780 DT, powers on when plugged in

That would appear to be a function of the power supply.  If it all works OK, after that, I would leave it alone.  

On the other hand considering its about 10 years old, a new power supply wouldn't hurt.  But you failed to note which version of the 780 you have and most require specialized Dell power supplies that are probably no longer available new.

DT means  is next size below  MT, with narrow case.

first things first, its old and the COIN call battery RTC is dead, caused BIOS to nuts. (as all will LIKE THAT)

so test the RTC below 2.9v bad, above and par is 3.3vdc new , using any voltmeter made . I see you did.

then in bios reset defaults, in bios page named that.

then see if power is set wrong, all PC  really most,  have page called what to do with power.

see what to do when power is list settings, do nothing is what you want I guess.

MY DELL 3020 does what you said,  and is normal but I never pull AC line cords ever. why do that?

just use the power switch.

The pulling the line cord kills PSU standby power and same on mobo.

this causes the next power event to get perplexed.  (BIOS or the intel south bridge runs off standby)

The PC has many WAKE features and doing what you will cause confusion.

Turn all those off? and even USB wake on?

That +5vdc_STANDBY rail goes many places, and my guess those places get confused,

The PSU  does not do this,  you can prove that, easy with PSU on bench and the add AC power, see no fan pulsing,... in the PSU fan inside, (surea  bad PSU can do any wild thing)but this is not it.

My PSU (MT class) has a test button and it turns the PSU on , glows the led green and spins the fan inside PSU.

as long a I hold BIST button. in.

I put the PSU back inside the system my MT PSU spins up the CPU fan, for a second or 2,  so is normal. IMO.

it sure  causes no harm. at all. if you run the PC for a year, it will do nothing wrong.

for sure the cause is the MOBO Power_on line from mobo to PSU doing that FAN PULSE , on MY PC.

your test button on PSU is on the side not the rear, on DT case PSU.

but works the same way.

If Am wrong"normal"not, then what causes this a, dumbb(sic) GPU card or some other card that has funky logic on the wake ON feature sets or on the +5vSTB line. (but wait , dells are not ATX spec)

like say a NIC card, getting confused,  I will do some tests later on my 3020 so see if when I pull all option cards out does it still do that AC cord track.

ok? I will do the full set of tests for you on my PC, to isolate the WHO DID THAT event.

test 1 with my 550watt huge PSU in my 3020MT , a non DELL PSU

The PC/PSU new, does the same thing so is not a PSU thing, infact   the South bridge chip is doing this.

but I will go to the stripdown next. (cards cause this?)

now my theory,  the DELL PC does not use 5vdc stand by (ATX spec, it is not) but does use a+ 12vdc stand by

that purple wire is it,(see) (use voltmeter see im correct)

so the DELL has a 12 to 5 VRM in the motherboard,  that creates REAL 5vdc standby to all things needing real 5vSTB. (lots of things do. in fact)

this is the cause, IMO, and is now a FEATURE< I BET> (bug/feature, endless cry)

The VRM wakes up late I bet, (a delay ) and causes the south bridge some confusion. (or really all things on that RAIL +5vSTBY.(can act up)

I will post more soon,  doing that strip down, (I've noticed this before but was not upset, knowing it DELL spec not ATX spec power supplies) I have converter to use non dell PSU. (even one my design)

next up, cards pulled, out. test.

if that fails (pulses, then its 100%^ motherboard caused)  no USB connected no cards, and PSU fully discharged at the test. (caps discharged to 0v)

next:> ok back again,

the PULSE happens with all things unplugged, SATA, USB, and ALL CARD.  only things  left is fans and front panel plug. with stock PSU and my Raidmax 550w

cause MOBO only related, guess that the south bridge has and A-RACE condition. https://en.wikipedia.org/wiki/Race_condition

so is NORMAL.  5min job done.

hope this  helps someone.

Hello!

I have same issue on optiplex 9010 with standart atx psu. Never have Dell original psu.

Interesterd is there any difference in cable arrangement of 24p connector.

Did you find how to remove that pulse on the start? I worried that work resourse of device will run out faster due to cold switching on off. There power outages in my region.

780 and 9010 psu 24 pin ATX pinout is standard.  No special arrangements.

"Pulse" can't be removed upon power being introduced.  It's a self-check that it does and is common in Dell's.

If you can afford it, you might want to consider a UPS (uninterrupted power supply).

UPS info from the Optiplex 9010 Tech Guide :

@Hermetic Locust 

Please press the blue Accept as Solution button below if this post answers your question.  9010 can use Pure Sine wave UPS to prevent damage due to power outages.

CyberPower CP1500PFCLCD Pure Sinewave UPS

760 780 960 980 uses non standard connector and pinout therefore the assertion that its standard is false.  Its not one to one or straight thru and uses non standard connector.

moddiy Dell 980 960 780 760  24 Pin to Mini 24 Pin

9010 uses 24-pin connector that's the same as on some aftermarket PSU's like EVGA, Corsair, etc.  780 DT uses mini 24-pin connector and requires adapter for aftermarket PSU's.  Hopefully that makes things a little more clear.

Same problem here, anyone found a solution? Thank you 

@dede24px 

As stated before in this thread, the 3-4 sec. run when power is introduced is a self-test that it does.

PC shouldn't be unplugged or power turned off after every use.  CMOS battery will drain faster.  Use Windows or other OS to shut down.

If you're having a different problem, please start a new thread.

That was not, is not a problem.  It works as intended.  When the system got unplugged from live electricity outlet, it was mimicking condition of a power outage.  When electricity is introduced to the PC, system does a quick check with BIOS settings.  If power settings was set to ON, system would continue with boot process to power on the system.  If the settings was at default, set to OFF, the system then would cut off the power and stays off.

Although it was not an issue to be resolved, if you do not want to see that power behavior, keep your PC always connected to a live AC outlet.