Dell Dimension 4300 ATX PSU Failure?

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Hi, I have a Dell Dimension 4300. It's worked fine for years until the other
day. I went to turn it on and it was dead. The fans don't come on and no
signal is sent to the monitor.

I opened the case and there is an LED lit on the motherboard.

I removed everything - RAM, PCI cards, graphics cards, DVD, HD, floppy. Now
when I press the power button the PSU fan and the CPU fan spin up. So it's
not the power switch.

I then started adding everything back in. If I connect either the DVD or the
HD or more than three PCI cards, it goes back to being completely dead - the
fans don't spin. In this state I measured on an unused HD power connector
1.5V and 1.2V.

I removed a couple of PCI cards, so the current configuration is MB, RAM,
graphics card and modem. Pressing the power button causes the fans to spin.
However there is no signal to the monitor. In this state I measured on an
unused HD power connector 5.5V and 10.5V.

I don't hear any beeps. There is no power LED on the front of the PC to look

Is it possible to narrow this down to a PSU problem without having to swap
the CPU and MB?

replace nospam with britishideas to reply personally, but it's better to
reply to the group

Re: Dell Dimension 4300 ATX PSU Failure?

Andrew Ayre wrote:
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Your symptoms (5.5V and 10.5V on the Molex), suggest one of two things. Something
in the computer, really is overloading the supply (in particular, the 12V rail,
meaning the CPU vcore switching converter, a hard drive or optical drive, a
cooling fan etc are doing it). Or, the alternative theory, is the supply has a
weak output or outputs, and a perfectly normal load on it, is causing the 12V
to collapse. (Your supply only has a 12V @ 14A rating, so it wouldn't take
much to crush it.)

If it was my computer, I have a clamp-on DC ammeter (Hall effect sensor), and
in 30 seconds, I can measure all the rails on a supply and tell whether the
loading is normal or not. But those are too expensive to justify for one time
use for a home builder. It is hard enough to convince people to pick up a $30
multimeter, to make a few voltage measurements. While you could pick up a
20 pin to 20 pin extender cable, cut the wires, and use your multimeter to
make current measurements, there is no way to predict whether the current
level is something within the range of your meter. Some meters are good for
10A max, and with a PC supply, I'd want something that can handle more current.
My clamp-on ammeter has a 40A and a 400A range, and because it is a non-contact
meter, there is no shunt to overheat while making a measurement. A multimeter
using a shunt has the limitation, that it can only handle so much current.
And an overloaded ATX supply can probably supply enough current to blow the
fuse in the meter, given the chance.

So the inefficient, pragmatic answer, is just replace the supply. If the problem
persists, you now have a spare supply. On my home builds, I've been in the
before, where I've had spares of everything, and fault finding is the tough part
of being a home builder or repairer.

Note that there were some model years for Dells, where Dell thought it would
be cool to use non-ATX supplies. They look like an ATX, but the wiring is
different. Plugging an ATX can damage stuff. And I haven't always been
looking for wiring information, when these Dell questions come up. This leads
to a lot of small outfits offering Dell replacements, but they *never* show
the actual wiring, because if they did, then the user would know whether a
standard ATX could be used as a replacement or not. Occasionally, I have
seen a Dell web page, with wiring info, but I don't think Dell has a
uniform policy on this. /

OK, there is a wiring diagram here - scroll down:

These two docs, are standards that define the older ATX PSUs with
20 pin connectors. The first one shows pin 18 has -5V on it. PCs
stopped using -5V a while back (with exceptions of course). The
1.3 spec shows pin 18 as reserved, and a number of supplies have
no pin on 18. Which means if you are unsure about pin 18, the supply
probably doesn't have any juice on that pin anyway.

So, maybe it will be possible for you to use an ATX for testing.

Also, before you go shopping, measure the outside dimensions of the
supply. I didn't check whether your Dell is ATX or microATX, and
sometimes it is hard to find a small enough replacement.

Good luck,

Re: Dell Dimension 4300 ATX PSU Failure?

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Thanks Paul for the detailed information! I will just go ahead and replace
the PSU. I think I am ok getting a standard 20-pin ATX unit (6" x 3.25" x
5.5"). Pin 18 on the connector has no wire. I'm looking at a PSU that is
0.25" longer than the OEM unit, so hopefully it will fit. My only concern
are the little tabs that help hold the OEM unit in place. If I can't use
them with the new unit then the new unit will be hanging on only two screws!


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