53XYT.. 

Dave, in your case the SUPPLY is hardly powering on at all... YOUR shutdowns are so quick, further 'supply' testing is more than difficult!

ALL of your efforts so far are very worthy tries and well beyond what other owners would try!...

After your efforts ~ my prime suspect is; your 'power supply'.  IF you have tried disconnecting all drives and removing memory ~ and your System STILL quickly shuts down... my prime suspect remains the supply.

Your supply's prime mission is to provide a small array of low voltages TO your system.  IF ANY ONE of those IS weak, lacking and/OR sometimes "impure" (as in "filters" going bad), this could cause faults you see now!

Some online "testers" check for missing voltages or other supply failures...  For the most part, DELL supplies are different.  That is, the 'connectors' (to the motherboard and drives) may not connect to your tester... so MOST eBay 'PC power supply testers' may not work for you?

IF you have a digital volt meter you SHOULD be able to test each voltage from the Supply  IF you could power-up.  Most power connections (and wire-colors) from the Supply are "standard", however some mfg change colors on us and that's no fun and not fair!  Still the voltages are fairly common... +5 / -5 / +12 and -12... and so on...

Most voltages are measured from chassis ground to the YELLOW wire (in example).  OR 'X color' (orange, green and red colors are also examples).  MISS any voltage and (most likely) this can cause the shutdowns you are seeing now.

Dave,  this is NOT  to scare you away from checking!  IF you set your voltmeter to 50 volts (or less)  you can test these voltages yourself!  Because your power connectors are 'well removed' from your AC line voltages... there should be no danger of shocks or other maladies. 

So missing +12 volts (or any other voltage), suspect the supply. 

UNPLUG almost all other "loads" possible (drives etc) from your Supply... STILL missing a voltage? ~ Suspect your  supply!

If all this fails, I would call a minister and sell the 'parts' left over.

Anthony

Hi @53XYT1G3R,

I answered your duplicate post in regards.

Gee, now I regret wasting my time and replying at all.  I add that I didn't cut n' paste anything in my comments to you... I was trying to be as helpful as possible!  

crank poster,  and troll. (and begs for help but is rude)  how can that be?

 the MOBO is bad, you have replaced all parts  but one. even 3  PSU replaced you said.

so why even post here, knowing full well the MOBO is bad.

or at new CPU, change out you bent the MOBO CORE socket Pins as MANY DO<, not trained, to do it right.

or even possible those spare you have are  all bad, if you need more bad spares let me know have boxes full.

all this sweat over $9 mobo sold on feabay, (last one sold for that)  pocket change, why sweat over 10 bucks?

you are correct the MT does not use the micros 24pin (12/2) power jack at all.

in fact only the 980 optiplex does in MT and DT. and SFF. (all 3 do) but is off topic in 380 post.

---------------------------here is my micro dell non ATX 24pin report---------------------------------

the SFF only versions of 760,780,960 , and 980 do   usff 760 is even worse.(dont ask)

other dell mobo run non ATX standby power,some dells  used 12vdc STDby and will not work with ATX spec, 5vdc standard ATX standby, so we have an  adapter for that too. (uses and inline upconverter device )

the small black connector too in the 760DT is non ATX and the MT is full sized ATX 24pin

all DTs too (in my list) are micro 24 (and wired different too , its not just size different)!!!

the 980 uses same mobo on DT and MT so both are Micro 24pin and colored black to denote that fact.

not cream , cream = full sized , my guess Dell parlance.

there you go .end micro psu 24pin jacks, not here.

All Amber LED on motherboard indicates is that the Power supply has 5v Standby.

The power supply is likely dead.

I believe this unit uses proprietary tiny 24 pin power connector.

https://www.amazon.com/COMeap-Female-Adapter-Optiplex-3-94-inch/dp/B07284WTMF/

Recommended replacement would be Corsair CX750M  Recommendation is based on the combined output of 3.3v/5v rails not a single spec of WATTS.  Thats why the same units from EVGA that are NOT B1 model Do not work.

https://www.bestbuy.com/site/corsair-cx-series-modular-cx750m-750w-80-plus-bronze-atx-power-supply-black/8324202.p?skuId=8324202

https://www.amazon.com/Corsair-Bronze-Certified-Modular-CP-9020061-NA/dp/B00ALK3KEM/

or EVGA 700 B1  100-B1-0700-K1.  NOT 700B2 B3 BR BT BQ N1 W1 G2  Etc.

https://www.amazon.com/EVGA-BRONZE-Warranty-Tester-100-B1-0700-K1/dp/B018JYHBE6/

Power supply is clearly BAD.

• DPNs: PW115 D326T FR607
• Mfr. PNs: VP-09500051-000 HP-D2551A0 PS-6261-9DA

This is a very under powered 255W unit.

Recommended replacements are based on standard EPS12v  24 pin

80 percent efficient and 142W minimum combined power on 3.3v rails.

Keep in mind 700W units that are B2 B3 BQ N1 W1 G2 G3 etc DO NOT WORK because the max power on same rails is 130W or less.  More power on these rails is fine.  LESS is not.  BEST BUY, AMAZON, OFFICE DEPOT ETC all carry these.

Note power on 3.3V/5V Combined SPEC

https://www.evga.com/products/product.aspx?pn=100-BR-0700-K1

https://www.officedepot.com/a/products/6811224/EVGA-700BR-Power-Supply-Internal-120/

700B1 (100-B1-0700-K1)  is also fine

with same restrictions on B2 B3 BQ N1 W1 not working.

https://www.amazon.com/EVGA-BRONZE-Warranty-Tester-100-B1-0700-K1/dp/B018JYHBE6/

Same problem with new Corsair HX850I.

Well dave,

I've been out of town enjoying my summer vacation, but I'm not so sure about you?...

"53XYT1G3R" for the record;  I AM Anthony and I also sign as "SOGLAD5" on occasions....

I don't understand WHY some persons have to be so HATEFUL and angry?  Thankfully it's just two or three in this Community ~ and in most Communities ~ and that's a shame.

dave, as another matter of fact, I just checked and found this today (7-27-2019):

"Sorry, 53xyt1g3r is either deleted, banned, or doesn't exist."...  

and that's on www-reddit-com!

dave, Reddit is a rather "liberal group", but apparently "Reddit" had it with you too, so I let you stand on your own merits ~ or lack of them.  And, dave  apparently there were quite a number of '53XYT1G3R' posts there, but apparently ALL have been deleted?... (Oh ya, also 1,984th in something else obscure?!)

While you are at it, you'd better checkout this guy "SOGLAD" on 'fixya dot com' too! 

Yes!  Seems THIS guy is also calling himself "SOGLAD"... and FIXIT said this about him:  "Your answers helped 137,895 people on Fixya!"... and dave, that was just  ONE  of the questions he answered!... Whoops.. I just found more!  "...helped 94,317"  and this too:  "... 127,540" with another answer... that rascal! 

If I was in error calling you "dave",  trust me, I can think of many words much more appropriate for you.  

How about doing everyone a favor by keeping ugly messages to yourself?... You so richly deserve them.  

Anthony aka "SOGLAD"

speedstep answer nails this topic solid! the horror of 2008 PCs. 101.

if you 3 TRIED, 3 spare PSU failed read this again.

read how this OLD PC CORE 2 from 2008, uses huge power 3 and 5vdc rails. see that.? CORE 2 tech.!

and you go to bestbuy and buy a new PSU, guess what , most PSU do not do not today. for CORE(tm) tech.

that is because modern motherboards (not yours for sure) have new VRMs on the mobo that generate there

own  3.3v , 5v and 1.8v,etc now, to run CORE(tm) PROCESSORS modern.  not CORE2.old.

OK  so not posting the 3 PSU (make/model or stickers posted) you slammed on there, is not going to work here.

next time use the correct PSU to start. (a good one)