380 MT, No power

You have pretty well touched all the bases and it would look to be a bad motherboard. If you pull all the RAM and try to start are you hearing a number of beeps? That would at least give the appearance that the MB detects no memory. Of course it could be as simple as the power switch is bad and in that case the system will not start. If you can trace the power switch cable back to the MB pins, try jumping across the pins on the MB and see if you get a start.

What does the BUILT IN SELF TEST do?

https://www.youtube.com/watch?v=udGOe7LQJEI

DIMM capacity in a 380 is only 2GB, total capacity being 4GB.

Optiplex 380 Tech Guide

Optiplex 380

dude you have serious issues, best you  post in fixya.com with the others.

see speedsteps rating, are you blind too>?

he is 99% right most the time, when not , its not his fault, as is the fact that some non dell parts flat do not work here, or you bios is old or new and does not like say a VBIOS card you have not from dell.

Also wrong is the DELL pages on BIOS do not tell anyone what as added, in full clearly, a fact.

nor does dell test  all 1000 models PC against all 20 years, of new memory, gee, get a grip , no?

learn that DDR specs evolve , and even bus  capacitances long ago were too high(ddr) and would not work THEN,.

then 10 year later that pF spec, (DDR sticks) drops and works now., gee learn now DDR works, ?

why not buy a spare dirty cheap used MOBO and stock it so next time you'd not have to insult everyone here.

OK? OMG $12 whole bucks on fleabay..

a fight over 12 bucks, (med time)

just the MT alone there are at least 2 different motherboard. (dell has no parts lists like HP.com) so,.... yah, that.

also learn that many motherboards have HW versions on the, ver1, 2,3, etc. and my not have same spec.

yah2.

this mobo is limited by BIOS and G41 express chip.

that supports both DDR2 and DDR3 sticks and will be limited by dell, what they chose. Dell has DDR3.

fact 2, some mobo called HYBRIDS, run DDR2 at 8GB and DDR2 at 4GB max.

so what this mobo does I don't know.

your sticks not once  told. in fact.

one more fact.(from toms) learn just how tricky  CHIP density is and is INTELS RULES the MMC not DELLS!!!

G41chipset mystery, some sources say 4GB DDR3, another 8GB DDR3 and another 8GB DDR2.
Seems, It does support 8GB DDR3, BUT i has to be double-sided,,low density,, RAM. That's mean 256MB modules x 16 per stick = 4096 MB * 2 stick = 8192 MB.
Tested today by myself, following works with my Gigabyte ga-G41mt-d3v

There are in fact lots pages written at Intel on old dumbb legacy MMC they made,.... logic limits, gone now, mostly. so.. there is that.

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@53XYT1G3R wrote:

Okay well, @bradthetechnut, you're completely wrong, the 380 supports up to 2x 8GB dimms with a BIOS update and I have 3 other 380s both running 2xz 8GB DDR3 1600MHz DIMMs. Instead of being wrong and quoting things from an old and outdated manual, you might want to try being helpful and post relevant personal experience instead. For someone who thinks they are a "tech nut" you lack a lot of experience with technology. Even some of the old DDR2 machine support high density 4GB DDR2 modules.

@speedstep Speedstep, yes, that was the first thing I tried. I'm not an **bleep**. I'm going to ask you to kindly never comment on my posts again because you treat everyone on these forums like they are stupid and it's extremely disrespectful and honesly I'm surprised you haven't been banned yet for being a disrespectful, grumpy old man. I probably know more about the mid 2000s optiplexes than you do, so please, either block me or ignore my posts because I think I've made it plainly clear that I don't like you and disagree with everything you post mostly because it's wrong. Using a big font "because you can't see" doesn't make you special, it just means you're too poor to get prescription lenses and you're too lazy to get a job to afford them. Grow up.

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Yes, from one answer you do know everything there is to know about a person. 

Yes, you are qualified you to hurl insults and assumptions, including based on just one answer because Dell had something wrong.  Haven't you ever heard?  Assumptions make an [bleep] out of you and me.

If you don't know more than speedstep, I don't know who else does.

As long as you know more, or are more knowledgeable than speedstep, why not tell us why your 380 isn't running?  Insults, assumptions, and accusations based on no proof or merit, and your 380 still isn't running?

This forum is mainly powered by unpaid volunteers.  Insults and baseless accusations have no place here.  This isn't the forum for you.  Goodbye.

If you want help, you need to be polite...

About the only things you haven't replaced are the front I/O panel and the cable from the motherboard to that panel...