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. 
(I have tested many  GPU cards here up to GTX1050 and R9s) and all run perfectly even 250watt cards !) I continue to test more GPU!
Runs W10-64bit all day long, no issues ever.

Many folks still use this great 790 , it was made in year 2011 and is not too old, it came new with W7 and I upgraded 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... but turned on  CPU advanced features in BIOS for sure NX (no execute enabled)
All tests done with SATA SSD and formatted, GPT mode. (64bit does that automatically)
Intel Q65 Express South bridge (chip set and very PRIME) and CORE gen 2, up to 95watt CPU.
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.
The F2 key 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 now,then that means you are too slow hitting F2, so try again.)
Do the long HARD reset if you have problems with BIOS acting dead or power button actingdead ,lamp or blinking oddly. (set BIOS defaults on any RAM/CPUchanges too)  The RTC COIN cell bettery is no good now, 9 years old +

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. 
(Learn that the PSU only uses power on demand, just like wall jacks in your home A.C, learn that if you have power lacking the OS will BSOD or the PSU SIMPLY SHUTS OFF ON ANY OVERLOAD)

See GTX1050ti run here

The beauty of my test bed jig is all PCI-expr. 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.
There are over 12 makers of cards and many have 10 versions of just one card !  Do not think this is like changing light bulbs, the card had to fit first and not overload the PSU 2nd.
The most easy PCs to own are full sized full height cards.  (if it can't fit or hits things , the card is useless to you)<< some guys cut out the DVD drive cage and HDD cage to fit larger GPU and or Larger PSU.
See My PCI-express card size page here. (full spec) for sure heights and more...

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 mobo may need its own front panel with correct Dell DP/N: model number, the cable end, Is keyed and most fit or you have the wrong panel, (DELL Hates parts list for PC's, so buy HP)

The secret 34pin jack for I/O panel.

See the DELL 380 PC with its odd ball 40 pin Power panel, (1/2 hacked by me, just to make it work, I did not trace out sounds(mic/lineout))

Some dells have  power switch others only have the one seen above, 380?  in the power panel.  (NO ATX standards here,  they do not exist)
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 sized , PCI-e GPU  card fan casings it.
 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  PCI slots at all.  Note the 4 PCI SLOTS , 3 are PCI express, the cream one is legacy only.

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.

Using the wrong fan: (Dell does not use Intel  fans and is wired custom to Dells needs)
The Intel fan works ok, wired correctly, but DELL BIOS will not like it  for RPM and sets errors in the BIOS 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. (To upgrade the PSU is easy,  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: (only LENGTH varies !)
This PC runs Windows 10-64bit,  1903,   )  Dell does not  support full  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  (well the limit is true but on practice , this is not a big limit)
 PCI-express.. slots. I tried all 3D tests to make it fail and it did not once fail or even hiccup.
Dell for better or worse does not have good spec. sheets, they sometimes warn you 2 ways for PCI-e power limits on slots, if in the manual or even those cute white painted words next to slot X16 (35 watts max)
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 seen below. There is   265watt PSU?  limit but lets  avoid that and use 700watt PSU.
 Dell limited SLOTS , due to there are  weak 3 PCI-e 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, rail)

A real MOBO (ATX SPEC) not this would have at least this sized trace here >  10mm  the question is what do we have.  (the below shows near 4mm sized) Yes that is very poor rail spec,. (but found out it works ok)
This mean this PC is NOT ATX spec, at all. !
Try to know that  a very high powered GPU , will have 1 or 2 AUX ports and only uses very tiny current flows in the PCI- slot, and even the GPU makers know to do that, after all there are DELL poor designs out here.
The GPU may in fact use huge currents.  28amps on the AUX rails or split 14 amps each and only 20 watts on PCI buss. 12vdc. just 2amps on PCI slot.  (there are no 2 cards made that are the same that is a fact)
NO LIE the SLOT X16 has  pathetic, 4mm power traces to the 16x slot and is a daisy chained mess.(means don''t put in a power sucking USB3 PCIe card here too !)
Dell did this to not have to pay for a power plane to the PCI-e slots. (cheaped out they did)
The above chart one can think if no other cards added I do have 60watts on tap, (25+45=) after all I did prove it is a DAISY chained power feed 4mm traces !
The PSU spec on +12vdc rails is +-10%  that is a voltage drop of 120mV allowed. The mobo and GPU both have huge capacitors help prevent that , V drop.
What I learned is all cards I own work perfectly on this PC over 20 cards  tried up to 250+ watts.

TESTING 1,2,3  I did real tests there to learn what is hidden in the PCB.(+12v) PROOF OF WEAK NON ATX MOBO IS HERE.
Tested at 35watt 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 (36watts)but most card do not hit this limit. 
The  best and correct trace  size (I dream) would be 6.25 amps (ATX 75w spec)  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?,  (even 250+watt cards)? even running the most GPU 3D modes (tests)
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 VRM's on AUX Jack 1 and 2, and tiny loads on PCI-e (B1 , 5pin set)  If 2 jacks both must used or the GPU goes dead.(not bad just goes dead)

Ohms law too can effect this. (and the least 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 a drunken Sailor to (J24,p10/p11).  (a super complex path) and yes weak. (no power plane used, sadly)
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. this helps with larger dynamic power surges. (same is turn inside the PSU and on the GPU card too)

GPU CARD TEST RESULTS:   See my full list of cards tested on all my Dells.

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 is not  about SPEED ! But first we overload the GPU, (means run it under max load)

Any card that uses over 75watts, will have 1 or 2 Aux power jacks on the side, (rear is common)  if you see 2 jack both must be wired to the PSU, for full power to the card or the card will BSOD, at full 3D load ! (read gaming)
    My old GTX260 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 GTX260 is a power hog and gets real hot at full 3D loads.
     The 260 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.
    Some high end video cards have this huge heat speader on the rear, and will hit the DRAM banks seen here. ( the FIT and not HIT #1 rule just got more complex)

    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, only 1 or 2 GPU cards supported or not sold now) In fact 1000s of  cards GPU work in this PC.

    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.

    This page is for those folks stuffing  any old Dell motherboards found on FleaBay? , into any newer alien cases
    Some Dells are BTX not  ATX mounting holes,  and some  USFF case type motherboards have no PCI-e slots !(x16)
    Avoid all DELL motherboards with the crosswise (45degree) PCU shock and even more alien crazy queer heat sink fan usage.

    Dell buying advice:
    The BTX avoid list is here:
    Optiplex, XPS and Dimension numbers, 755,780,380,760,3100-e310,3100,5150,9100,9150,9200, xps 200s/210/310/400/410/420/700/720, E520, E310, at the least.
    Using a DELL mobo NEAT like that , it complains (POST) bitterly about missing front panel, wrong fans, missing fans and missing thermal sensors and POPS  F1 errors until you fix that.
    Run for the hills ! BTX motheroboards boards are junk, do not try to use them in modern alien (to Dell) cases. The Heatsink is BTX ALIEN !
    Buy the whole PC and be happy, but never buy just the MOBO seen above.  See the crosswise CPU , RUN  away !

    I have a special PSU inductive ammeter so I can measure AUX power(current in Amp.s) directly.  (To use this with GPU-z and the Shark to double check which cables use what amounts of current) Furmark has all that build in.
    See my new W10 install limits page here, (shows even the above MoBo  runs w10 64 bit !)

    A better low cost PC is the DELL Optiplex 3020 MT for low used cost of  $50(no HDD). Best bang for the buck and very much upgradable, and most GPU cards fit inside and even RUN right.

    version: 3 .  7-9-2019