Broken Computer Curiosity

[BrianG]

One of my computers at home has been down for a while now, but it was the kids' computer, so I wasn't too much in a hurry to work on it. Initially, it would sort-of boot up. Sometimes it would make it past the BIOS, sometimes it would get to the Windows loading screen. But at some point, the video would get scrambled and stop booting. I had tried a new vid card, HDD, etc. I removed all non-essential items (add-in cards, CD, floppy, etc). Nothing seemed to help so I shelved it for a while.

Well, I decided to take a look at why it wasn't working last night. When I took everything apart, I found that all the 5v slots on the main mobo connector were melted and blackened! All the rails on the PS (Antec TruePower 450) still work fine, even under load. There are no discolored areas on the mobo, no bulging caps, no discolored CPU/memory/AGP/PCI connectors, and all ICs look fine visually. There was no loose metal parts floating around inside the case either.

I'm kinda stumped. It seems as though nothing was wrong. The only thing I can think of is maybe the contacts for the 5v line were not perfectly connected and overheated from the current draw.

The mobo connector is pretty much toast, and it's an old GigaByte GA-7VRXP, so it's not really worth fixing, but I'm curious what happened. The highest current draw on the mobo would be from the 12v line (uses switching regulator to power CPU core), but can't think of what would use the 5v line so heavily?

Don't know if I want to replace the PS connector, or convert the PS to another 12v PS for charger duty.

Actually some systems, such as AMD XP platforms, and VIA C3 systems, power the CPU from the 5v rail.

It's really hard to say what went wrong. Perhaps there is some internal trouble with the PSU. If that were the case though, I would expect you would have seen similar symptoms in the 4 pin molex connectors. They should have overheated as well. I would pin it on either Mobo or CPU.

Let me know what the platform is, I might have a compatible mainboard kicking around in the basement that is not in use. I have at least a dozen. AXP, PIV, PIII, etc.

[BrianG]

There wasn't a spot for the 4 pin molex connector on this mobo, and besides, that connector just has 12v and ground. I never measured the input voltage on the switcher section, but it is your typical 3 phase converter. I just assumed it used 12v as the input simply for efficiency...

The CPU is an AMD AlthlonXP 2600+. Worked quite well for what the kids did with it until it went belly-up. The CPU and memory still looks fine too, so it would be nice to find some way to still use those parts.

Do you also happen to have a bad PS unit? All I need is the mobo connector and about 6" of wires. It'll be a PITA to connect all those wires, but the PS is actually in good condition except for the blackened/melted spots. Holds stable voltages even under heavy loads (most PS units drop v quite a bit when near their rated output).

I'm still thinking it was just a poor connection that simply got too hot. And once one connection went bad, the load was forced to the others making them get hotter. A sort of domino effect.

I actually have a shuttle AK31 that will run that proc nicely, and plenty of old PSU's laying around too.

If I'm not mistaken, that AXP definitely pulls cpu power from the 5v rail.

[BrianG]

Hmm, that makes sense then. It's funny though, because when I had that board in my box, I was using an XP 2800+ O/C'd to 3200+ speeds and it never had issues. Oh well. Yeah, let me know via PM what you want for the board and a broken PS (or just a connector with ~6" of wire) and we'll work it out. I appreciate it!

[BrianG]

I found out an interesting thing about the PS; each of the main rails (3.3/5/12) has a sense wire. This wire is smaller gauge and connects to the voltage rail at the mobo connector. I did a little reading and this sense wire is supposed to monitor the voltage at the connector to provide feedback to the regulator. So, this PS individually regulates each rail; nice!

Well, I got to thinking and discovered that if you use a voltage divider, you can trick the sense wire into thinking it is seeing less voltage than there is and needs to boost that rail. The resistor values you need depend on the rail you are trying to boost. Use two resistors in series (so total is around 100-150 ohms; higher values don't work well). One end goes to the voltage rail, the other end other goes to ground, then hook the sense wire to where the resistors meet.

Most PS units use only one of the rails to regulate all three rails, so it'll take some playing around depending on the PS unit you're using.

I love playing with this stuff!