Dell complaints

[STSCVILLE]

Hello there,

I am dealing with a cantankerous Dimension 4700 with a 3.0ghz P4, 1.25Gb DDR2 RAM and running 2 SATA HDDs that has what at first look seems to be a power supply problem but after trying 3 different power supply units(2 of which were ordered from dell as replacements for the OE unit that was initially in the machine) the problem keeps coming up so I am wondering if it may be the MOBO.

What it has been doing is just shutting down completely following a single well defined click from the area near the top rear of the tower and the power LED blinking yellow after the shutdown. (Hence the initial thought that it was a power supply issue.) After the involuntary shutdown the unit will not restart until the power to the machine is totally shut off either by unplugging the power cord or shutting off the surge protector and dumping the static charge by pressing the on button with it unpowered.
If the machine is then restarted a short time after the power interrupt it repeats the involuntary shutdown after only 1 or 2 minutes of operation. If the machine sits for a longer time (An hour or better) then it will last 15 - 20 minutes or so till blinking out again.

I am trying to find if this is a common problem with this particular model and if there is anybody out there that has been successful in resolving it.

Thanks.

[Rocke_T_Sinetist]

The onboard regulator is latching off on overcurrent. The power has to be completely removed to reset the latch. This is most likely attributable to a capacitor downstream of the regulator losing its capacity, making the regulator work harder.

The capacitor could also be leaking electrically, consuming current itself, though if this condition progresses very far the capacitor will overheat and push its contents out of the can. You may even be able to see this, look for any bulges in the capacitors. If you find one, it may be possible to replace just that part.

BUT, it's not all that easy to change parts on a 7-layer board AND the other capacitors are just as old and likely to fail in the same manner before all that long.

Most practical solutions are 1)replace motherboard or 2) replace entire system.
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Rocke T Sinetist
as in, 'it doesn't take a...'

[diashto]

As Rocke said, check the capacitors on the motherboard. Usually its the big ones around the processor that get the domed/split tops and the swollen sides. This is a known issue with the Optiplex 270 series, and I've seen it on many of the other models, especially in the clamshell-style case.

More than likely, there's at least 3 or 4 that are swollen, possibly one or more that are leaking.

Tom's Hardware recently had an article on how to fix just this problem:

http://www.tomshardware.com/2007/05/18/fixing_your_motherboard/

They say you can do it for $15, but that doesnt count the $500 soldering station, apparently.

[wintech]

The previous suggestions are a possibility, but it has been my experience that what you are describing is caused by the cpu overheating. Check the heatsink and see if the plastic retention module that the heatsink clips latch to is broken. If one of them is broken, the heatsink is not flush on the cpu and it will overheat. Since you describe that the machine runs for 10 - 15 minutes from a cold boot, I would suspect that just one of them is broken. I see it quite often in my shop and I stock replacement retention brackets just for that reason.

[KE7EHA]

It could be a cooling fan problem.

On my dim 4700, the damn thing would shut off because of a fan failure.

Also, could be heat. I'm doubting it, as there is very little that heat can do to make an audible click in the case

Caps: there are decoupling caps on the processor and below the socket itself. These decoupling caps will remain intact even iff the big ones go. The big ones just make sure there is enough energy to support the voltage even under high current drain. I do believe that you might be able to get by with just the decoupling caps, but don't take my word on it. I'm not 100% sure.

This also could be a hard drive issue, but that comes from the front bottom of the case. It's probably not a big deal, but you may want to run some diagnostic tests [hey, it can't hurt]

Here's what I would do: check the fan first [cause it's the easiest] and while you're at it, check the heatsink assembly. If the fan is bad, get a new one and see if that helps. Oh, while you're looking at the heatsink, check the caps for any popped tops or bulging. It'll be apparent if any are damaged [capacitor electrolyte stinks too]. If none of these turn up, run a diagnostic test on your hard drive.

One more thing, you may have put three lemon power supplies into this computer. Statistically, it is possible, so you might want to check it out.
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"The art of Engineering is knowing what you can safely ignore." -Dr. H. Hess

[Rocke_T_Sinetist]

Thanks for the suggestions everyone, but 4700 was one of my platforms.

Overtemp shutdown does NOT require draining all power to achieve restart, forget that. However, overall cooling is greatly improved by remounting the CPU fan so that it blows outward. Having it blow inward was an ill-conceived compromise trying to reduce fan-noise complaints.

Even if you took out ALL the little decouplers, that wouldn't cause the regulator to sense overcurrent. Forget those too. The little ones decouple machine-cycle loads, the big ones decouple regulator-cycle loads (much lower frequency).

It is possible for 3 replacement supplies to be defective, but extremely unlikely they would all 3 produce the exact same symptom.
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Rocke T Sinetist
as in, 'it doesn't take a...'

[KE7EHA]

Well, what I was saying is that the decoupling caps are probably big enough to support the voltage on the processor loads even if the big ones are gone. Really, they probably run their converters at a really high frequency [into the MHz] to reduce cap size requirements.

Oh, and losing the big caps effectively removes a lot of energy storage from the power converter. THis requires it to pump more energy into the system, increasing current and such. That's why they have those massive ones and the decouplers. The big boys are there to reduce the current requirements on the input. Granted, you're stepping down from 12v to 1.2v, which will reduce the current requirements on 12v based on transformer ratios, but you still need plety of current. Losing part of your energy storage will make the converter have ot work harder, potentially over-currenting the system.

Really, to me, this sounds like something mechanical, not electrical. There's not much electrical-wise in the 4700 to produce a clicking noise.

Just a side point: how much stuff are you trying to run off of this power supply, STSCVILLE?

Oh, as for changing parts on a multi-layer board, it's not hard. Most of the through hole parts [caps included] are in lead-plated copper barrels called vias that connect each layer to the other layers. I wouldn't recommend it, as you have to be careful [i.e. know exactly what you're doing i.e. worked on PCB rework before] about temperature and force or you could delam traces and ruin your board.
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"The art of Engineering is knowing what you can safely ignore." -Dr. H. Hess