Electrical Engineers / Computer Hardware Enthusiasts: Replace fuse in computer power supply?

June 27, 2007 11:57 PM

Electrical Engineers / Computer Hardware Enthusiasts: Replace fuse in a computer power supply?

I was recently given a 1U rackmount P3 server from a place where i do some computer work.
The fuse in the power supply was blown. You can actually see a film on the inside of the glass where the fuse element melted.
The was a glass BUSS fuse, must be a 5x20 mm size, with markings 'F6.3A L 250V'.
Is this a long-blow or a short-blow fuse?
(I was thinking the L might mean long, but not sure)

The fuse was in a holder (meant to be replaced?), none of the other components in the power supply have visible damage, and I had the computer going for just over an hour on another (desktop) power supply.
(There are also three of those little blue disks (MOVs?) right next to the fuse holder... if i recall, those provide some sort of surge protection?)
The computer itself is a clone, with less dust than i'd expect to see in a year old desktop computer - let alone a computer that was built in late 1999.
If i can get the power supply fixed or replaced ($50 +... ouch), the unit would make a great storage server or media player (think .avi on a tv).... but i'd much rather spend that $50 on a pci to sata card or a pci video card with tv out... and i'm not willing to jurry rig the thing up to a full size power supply on a permanent basis.


The most recent 'date modified' i could find on any of the files was 12/10/2006, which would lead me to believe that computer was running fine until at least that date... and that winter weather may well have caused a power surge.

Radio Shack has a '6.3A 250V 5x20mm Slow-Blow Glass Fuse (4-Pack)' Catalog #: 270-1068...

I was thinking of replacing the fuse,
testing the power supply with a paperclip (green to black), a dummy load (10 ohm 10 watt resistor from red to black), and an led (grey to black),
then using it on a junker computer,
and then using it if that works...

Once back in the computer, the supply will be enclosed by two layers of fairly thick steel... as long as i dont have it near carpet or drapes, i'm not going to burn down my apartment complex... right?


(Specs on power supply at: http://www.sparklepower.com/pdf/MPW-6150F.pdf)

posted by itheearl to technology (10 comments total)

You could verify this with a toll free call to The Fuse Company, giving them the markings data you've supplied here, but for my money, yes, your 5 x 20mm fuse is a 6.3Amp slo-blo fuse, capable of extinguishing current flow in circuits of up to 250VAC RMS. That said, I'm going to argue that it is as likely that your power supply has been damaged, as not. And that, you'd be much wiser to invest in a modern, high efficiency (high power factor) supply, if you intend to use this machine as a home server in 24x7 operation, for a number of reasons.

First, modern high power factor supplies can save considerable energy cost over older low power factor supplies. How much you save depends on how much power costs, and how inefficient your existing supply is at average load (but since at full load, it's only 60% efficient, so at partial load I bet it's maybe 45-50% efficient at best), but for PIII generation machines, you could possibly save the cost of a new supply on your electric bill in one year of 24x7 operation. Good HPF supplies are about 95% efficient at full load, these days, and never drop below 90% efficiency at any point in their load curves. They'll also have significantly better "hold up" specs for +5 and +12 rails than your current supply, in equivalent package sizes.

Second, modern high power factor supplies tend to have better (faster, more robust, and un-fused) protection circuits, than the supply you're trying to repair. Unless you're going to be operating this machine on some heavily filtered, in-line type UPS, such improved protection circuitry can save you considerable aggravation, particularly if you live in the Southeastern US or other areas prone to summer thunderstorms.

Third, higher power factor supplies, because they are more efficient, generate less waste heat, requiring less cooling themselves, and radiating less heat to other components. Thus you get quieter operation, and longer life of your server.

I doubt you'd have to spend $50 these days for a supply of such modest output as your current Sparklepower - more like $30, before tax, shipping, etc. Way worth it!
posted by paulsc at 2:16 AM on June 28, 2007 [1 favorite]

Replace the power supply: I have to do it all the time (they fail almost as often as fans do).

Welcome to computing ;-)
posted by chuckdarwin at 3:28 AM on June 28, 2007

I'll disagree with you abit, paulsc - from the designations, that's a fast-acting (F) 3.6A low rupture capacity (L) fuse.

But I will agree with the rest - particularly about the power supply likely being kaput; if the fuse has blown violently enough to deposit a film all over the glass, then there's a high likelyhood of a dead short there - particularly in a switchmode PSU, that meads a shorted rectifier diode or chopper transistor.
posted by Pinback at 3:33 AM on June 28, 2007

Don't mess with it. Just replace the power supply. A bad power supply can easily take out anything and everything else in the computer. If you replace the fuse, you're just giving it another chance to fry things.

Fuses don't just go out in a power supply. I've never seen that happen. Whatever the problem is, it's major, and it's very likely that the machine will have substantial internal damage. I would suggest hooking it up to a big power supply to test it very thoroughly. Only after you're really, really satisfied that it's working well should you buy a supply that will fit in the case.

Do NOT replace the fuse.
posted by Malor at 5:09 AM on June 28, 2007

Oh, and: never, never plug that supply into any computer ever again. It's radioactive and dangerous.
posted by Malor at 5:11 AM on June 28, 2007

Replace the fuse. If a transient blew it, it'll run fine. If a catastrophic failure blew it, it'll blow again.

Fear not! Move ahead. See what happens. Gather info. Evaluate.

You have done a first class job of observation. Follow it up with a little creativity and initiative and you'll redeem some utility from what would otherwise be a very fine piece of discarded hardware.
posted by FauxScot at 6:12 AM on June 28, 2007

I've tested the computer itself - it's fine. Grabbed a power supply out of an old desktop. I havent tested it under heavy load, but i did install vlc and watch an episode of family guy off a usb drive. If it can do that, the hardware's probably fine.
I'll be replacing the 10gb hard drive.

The problem is that $40-50 is a lot for me to put into such a dated piece of hardware. I calculated that i could build a cheap file server with usb2 and sata 150 for under $200 (no hdd).
As i mentioned, i have some test hardware that i dont mind frying.
posted by itheearl at 8:15 AM on June 28, 2007

The safest course is to replace the power supply. The problem with multivoltage PC power supplies is that they are a fairly complex circuit, made VERY cheaply. There's a ton of different ways they could fail, many of which can destroy the mobo they are connected to.

It would probably be cheapest to steal a working PSU from another retired PC.

If you are willing to risk the P3 server, or you have a similar mobo to risk, then I would consider just replacing the fuse and trying. Slow-blow fuses most always have a wire coil and some other bits, so if all you see in the blown fuse is vapourized metal and wire ends, it was a fast-blow. Use another fast blow fuse, it's safer while testing. If the PC seems to work after the fuse change, quickly measure the DC output voltages under load to confirm they are on value - if any values are more than 0.5 v away from where they should be, toss the PSU. Next, see if the thing will stay up for 48 hours.
posted by Artful Codger at 8:43 AM on June 28, 2007

If you're the hacker type and enjoy fooling around, follow FauxScot's advice. Sometimes you save a lot of money and sometimes you end of spending more money. That's part of the game. You're not going to set your house on fire. If you are the practical type, follow paulsc's advice. Power supplies are probably the most common point of failure for computer systems.
posted by JackFlash at 9:08 AM on June 28, 2007

The best thing that this computer has going for it is it's form factor. If that goes away, it's just spare parts.
If it was a desktop instead of a 1U server, i'd just replace the power supply with an old one or a $20-$35 new one. (Or upgrade my desktop's power supply and put the hand-me-down in the server)
I could extend an old desktop power supply and put it outside the case, but that would remove the ability of the computer to fit underneath a television.
I'll try a replacement fuse. If it can run a dummy load for a day or so, then i'll try it on the computer and check the voltage readouts in the bios. I could probably even get a program that would keep tabs of the voltages from inside windows... and if it runs a day or so under load and the voltages stay ok, i'll deem it fixed.
I'll put in the smallest ram module that i have on hand and unplug the cd drive, so the most i could loose is a 10gb hdd and a motherboard. Processors are robust in my experience.

hm... now to find a replacement, fast-blow fuse for $5 or less...
posted by itheearl at 7:44 PM on June 28, 2007