Dell 790 Optiplex PCs  (MT/DT/SFF/USFF)           

This is only a test bed (jig) PC and not used, to do real PC work of any kind it is for my test lab.

Many folks still use the great 790 , it was made in year 2011 and is not too old, it runs windows 10-64b quite well. (It came new with W7) and with BIOS v:A18 is UEFI certified
I turned off UEFI fully off in BIOS, (fastboot, secureboot, safeboot,TPM and boot UEFI all disabled.) but...
I did test w10 for full GPT boot drive and it works perfectly.
Intel Q65 Express South bridge (chip set) and CORE gen 2, up to 95watt CPU, but not the i7-2700K with wrong HD-3000, iGPU inside. Intel HD 2000 is supported only.
CPU's Up to I7-2600 and 3.4GHz. clock. 4cores and 8 threads.   (jump to GPU test results)

Here are some facts about this PC. ( this is my GPU card report)
The PC needs to be BIOS upgraded to A18 version, I use FreeDOS(tm) to do that and run O790a18.exe via a FreeDos boot USB stick. Done. (sadly modern F12 BIOS ,BIOS Upgrade method is missing here)
The PC will have dead BIOS screens if he RTC COIN cell is dead, Below 2.9vdc is a bad Cell. (at 0-vdc the power button is dead)
Plugging in the monitor to the wrong video port? mine has 4 ports, only those in the X16 are live below(or using lame VGA adapters is prime #1 to fail)
The BIOS hotkeys, F2 and F12 keys seem dead? check the coin cell first. (seeing F1 warnings is not a dead PC and for sure not DEAD BIOS)
If you have BIOS set to fast boot (mines off), it can be hard to see, use HOT KEYS,. use FASTBOOT (in BIOS) set to "Thorough" (if you can reach it) by you hammering the F2 key quickly after power on is pushed.
F2 is best hit at DELL 790 logo instant !
To attain BIOS screens more easy unplug the HDD 0 (SATA port 0) now you have more time. (easy no?) (if you see , no boot media errors that means you are too slow hitting F2)
Do the long HARD reset if you have problems with BIOS acting dead or power button acting dead. (set BIOS defaults on any RAM changes too)

See all GPU cards tested by here at the end of this short page.

Using my test bed frame , Bitcoin sucks frame, I call it.
This lets me test any Mobo easy, and any GPU card I want,  with a 700watt PSU. Both 12v rails below are 28 amp rated. 

See GTX1050ti run here

The beauty of my test bed jig is all cards made fit the X16 slot and no hitting. I can even plug in 1/2 height cards and not need to remove the J-bracket to do that test.

The full sized MT desktop uses the full sized tall card. ONLY !  No 2 VIDEO cards have the same 3 dimensions,  that includes depth ( even 1 ,2, 3 deep cards are made) Do not guess, but do MEASURE your space with a RULER.
The most easy PCs are full sized full height cards.  (if it can't fit and not hit , the card is useless to you)

Best and good to have is having  a front panel I/O below,  this so the 4 P.O.S.T error codes at he 4  LED work.
The BIOS will nag you with endless F1 errors. if missing. (but can be skipped) 
The dell myth that this SFF panel does not work on MT/DT is wrong, sure won't FIT But works 100% perfectly. (DIY systems might use this... so.... I tested it ...)

I offer this to you if your power switch is bad or missing and you need to jumper pins 1 to 4 to turn it on.
The SFF 24pin power jack hits many GPU double wide , PCI-e  card fan casings.
 The DT/MT mobo a wide card can hit the 2 SATA ports., but 90degree angle ended, cables may mitigate that issue.

The below card is  DT/MT has old PCI legacy slot (cream color)  unlike the SFF with none. The USFF has no slots at all.  Note the 4 PCI SLOTS , 3 are PCI express, not the cream one.

Mobo bottom here?

The bad case to own is SFF or USFF (small form factor) see GPU slot no room. (and weak PSU) USFF has no PCI-e slots at all. (horror?) Avoid PC's like if you plan to upgrade one.

The secret 34pin jack for I/O panel.

Using the wrong fan:
The Intel fan works ok, wired correctly, but DELL BIOS will not like it  for RPM and sets errors in the log, but can be used on a pinch. (for sure ePSA fails but we can test with the wrong fan)

Buy the MT and then most GPU card fits. ok?
his  PC will run with no I/O front panel, .(but will nag you about the missing panels, missing thermal sensors, and missing power LEDs and wrong fans.)
The other nags besides thermal sensor 1, missing is HDD fan missing so I put yellow jumper wire from CPU yellow pin to main chassis yellow pin 1(taco-sig) and bingo ,it work no errors. (Dell secret told)
If it nags, you ? then hit F1 key then F2 or F1 and F12 and use F12 to boot any device you want.
F12 boot on the fly menu, lets me  load W10-64bit no problems.  (f1 then F12 and boot to USBs stick)
I do not USE UEFI modes. (all are turned off in BIOS) to gain my needed full usage as a test bed for hardware.

All tests with 700watt PSU. (Buy  MT 790 so you can upgrade the PSU, and cut out the silly PSU lock and trash it) Up to 750w easy ,over that you get extended length PSU and will not fit many PC's. (use a ruler)
ATX spec size is here:
This PC runs Windows 10-64bit,  1809 no problems. GPT HDD formatted, UEFI turned OFF !!! BIOS "FastBoot" set to Slow boot (thorough)  Dell does support UEFI on this PC.(proof)

The GPU cards tested below by me.

Tests done are , VGA and DVI-d both work in BIOS and W10, and runs FurMARK and passmark 3D. (Results all work perfectly)

The myth that 35watts is the slot limit   PCI-express.. slots. I tried all 3D tests to make it fail and it did not once fail or even hiccup.
In truth most cheap cards never go that high, or if using a high end card the slot power is very low but AUX1 and 2 may draw the lions share of power off those and never the PCI-e slot, (minor there)
The full blown spec, shows 35watt slot limit , and I call    on that. I guess they did that, to say the PSU is maxed out and only 35 watts remain  from the gutless  265watt PSU?
and or Dell limited it due to there are 3 PSI slots all sharing that skinny +12vdc PCB trace, so the must share that 1 feed, but in reality if you don't use the other 2 slots, the x16 gets all that power , fans also share this)
This stock PSU runs high powered 3v/5v power rails which limits GPU 12vdc rails.  (now upgraded to 700w)
A real MOBO not this would have at least this sized trace here  10mm (9.3mm) the question is what do we have.  (the below shows near 4mm sized)

TESTING 1,2,3  I did real tests there to learn what is hidden in the PCB.(+12v)
Tested at 36watt limit (3a  x  12v)
The MOBO test I did  at 3 amps current  end to end , was : 0.077vdc drop. (this data allows computing what is hidden inside the PCB) R= E/I = 0.026 ohms, P= IxR = 0.23watts (inside pcb trace) (hard fast data !)
This test is the voltage drop from pin 10 (P24 (12/2) power jack and B1 pin on the X16 slot. (exactly this)

 IPC - 2152 rules,would tell you 10mm is the minimum here. (for 75watts to just 1 slot)
  150mil wide trace is what I computed. (0.150wide/ 3.8mm)   not a vary large area at all, and yes 3amps is  good limit but most card do not hit this limit. (or if do, go over this, the system is happy)
The  best and correct trace  size (I dream) would be 6.25 amps  and  would be 385mils  (a trace nearly 3/8" wide or (9.8 mm wide) at 0.0105 ohms. 
Why does the board  work so well with all GPU tested?,
Answers, many:  4 slots.  I do not know the spec, on X1 slot but my  guess is 10watts. (but I will discount this slot)
  1. Slot 2 and 3 are empty freeing up 50 more watts for sure. (or more) (25w perslot,  2 and 3,  is 50watts and 35w more of slot 1, is 85watts total on tap) The math is correct.
  2. Or the Card you use ,does not use 35watts ever,  but my 75watt card does (running any GPU/CPU overloading program I can find) Furmark is best.
  3. My GTX260 Card uses 3 rails and the brilliant designer runs huge power on AUX Jack 1 and 2, and tiny loads on PCI-e (B1 , 5pin set)

Ohms law too can effect this. (and the lest path of resistance rules in current flow laws) but not always and my 260 card runs 3 isolated rails.
But do know my GTX1050 does use a full 75watt slot currents for sure, and at full tilt (Furmark ,2d or 3d passmark , you name it , it works) Others besides me on other forums report the same thing.
See the way VRM'swork and how the low power VRM is on the PCI-e slot side.(  read how power really flows in the card, it is not LIKE YOU THINK, in most cases)

  The 5 pins are on the rear end of the slot    (the B1 , +12vdc group I call them)

These 5 PCIe pins to go pin 10 and 11 (24 pin power) and does not go to 12vdc CPU power 4pin jack.
The 5pins are on one +12v fat trace but the path , seem to follow  drunken Sailor to (J24,p10/p11).  (a super complex path)
The path is complex and for sure my test above proves it averages 4mm wide. (ending this investigation, on the power myths)

There is a 470uF  cap on pin B1.(group) at least.

GPU CARD TEST RESULTS: (I will keep testing until all  cards I have are tested) (some are new)

700watt PSU not that  Dell gutless wonder PSU.

(I tested with the card PASSmark and FURMARK) to overload this MOBO and failed to cause any issues at all) and max resolution, 1600pixels.
Before testing any thing like this, CMOS logic, the faster you clock it the more power it wastes, (laws of pure  physics, not SciFi) The more logic at work the more heat, (intel super scaler , or parallelisms, etc...)
The windows desktop needs about 10watts, so is not any kind of test ever.

My test jig allows me to test any card I wish, lacking restrictive non MT  cases) why in the world buy a small cased desktops, then wonder gee, I can't upgrade?
The stock VGA port runs 2 FPS (Furmark, slow as a snail) but this is power/functional, report not  about SPEED ! But first we overload the GPU, (means run it under max load)
  1. Brand new GTX1050ti (Zotac brand 2018) 100% and only needs 75watts max to run (way less with windows idle) Runs great despite Dell warning on 35watt limit!) (but keep all other PCI-e slots EMPTY)
  2. R5-240 Radeon (November 1, 2013), works perfectly at 30watts Despite  lots of false rumors about  dead R-series, this PC on the Dell forum about R series is dead (  and the false non UEFI canard, proof here)
  3. GTX260 (2008mfg) double wide power hog, 200watts, works perfectly BIOS and DVD-D mode (but needs both AUX power cables or it will not run ! with just one AUX power cable.)"amazing this works !)
  4. GTX650 works perfectly; (2012mfg) 110-134 watts. (with aux connector) runs perfectly.
  5. A lot of 5 random old cards year era 2007 Geforce 8400 GS ,   (all flea power) all function flawlessly and slow as should be. 5FPS in 3D test, slow.  even Runs in DUAL monitor mode, all by it self.
  6. ATI R9-380 a 190watt card ! Runs perfectly and FurMark at 67FPS, at 61C and really gets hot. on cooling tubes and on back of card. Runs great. Idle temp.'s = 33C @5watts. (GPU-Z info)
  7. (I don't own any GPU cards at  300-365watt  nor ever will)
The 260 above has 3 isolated 12vdc power rails, and are 3 VRM's separate,  PCI,(5pins) Aux 1 and Aux2, if any are weak or dead the card fails. (mine flat refused to FAIL no matter trying every  tick to do so.)
 The card hits the SATA port cables. (90 degree cables cured this... easy)

I run it  a very hot 90f garage the GTX260 under Furmark and passmark 2d and 3d.  runs perfectly even with the GTX belching out huge hot air from the rear, and 3D did 25 to 35FPS rate,  so much for 35watt sockets.
Myth Busted. (or at least 2 other empty slots = 50 more watts on tap, (gee, ohms law say so...) the 3 slots are on a 10mm wide  12v trace, the feed is 4mm to here. (my data)
The main motherboard, VGA port even warns you that the GPU card added is failing, (so be sure to try this jack VGA iGPU test at any black screen  failure)
The reason it failed in all my long tests was this adapter just below. (a $1 adapter DVI to VGA , is Brick dumb)
Watch  out some GPU port adaptors may fail,   See more here, more tests in adapters, by me.
Use only a proper monitor and port to test the GPU card or get super confused.

DVI-I to VGA for monitors that have no DVD-I or DVD-D ports. (love to fail, if the PC can not see the DDC/edid data or makes no sense)
Only this fat double wide card GTX260 hits SATA cables, but there is a cure. (but with the a normal case there will be MORE HITS)  (this is cheating, it is only to prove the MOBO does not fail with this power sucking hog.)
The SFF mobo will not work at all due to 24pin power hits this huge fan casing and you are sunk.
This card sucks 200watts total.(sucks,draws , and burns that power at full tilt)  It needs 17amps total to run.

My parts list: (Dell fears parts lists and I don't know why, HP wins that Quality metric big time ! at HP parts surfer)
Dell also has very limited parts to sell you, even owned for 1 day. (no PSU upgrades, limited grossly GPU upgrades) In fact 1000s of  cards GPU work in this PC. but Dell is clueless how or way.

See adapter horrors here (and lots of cures)

Here is proof, GPT boot drive works on this PC. (just as a test) And the W10 install F12 screen pick to get GPT.  For sure BIOS version A18 up.

version 2.  7-9-2019 (for those stuffing old Dell motherboards , in to newer alien cases) The D do not have ATX mounting holes,  and  the USFF has no PCI-e slots !