Dead PC Troubleshooting

My computer died yesterday, and I’ll be spending today trying to figure out what replacement parts to order. Yesterday morning, the system just started shutting off randomly. I blew out dust, ran virus checks, and was monitoring power supply voltages and temperatures (no notable problems) when it shut down last night for what seems to be the last time. When I hit the power switch on my machine, case lights & system fans start up for half a second, and then everything quits. I don’t even get to POST beeps or red-green lights.

My initial hunch is that either the power supply or motherboard is bad. My plan is to pull memory & cards, reseat everything, and then curse when that doesn’t work. Next up is swapping in my wife’s power supply to rule out that as the culprit. If the power supply swap doesn’t fix it, I’m assuming it’s the motherboard. Based on the symptoms, is there any reason to suspect the CPU? I don’t have an extra, so I can’t isolate it from the rest of the motherboard.

Any suggestions for diagnosis would be appreciated.

I had something similar a couple months ago on a machine I’d just ordered from E-Bay. I even went so far as to order a replacement CPU, but that was no help, so I returned it to the seller.

If it’s a moderately old PC (>2 years old), then your best bet is to get another similar PC, swap parts to see if you can isolate the problem, and if you can’t, just write it off.

If it’s newer/more valuable, you can do the same steps, but if they fail, take it to a screwdriver shop, who will have a wider variety of diagnostic tools and parts to swap in. It’ll be more expensive, which is why it’s not worth it for a 3 year old PC that’s probably only worth $100-150 in good working condition.

That sounds like power. Your bios/cmos will beep when various motherboard problems are discovered, even if it doesn’t boot up, you have bad RAM, etc. But the fact that it doesn’t come on at all would indicate your power supply is trashed.

A good test would be to disconnect your power supply from the motherboard but leave it connected to your drives and then cycle the power. See if it powers them and stays on. If it doesn’t, you know your supply is bad. If it does, I would try a different supply to rule that out first. (I should qualify that by saying, some supplies won’t power if they aren’t connected to the motherboard. In which case, just try your wife’s supply since you mentioned that’s available to you.)

K

Have you tried resetting the CMOS? Usually it’s a jumper on the m/b. Not sure if that would solve anything, but worth a shot.

I disagree, it doesn’t sound like power. Power supplies are usually a binary thing, they either work or they don’t. The fact that your getting a small hit of power suggests that something hardware is either fried, shorted, or misseated. The first culprit would be any expansion cards and daughterboards you might have (I’ve seen the same behavior dozens of times with defective Dell SCSI backplanes) so go ahead and strip it down and see what happens.

If the problem persists with just mobo and CPU, I would unscrew the mobo from the case, set it down on a nice piece of plastic, and try to start up. That should eliminate any case shorts. Keep in mind that some motherboards need RAM to boot at all.

H.

Thanks for the input, everyone. I’m pretty sure it’s the power supply – the system booted up with no problems when I hooked up another supply. I can’t keep it out of my wife’s machine for an extended period to test long-term stability, but I feel fairly safe ordering a new PSU.

The one strange thing is that Speedfan, the application I used to diagnose my wife’s failing PSU last year, was giving me no abnormal readings on my machine yesterday. Speedfan found that my wife’s 5V line was substantially lower than the rated value, but all my voltages (3.3/5/12) were within 2% of rated specs. Supposedly, Speedfan uses the same sensors the motherboard uses when you check voltages in BIOS. Can PSUs crap out with no preceding voltage deregulation, or are the sensor values from Speedfan unreliable?

I’d love any opinions you folks have in my follow-up post to the Hardware forum, since I’m using this setback as an opportunity to upgrade my system.

Well, the voltage comes out of the transformer (Or transformers), and the transformer may not be what is broken. A bad capacitor might give a short burst of power for a moment and then kill your system. It’s basically a short. A bad capacitor isn’t binary and can slowly get worse without any obvious indication without physically looking at or testing it. But typically one of the first things you’re taught is to test voltages with a multimeter and look for problems there. So you went down the right path, but there cold be other reasons.

K