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Psu or mobo failure


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#1
ninja88

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Trying to repair my old hp slimline s3200n. It has been collecting dust for over a year so I decided to take another whack at it. The thing won't turn on. No boot, no beeps, no sounds.

I've completely dismantled the whole system and am bench testing. Here are the few confusing results I've been able to gather thus far:

1. When everything is plugged in as normal the system does respond to the power button at all. The psu light is on but is dim. It also does not respond to a rear short of the green and black wires on the atx connector, or a short of the power switch leads on the mobo, except for the psu light turning off until ive removed said short.

2. When I disconnect the 24 pin atx connector from the psu to mobo, I can get the psu, hdd, and optical drive to all turn on with a paper clip shorting the green/black wires from the back. While the psu and drives are all on, I tested the unplugged 24 pin atx cable coming from the psu with a multimeter and the voltages on each of the colored wires is correct according to the charts I've found online. Does this mean the psu is ok? Or does the multimeter testing need to happen while the mobo is attached? The plug is rather tiny and I can't get my multimeter test leads into the back properly.

3. When everything is plugged in AND I leave the paper clip in the back of the atx cable while it is also plugged into the mobo, nothing I do will cause the system to respond, however if I leave it there for about 5-10 minutes without touching it (for example as I write this forum post) it will suddenly turn everything on. There is no signal going to the monitor however. This happens consistently. It actually happened by accident once when the cpu heatsink/fan was removed and I quickly unplugged power to prevent over heating. When I touched the cpu it was warm which leads me to believe the cpu was either on, trying to turn on, or at the very least was receiving power from the mobo.

I wish I could test with another psu but apparently this "slimline" motherboard is wonky and will only fit this psu's plugs.

Can anybody provide any insight as to what the problem is, or suggest any further testing to isolate the problem? My guess is it's either the cpu, or the motherboard but I might be wrong about the psu being fine (please tell me if I am! )..

I might be willing to replace the psu, but a mobo replacement is too expensive and I have no idea how much a new cpu is. But I still don't know which component is causing the problem and I don't really want to replace something just to find out it's the other thing that's broken!

Thanks a ton!

Odd update: Okay new development. For some reason, Now I can turn the system on with a paper clip in the back of the atx connector. Weird thing is, when it powers up, it powers up to an orange led light (normally blue). If I'm remembering right, this is hibernate mode. I then have to either jump the power_on mobo bins if the switch is unplugged, or press the power button if the switch is plugged in (I've tried both ways) and the computer will boot up into windows. Once it starts to boot, I can remove the paper clip from the back of the atx connector and everything stays on until I do a regular shutdown, then everything shuts off and won't turn on again unless I put the clip back in. If I remove the paper clip while it's still hibernating, BEFORE I press the power button to make it boot up, everything just immediately shuts off. This is fairly consistent now. I've reseated the atx cable a few times and sometimes it will go completely dead as above, but I just have to reseat it once or twice again and this paper clip method will work again. I can't just leave the paper clip in indefinitely because when I do a shutdown with the paper clip IN, everything shuts off except for the psu and it stays on until I unplug it! What's the deal! Is my psu on drugs?!

Edited by ninja88, 02 September 2011 - 06:29 AM.

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#2
Macboatmaster

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Firstly - here is the opinion of our expert on PSU issues. - Staff Member - Digerati
To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive ripple and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

Fortunately, there are other options that are almost as good. I keep a FrozenCPU Ultimate PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within specified ATX Form Factor Standard tolerances. Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. All these testers contain a "dummy load" to fool the PSU into thinking it is connected to a motherboard, and therefore allows the PSU to power on, if able, without being attached to a motherboard - great for testing fans, but again, it is not a true load or suitable for conclusive testing.

Note the required voltage tolerance ranges:Posted Image
NOTE: Disregard the -5VDC reading. It is no longer used.

Swapping in a known good supply is a tried and true method of troubleshooting used for years, even by pros. If you have access to a suitably sized, spare power supply, carefully remove the suspect supply and replace it with the known good one, and see if the problem goes away.

I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.

Note the ATX Form Factor standard does not "require" specific color coding for power supply connector wiring. It has recommendations but manufacturers often do not follow them. Sadly, many testing guides or tutorials will refer to wire color only and that can lead to improper testing.

The voltages can be checked in the BIOS Setup Menus of most motherboards but they do not reveal ripple or other anomalies either.

As always, before working in the interior of the computer case, take necessary ESD precautions to ensure static buildup in your body does not discharge through and destroy any sensitive devices. Unplug from the wall and touch bare metal of the case before reaching in. And remember, anything that plugs into the wall can kill. Do not open the power supply's case unless you are a qualified electronics technician. There are NO user-serviceable parts inside a power supply. If you do not have a tester or a suitable spare to swap in, take the PSU to a qualified technician for testing.

Acknowledgement to Digerati

Secondly shorting the terminal 16 Power On - GREEN and a BLACK EARTH is a most unreliable method of testing the voltages as there must to reliably test be a load, ie: a current draw on the PSU.

Without commenting on each and every point on your post, but in view of the last paragraph

I've reseated the atx cable a few times and sometimes it will go completely dead as above, but I just have to reseat it once or twice again and this paper clip method will work again


I suspect a faulty solder joint/PCB track on the motherboard.

However - that is and ONLY ever can be an analysis from what you have written in the post. I know you will appreciate this and diagnosing these sort of faults without actually having the computer in front of you is always difficult.
If it were mine I would either get the PSU tested or try another PSU, However that of course depends on if you can borrow one.
I would also test the case switch - no power is NEEDED - simply a multimeter on continuity/resistance. - open circuit - depress switch - circuit made.
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#3
ninja88

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Thanks alot! I know its impossible to diagnose the problem without having the setup available to test with your own hands. I suspect you may be right in your guess about it being faulty soldering or connections on the mobo. I was just hoping to have a second opinion on what I've found as I'm very much a beginner at this stuff and maybe there was some kind of pattern or tell-tale sign that I was missing.

All odd observations and paperclips aside, the fact that I can get everything to turn on, boot, and run windows normally (even for extended periods of time with multiple reboots)and quite consistently too so long as I don't completely power down the system, leads me to believe that the mobo, cpu, psu and all essential components are actually fine once I get the juice running to them... and it's just a connection issue somewhere. I may be entirely wrong with this, but that's just what logic is telling me currently. Again please let me know if this is a stupid assumption, doesn't hurt to learn for next time!

Is it more likely that the problem lies somewhere on the board itself, or on the atx connector? If atx cable, I would consider replacing the psu to get this bugger up and running...

Unfortunately I can't get my hands on a spare psu to test out. Rather limited in the spare parts/friends I can ask for spare parts department.

This machine really isn't terribly important. It's just been sitting in my office for over a year so I figured it couldn't hurt to play around with it for a while and see if I could get it working again if it wasn't too difficult or expensive. Since it's junk anyway, if you had any other suggestions or ideas feel free to post them up. If I fry something I'll just be left with a scorched paperweight rather than an unscorched paperweight. Just as long as I don't electrocute myself! :)
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#4
Macboatmaster

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Good Luck with it.
We would be interested to hear how you progress.
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