Need help trouble shooting a PC start-up problem

I just replaced my venerable Dell 8400 (I know, I know, but it was a workhorse) with a new tower. I was planning to give it to a friend. I air dusted it, but then when I plugged it back in it won't start up. I have a blinking green and amber light (which usually indicates a PSU problem), but it worked 10 minutes before I cleaned it. Figuring I knocked something loose I checked all connections, still nothing.

Well, not totally nothing. It constantly whirls up and then dies immediately, like it a car with a bad starter that can't catch. The PSU fan and all other case fans spin up for half a second, then die. Spin up, then die, spin up, then die, and on and on. The mother board light is solid green. I detached the power from everything and connected things one at a time trying to pinpoint something but no luck.

It's a Dell 8400, 3.8Ghz. Been running an upgraded PC Power & Cooling PSU (Silencer 470 Dell) since 2006 with no problems. Powering an 8800GTS 640MB, sound card and two SATA drives.

What's my next step? Thanks for the help!

Probably the PSU - you can do the paper clip test to be sure. I had an Enermax that was rock solid until it died a silent death. PC turned on the night before, next day nothing. Motherboard showed a green light as well. Maybe it was mad at me for butchering it's +12v wire while it was still under warranty?

Air dusting can do some weird things if a piece of dust gets somewhere it shouldn't - inside an expansion card slot, for example. If you dusted it with everything still plugged in it's probably not that. Otherwise, check all the slots/sockets to make sure there are no bunnies in there.

Yeah, that might be a short, the power supply refusing to power up because it'll fry something if it does. I'd check very carefully that you didn't blow something where it shouldn't be, and also that all the cards, memory, and cabling are firmly seated.

It could also be that the supply just chose then to give up the ghost. This would be especially likely if you leave the machine on all the time, and this was the first time you'd powered it off in awhile. This use pattern isn't hard on supplies or anything, but it could have failed somewhere internally during its last extended run, and then is only detecting the problem now.

If you power it up and down frequently, then that's still possible, but far less likely.

Malor wrote:

This would be especially likely if you leave the machine on all the time, and this was the first time you'd powered it off in awhile. This use pattern isn't hard on supplies or anything, but it could have failed somewhere internally during its last extended run, and then is only detecting the problem now.

If you power it up and down frequently, then that's still possible, but far less likely.

Actually, that is exactly the case; this machine was left on all the time. Very interesting. And thanks for the paper clip test thing. That's cool! An interesting note, for some reason when the video card power supply is unplugged, and I try to start it up, I get a "beep code" from the mother board. It's a steady, slow repeating, "beep.. beep... beep, ect." That doesn't match up to any of the beep codes in the Dell user manual. Does that mean anything?

EDIT: Looking at some cheap PSU replacements, best I can find that will fit in the 8400 is a 480W PSU that does 24A on the 12v rail. The 8800GTS requires 28A. Think I'll be okay (won't be running the video card maxed out or anything).

An interesting note, for some reason when the video card power supply is unplugged, and I try to start it up, I get a "beep code" from the mother board. It's a steady, slow repeating, "beep.. beep... beep, ect." That doesn't match up to any of the beep codes in the Dell user manual. Does that mean anything?

Not sure. It might be the motherboard detecting no video; I don't remember if those old 8800s would work at all without the power plugged in. The power supply may be starting up because the heavy load of the 8800 is gone, and then the motherboard is noticing that the video card is gone, but there's no way to to be certain from here.

Remember that when you lose a power supply, it's quite common to lose a bunch of other stuff. And components can be weakened by the zorch, and then fail sometime in the next couple months.

For the replacement: according to Wikipedia, that's a 150-watt card. The TDP on your processor is 115 watts. Assuming you don't have an unusual number of peripherals, 50 watts should cover the motherboard, memory, and a hard drive. So you're looking at about 315 watts, roughly, or about 27A.

edit: oops, I misremembered which figure was which, I thought the new supply was 28A, not 24. 24 is not enough, and it's not really safe to use. You'd want 28A if possible. But it would be safest to go a few amps higher, because power supplies lose about 10% of their rated amperage over time.

Don't forget that Dell used oddball supplies back then, and you'll need something Dell-specific.

Ooh, yet another edit: I had a sample supply listed, but I forgot something important: computers that old pulled a lot more power from the 5V than they do now. Modern machines pull almost all their juice off the 12, but those older computers didn't. If that 480 is specific for Dell, it will probably work... it will have the power balanced between the 5V and 12V in a way that made more sense for the era.

That machine is almost ten years old!

I edited the prior post very heavily, multiple times, so if you read it before this post was up, check it again.

Is the CPU fan cable firmly connected to the motherboard?
Is it possible you unseated the CPU cooler slightly? Might be worth removing, reapplying thermal paste, and reseating.

Either of the above issues would force an immediate shutdown.

The steady beeping is definitely the "no video card connected" warning.

Yeah, I think the beep beep beep is the no video card installed standard error. Interestingly, on my mobo, the blinking green LED is for lack of ram. Have you tried reseating this?

Not sure about the CPU cable fan, that may have come loose I guess. I think I may have missed that one when I was unplugging everything. I'll check. I did try reseating the RAM and then just inserting one at a time.

I know it's old, but it still works great for word processing, basic photo editing and web browsing, and my friend has absolutely no budget for a new machine (he and his family have fallen on hard times). So I'm trying to make this one work for as cheap as possible. I couldn't believe it when I just dusted it and it wouldn't turn back on.

This is the highest wattage PSU I could find that will fit the Dell 8400 (they can't have an on/off switch or they won't fit in the case, short of cutting a hole out of the rear of the case:

24a on the 12v (8800GTS needs 28a)
http://www.ebay.com/itm/480W-POWER-S...

This Athena (20a on the 12v)
http://www.newegg.com/Product/Produc...

24a on the 12v:
http://www.ebay.com/itm/NEW-300W-Upg...

I think the first one on your list will probably work okay. The 8800GTS itself doesn't actually need 28A: it needs 12.5. The reason they say 28A is because they assume that the rest of your system will be pulling all of its power from 12V as well. If you'll note, when I ran the calculation, I came out at 27A.

But your system is really old, and it will have a lot more of the load from 5V instead of from 12V. So that 480-watter should be tuned about right to keep both your motherboard and your GPU happy. Should. I can't absolutely promise. But I think it will work.