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#1
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Small Form-factor Power Supply Failure
I've got a Dell Inspiron 531S -- one of those slimline desktops -- that
will not power up, nor show any sign of life. Pulling the power supply and hooking up a known good unit permits the machine to boot normally. Putting one of those power supply testers on the suspect PSU shows normal readings. I know that those PSU testers are not infallible, but I'm surprised that a unit that is so dead *in* a machine, will show any readings on the tester, much less nominal ones. The reasonable move from here, I guess, is to replace the power supply, but I invite you to speculate as to what might be going on to give these apparently discordant readings. Thanks. |
#2
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Small Form-factor Power Supply Failure
Grinder wrote:
I've got a Dell Inspiron 531S -- one of those slimline desktops -- that will not power up, nor show any sign of life. Pulling the power supply and hooking up a known good unit permits the machine to boot normally. Putting one of those power supply testers on the suspect PSU shows normal readings. I know that those PSU testers are not infallible, but I'm surprised that a unit that is so dead *in* a machine, will show any readings on the tester, much less nominal ones. The reasonable move from here, I guess, is to replace the power supply, but I invite you to speculate as to what might be going on to give these apparently discordant readings. Thanks. You need +5VSB to run the front power switch. If the motherboard overloads +5VSB, and the power supply shuts it off, then you won't be able to start the computer. Asus motherboards have an ideal feature for this issue. They have a green LED wired directly to +5VSB, for monitoring purposes. If your motherboard had that LED, you could immediately identify whether the +5VSB was present before, during, and after you press the front panel power button. The green LED should remain steadily lit, and not glitch while doing the test. Other brands of motherboards lack that convenient LED. (My Asrock doesn't have the LED, and I miss it.) The power supply tester likely doesn't have a dummy load for +5VSB. It has a dummy load for a rail like +5V or perhaps +12V. And as such, since the power supply tester puts next to no load on +5VSB, then it isn't a reliable test that the supply is ready to go. In addition to delivering power, a power supply also delivers the Power_Good signal. You can have fans spinning, and solid meter readings on +3.3V, +5V, and +12V, and yet the motherboard won't actually start. Without Power_Good asserted, the motherboard stays in reset. But this isn't your situation, because you can't even get the fans to spin. So you're missing +5VSB - that's my guess. And if there was one of those green LEDs present, it makes it dead easy to check out +5VSB. If you don't have the LED, you'd need a multimeter. Paul |
#3
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Small Form-factor Power Supply Failure
On 6/22/2011 1:09 PM, Paul wrote:
Grinder wrote: I've got a Dell Inspiron 531S -- one of those slimline desktops -- that will not power up, nor show any sign of life. Pulling the power supply and hooking up a known good unit permits the machine to boot normally. Putting one of those power supply testers on the suspect PSU shows normal readings. I know that those PSU testers are not infallible, but I'm surprised that a unit that is so dead *in* a machine, will show any readings on the tester, much less nominal ones. The reasonable move from here, I guess, is to replace the power supply, but I invite you to speculate as to what might be going on to give these apparently discordant readings. Thanks. You need +5VSB to run the front power switch. If the motherboard overloads +5VSB, and the power supply shuts it off, then you won't be able to start the computer. Asus motherboards have an ideal feature for this issue. They have a green LED wired directly to +5VSB, for monitoring purposes. If your motherboard had that LED, you could immediately identify whether the +5VSB was present before, during, and after you press the front panel power button. The green LED should remain steadily lit, and not glitch while doing the test. Other brands of motherboards lack that convenient LED. (My Asrock doesn't have the LED, and I miss it.) The power supply tester likely doesn't have a dummy load for +5VSB. It has a dummy load for a rail like +5V or perhaps +12V. And as such, since the power supply tester puts next to no load on +5VSB, then it isn't a reliable test that the supply is ready to go. In addition to delivering power, a power supply also delivers the Power_Good signal. You can have fans spinning, and solid meter readings on +3.3V, +5V, and +12V, and yet the motherboard won't actually start. Without Power_Good asserted, the motherboard stays in reset. But this isn't your situation, because you can't even get the fans to spin. So you're missing +5VSB - that's my guess. And if there was one of those green LEDs present, it makes it dead easy to check out +5VSB. If you don't have the LED, you'd need a multimeter. Thanks, I'll check it no with my multitester (no LED on the board.) Is it a reasonable presumption that the power supply is the fault if another known good power supply can get the system to boot? I would had to get a replacement PSU only to find out the fault is in fact in the motherboard. |
#4
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Small Form-factor Power Supply Failure
Grinder wrote:
Thanks, I'll check it no with my multitester (no LED on the board.) Is it a reasonable presumption that the power supply is the fault if another known good power supply can get the system to boot? I would had to get a replacement PSU only to find out the fault is in fact in the motherboard. The fault isolation isn't that easy. A motherboard can fail to drive PS_ON# low, when you press the button. So a failure to start can be a motherboard defect. First you check for +5VSB, and then you contemplate "dead motherboard" and an inability to drive PS_ON# properly. The power supply itself can be "deaf" to PS_ON#. Again, with your meter, you check the voltage on PS_ON# both before and after you push the button. Before you push the button, the voltage on PS_ON# should be close to the same value as you observe on +5VSB. After you push the power button on the front of the computer, the level on PS_ON# should drop to logic 0. This would be in the vicinity of 0.4 to 0.8V or so (it can't be exactly zero, due to the drop across the transistor driving the signal). So now you have two things to check. 1) Voltage on +5VSB before, during, and after pushing the button. Should stay "solid" 5V the whole time. On an Asus motherboard, just use the green LED, as a "power quality monitor". The LED should stay solid, indicating +5VSB is stable at all times. 2) Voltage on PS_ON#. If it stays at 5V, before, during and after the button push, then the motherboard may be failing to drive the signal. Perhaps a bad driving transistor or chip. If the voltage is 5V before pushing the button, and close to zero volts after pushing the button, then the motherboard is sending the correct signal to the power supply, but the power supply is not listening. When the power supply has an internal fault, it may "latch" the fault condition, until all power is removed. If the power supply is "not listening", you turn off power at the main switch, wait two minutes, and turn it on again. Then try your test again. Watch for a "fan twitch", which is symptomatic of the supply halting operating early after T=0. If the power supply appears not to be listening (is getting good levels on PS_ON#), change it out and try another. If the motherboard can't seem to drive out a proper PS_ON# signal, then another motherboard is probably the ticket. There are yet other fault possibilities, and for those, I might include the power supply swap test first, before putting in another motherboard. I'd do that, because the power supply swap is easy to do, and since you'll be tearing it apart anyway, probably isn't a lot of incremental extra work. (You tear out half the cabling, to make room for the power supply swap. You won't be wiring all the PSU cables for a quick test, just the main and ATX12V. And if it doesn't work, it's just more tearing to get the motherboard out.) If you need a waveform diagram, to put the above words in context, try page 25 "Figure 7. Power Supply Timing" here. There is information on levels and such, on subsequent pages. http://www.formfactors.org/developer...public_br2.pdf Paul |
#5
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Small Form-factor Power Supply Failure
On 6/22/2011 10:30 PM, Paul wrote:
Grinder wrote: Thanks, I'll check it no with my multitester (no LED on the board.) Is it a reasonable presumption that the power supply is the fault if another known good power supply can get the system to boot? I would had to get a replacement PSU only to find out the fault is in fact in the motherboard. The fault isolation isn't that easy. A motherboard can fail to drive PS_ON# low, when you press the button. So a failure to start can be a motherboard defect. First you check for +5VSB, and then you contemplate "dead motherboard" and an inability to drive PS_ON# properly. The power supply itself can be "deaf" to PS_ON#. Again, with your meter, you check the voltage on PS_ON# both before and after you push the button. Before you push the button, the voltage on PS_ON# should be close to the same value as you observe on +5VSB. After you push the power button on the front of the computer, the level on PS_ON# should drop to logic 0. This would be in the vicinity of 0.4 to 0.8V or so (it can't be exactly zero, due to the drop across the transistor driving the signal). So now you have two things to check. 1) Voltage on +5VSB before, during, and after pushing the button. Should stay "solid" 5V the whole time. On an Asus motherboard, just use the green LED, as a "power quality monitor". The LED should stay solid, indicating +5VSB is stable at all times. 2) Voltage on PS_ON#. If it stays at 5V, before, during and after the button push, then the motherboard may be failing to drive the signal. Perhaps a bad driving transistor or chip. If the voltage is 5V before pushing the button, and close to zero volts after pushing the button, then the motherboard is sending the correct signal to the power supply, but the power supply is not listening. When the power supply has an internal fault, it may "latch" the fault condition, until all power is removed. If the power supply is "not listening", you turn off power at the main switch, wait two minutes, and turn it on again. Then try your test again. Watch for a "fan twitch", which is symptomatic of the supply halting operating early after T=0. If the power supply appears not to be listening (is getting good levels on PS_ON#), change it out and try another. If the motherboard can't seem to drive out a proper PS_ON# signal, then another motherboard is probably the ticket. There are yet other fault possibilities, and for those, I might include the power supply swap test first, before putting in another motherboard. I'd do that, because the power supply swap is easy to do, and since you'll be tearing it apart anyway, probably isn't a lot of incremental extra work. (You tear out half the cabling, to make room for the power supply swap. You won't be wiring all the PSU cables for a quick test, just the main and ATX12V. And if it doesn't work, it's just more tearing to get the motherboard out.) If you need a waveform diagram, to put the above words in context, try page 25 "Figure 7. Power Supply Timing" here. There is information on levels and such, on subsequent pages. http://www.formfactors.org/developer...public_br2.pdf Thanks Paul, abundant as always. |
#6
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Small Form-factor Power Supply Failure
Grinder wrote: Is it a reasonable presumption that the power supply is the fault if another known good power supply can get the system to boot? Yes, unless the mobo has something like a solder crack at one of the header pins where the power switch connects, and the pin just happened to make contact with the mobo when the second PSU was tried. I know that those PSU testers are not infallible, but I'm surprised that a unit that is so dead *in* a machine, will show any readings on the tester, much less nominal ones. Assuming the PSU tester measured the voltages accurately, then maybe the PSU has a bad diode pair or bad capacitor connected to it, as that could make the voltage from them seem OK under light load but droop under the mobo's load. OTOH it's not reasonable to assume the tester is accurate, unless it gives a numerical readout of the voltages. I tried a CompUSA brand tester equipped only with simple lights, and it said my PSU was fine, despite its +12V rail putting out only about 10.6V, not enough to make hard disks spin. |
#7
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