circuit you're looking at. It was wired-OR logic and open collector driving chips. I took one look at it, and I could see immediately, he was forcing the driving chip to sink 3x the rated current. The chip would last pretty well three days doing this, before it would "take a crap". He would put in another spare, and it would run for another three days or so. That's an example of "abuse" and since this was a prototype, nobody got hurt. But it was one of my first experiences with what it takes to kill a circuit like that. And I appreciated the opportunity to see theory turned into practice (just what could an abused chip take). I was surprised it was that sensitive. This should not be happening on a modern motherboard, and the chip is nowhere near the abuse level. At one time, boards around the 2000 era, were using stuff like 74F07 or the like. Huge drive capability. Now, the driver is gutless, but should still be well suited to the task (in other words, no spec violations should be present on either end of the implementation). Which is what makes it hard to understand why the drive is defective on that signal. Sticking your multimeter on PS_ON#, should tell you whether it's making it all the way down to 0.4V to 0.7V or so. The voltage is the saturation voltage of a bipolar junction transistor. That's what that voltage represents. And chances are, the transistor involves is "kinda" working, and you'll see 2.0V on it or so. Which isn't sufficiently low to kick on the PSU. If the power connector is 24 pin, you can get the pinout info here. Or here. http://www.formfactors.org/developer...public_br2.pdf http://www.playtool.com/pages/psucon...onnectors.html HTH, Paul |