Dell Optiplex series 4,  3020 MT, setups, modifications and secrets told .        
Sold in 3 case form factors MT mini tower , SFF small form factor and Micro.  (light weight aluminum case is nice !)
Below is MT case coverage only and is the best, for full expandability.  (the wiki covers this)
OEM Dell Build dates 2013 to 2015.
This PC has one of the finest BIOS ever designed with amazing ePSA ( extended Pre-boot System Assessment.) runs even with the HDD missing! (a fact key to how good this is)

  
Click to zoom images above.
Featuring these key components:
Intel chip set H81, DDR3-1600 MHz. 16GB max. 4x PCI-express slots, and two USB v3.0 ports, Realtek™ sound chip ACL3220,  and supports most HASWELL Intel CPU processor (Desktop class up to 84watts TDP)
TDP means thermal design power, and is max power (in the form of heat) the CPU uses (at max load)
The 3 minor limits on this PC are , 300watt PSU (no larger sold at Dell and is non ATX spec .8pin connector and non standard standby power voltage +12vdc.)  (SOLVED)

This PC uses a  OEM designed  micro-ATX  motherboard, with 6 screws.  (the manual on it is here)
This PC can run the newest Windows 10 64bit no problems and is fast and very stable. The W10  and older drivers are here.
The Plug and play did all that, took no extra work by me to install W10. (from ISO built USB stick and working Ethernet cable to my router and web connected so PnP can do its job)

All PCs OEM have some non standard ATX spec parts inside, seen here (PSU) is one surprise but is a simple $7 fix to end that needless worry etc.



Listing of topics :
  1. A Slide show
  2. Power switch secret 1: (pin outs)
  3. PSU upgrade to standard ATX12V  version 2.3 or higher only !,.and the special cable for $6(and The adapter and  pin names on all jacks and voltages (Pin4 and 8)
  4. How to run Embedded Firmware diagnostics spa
  5. Supported CPUs, if BIOS is upgraded.
  6. FAQ's on this PC.
  7. What are my beep codes and or power switch amber blink codes?


Front panel power switch has 4 wires. (3020)   the real power button cable jack is here  a 6 Pin JACK, one pin is missing for 5 total.
For those lacking  real Dell CASE (chassis MT) or lost the power button?

The below is how I test these simple circuits.
See how a generic power switch is wired up here.  (ACPI is complex but not the PWRSW wiring !)  It is just 1 wire and 1 PULL-up resistors to make the power button work.
 
We use this simple fact (current measurements and voltage)   (math ohms laws ) and a meter to find which pin is which.?
What ever you do, do not apply 12vdc to any pin, in fact only ground pins to test them.  5VSB means +5vdc standby power, this is the power that is always there, PC off, and line cord HOT to wall AC jack.
The PSU  has  +12vSB pin at 1.67amps that is not standard at all.  (data: I measured the trip point and is at 4.95vdc exactly so is in fact a LDO regulator.  (but marginal for any real PSU ATX supply.)
 
This shows  how electronics works in relation to the ICH7 south bridge.
All ACPI power modes are in the ICH chip.(called power states and is complex but the power button is simple and works as show.
The PSU or the mobo can auto trip to power off , if any VRM overloads in the PSU  or  (the VRM onboard any MOBO)
The above is primer on power wiring in a generic PC.
Next up is the actual 3020.

Reverse Engineering(steps): This works on any PC. (desktops)

With my volt meter (DMM) I find the 2 pins on the 3020 power on/jack jack that reads at power supply voltage spec  3.3vdc. those are LED and the  power switch pins. the other 0volt pins are ground.
Then I short all pins 1by1 that were measured at PSU Standby voltage 5vdc to ground using any 100 ohm resistor  jumper and if the PSU is plugged in to the wall AC then the PC turns on. (fans spin ,etc) bingo you found it.
In fact all top brand modern MOBO have protection resistors on each pin here, and can stand up to any action of grounding any pin here directly even the great and common practice of using a screw driver tip shorting trick.
wire colors can vary at Dell, so  I will do my own jack pin drawing below.

To power switch simply  jump P3 to P6. ( all you need to know is here)
The LED is a bi-color LED (polarity reversal)  yellow plastic but glows white or yellow. (confirmed on P2 and P4 this is the power button switch LED pins)

P1   empty cut off pin keyed.
P2 blue 0v, LED cathode power on.
P3 black power on, 3.3v .04ma 8.2k
P4  Orange ,3vdc-on, LED anode
power on LED.
P5  Red.  2 ohms from here to P4
disconnected test. 3vdc on.
P6  yellow 0v, 0-ohms to ground
The pins with voltage present are:  3,4,5
The power on pin is: (P3) if you ground this pin PC will turn on. (jump P3 to P6 ground)

 And Acid test for the Power LED  is force a dead PC errors (like unplug all RAM and make sure you get the dead ram blink codes) The factory manual covers all this.
The Power LED blinks 3,3, amber code,  but is code 3, with NO RAM, no Memory installed. (the VGA ports are dead due to NO RAM)
Short of destruction could not figure out P5 usage.
This is the bi-color L.E.D. used, (In he power button actual)

CPU  list. all known to work correctly, except the red marked Xeon.
Learn that Intel built all CPU below, (not Xeon) to be fully working i7 top value and featured, processors, but maybe only 20% made that grade, the  chips are graded for speed and for working features sets !
Intel generation 4 only work on this DELL MOBO, like   i7-4nnnn the 4 means gen 4. (Intel coding) 
To see your processor in Windows, open the RUN box type "system information" and behold what you have now in your PC.

CORE™ CPU only and only Intel Generation #3  (eg: I5-4nnn)
The cheaper CPU's sold new, are all defective i7 processors , that means some i7 features are dead like dead cache sections or dead cores,! Not just slower clocks) You do get what you pay for here.

The LIST: (avoid dash  -T ,-R and -S CPU they are for laptops )
i7-4790 (or 4720)  has all 4 cores working all CACHE good. 8MB  full sized  +  proof it runs on this box here.
i7-
is best with hyper threading and TXT (trusted execution technology) the i5 and below do not have this..  but i5's are not really chopped liver...
i5-4570/4590/4690  first CORE that has mostly has  4  good Cores. and full 6MB cache. (2MB of cache is dead, found at final testing.)  The 4690 has 4 CORES.  i5-4nnnT (The T at the end means 2 Cores)
i3-4130 Gen3, only 2 cores work,  (But does have   hyper-threading working  and dead other sections of the CPU are (2cores  and 1/2 the cache 4MB are good at final test, Intel.
Pentium Gen4, G3320  all defective i7's  The 3MB cache and only 2cores functional (ie: 1/2 dead) and Lots of  "NO's" in the advance technologies section unlike above .
Celeron Gen4, G1829  ($20 used value) this chip has the most defects of all, the above. 2cores only 2MB cache, The largest list of NO's is here. (this is the super dog chip, below this is scrapped by Intel)
i7 processors sell used at a premium ($$$) due to gamers bidding like mad for them... ($150 used is big money this old)

The name Pentium "P5" no longer means 1993 SILICON, it now means even brand new defective i7 CORE™ processors.  (now that is cute)
Look here in the wiki for CORE gen4, and look only at Desktop CPUs at  95watts or less( only in G4 Haswell and only Desktop)
Xeon processors can be a cheaper to buy used, after expulsion from Server farms vast the Haswell Xeon is,  E3-1225 v3 (NOT V2) SR-14V OR SR-1KX (this is not confirmed to work yet,
hypothesis !

The H81 chip  spec.

The block diagram is like this.
In all cases above for CPU upgrades, update  the BIOS firmware first, so the PC supports all Gen4 microcodes.

Processors are sold just like CUT diamonds, the more perfect the cut , the more valuable they are  (given: size /color all same just cut)
Final testing at Intel production plants,  grades the CPU, with very expensive ATE test equipment and if any feature is dead, it is Down graded, call BINNING down. (BIN#1 is i7, my guess)


Core i5 & i7- processors   use 84watts CPU and the DELL PC has larger Heat-Sink (HS) for it.
I show the two sizes of  HS here, and full part numbers.
The 
X3JDD  fan is 95watts, this is the best Heat sink by DELL , the tag on the connector must be marked like this.
A used i7 goes for $130 and up, and is not really warranted. (on my $50 PC)
 

All CPU's found in ANY 3020 will run Window 10 -64bit  (proof is here) 
Even the lowly Pentium.




The PSU  funny power cables secret adapter colors and wiring,
Rumor has it that this Adapter ,works with T20,T1700,7020 and 9020 and not just 3020.(and does for sure on mine)
SOLVED !
I check this out very carefully so I did not blow up my system as so many have.

I gleaned all these facts from wiki on ATX spec, and reverse engineering every inch by hand.
I needed a bigger (wattage) PSU for my Video GPU card.
See my   Nvidia GPU cards  hogs cards list, this is only the hogs , all others run with stock PSU.
The small 4 pin is ATX standard v2.3 up is best , but not the 8pin seen on the Mobo. , it is Dell 100% unique.
The  Dell key facts to know are:
  1. The stock DELL PSU is 12vdc only PSU,  at 300 watts max. Plenty but not for the Nvidia hogs listed above.
  2. The stock PSU has no +5vSB standby pin but in the stead, a 12vSB  standby pin. (non ATX)
  3. I tested said pin at the Mobo purple and the threshold is +4.95vdc, ( too low  for normal 5vdc)  < my Data and tests.
  4. The wire color are different on the  2 white wires, and are 12vdc. ATX has no such color and should have been yellow. (as the adaptor below cures)
  5. The stock PSU 4 screw mounts are ATX spec.  (so all ATX PSU fit)
  6. To upgrade buy the adaptor and new PSU and plug it all in, end of story. (If new GPU card needs power to the 6 pin jack, then use the new PSU cable for that PCI-e cable)
The below is reverse engineering steps to glean the above.

Next up the adapter card that cures all my power problems:  (do not read this ,it is only supporting evidence , testing and proofs)
 
Below are the imported $4 china cable adapters, that gringo's in USA sell for $7 to $20 +shipping. Seen on Ebay or Amazon. 



ATX12V SPEC, AND SPEC, COLORS.                                              ATX spec. and the PCIe-8/6 pin or EPS must never be used on this mobo,  the atx12v is correct 4 pin. (v2.3) CPU power jack .


If standby is working okay, a tiny Motherboard Orange LED glows 1/2" inch from left the 8 Pin PowerCON  connector, if not the PC  will not turn on until 12vSB power is restored. (bad PSU?)
The Orange LED glowing means do not unplug internal parts until all AC line power is removed (do not HOT plug devices inside any PC)

FACTS to skip , (only for me)
The Molex P/n for this plug is (Minifit jr. 4.2mm pitch)) 5566-8 socket or 5557-8 plug (seen here)   or here at Mouser.com( I say this only if you want to make your own cable from scratch from a scrapped PSU cable)
My Pin number # are molex spec, not Dells. Male 5557-8-BL end.(5557-08R-BL) First sold by molex year 1990.
The only potential problem is here:
Looking INSIDE THE PSU  (Dr; Evil)
Both White/yellow wires go inside the PSU and connect a single RAIL, and is 0.00 ohms between them. (and voltages exactly the same, so again one rail,  and rated at  near 25 amps  and the limit is the wire gauge not the VRM.
The PSU may in fact violate the 240VA limit rules(ATX) on each 12vdc rail.   (I'm not sure and have no 250 watt load bank here)
The truth of all this is covered here. by Mr. Mark Allen. (thanks Mark !)
The PSU has an  8 amp fuse inside.(pig tailed cylinder normal looking fuse)
All black wires also go to a single point. (ground)inside.
Now back to the business at hand, the upgraded PSU:

Actual:  Top photo view is right side. (note the VRM chips just below the molex 8jack )
The actual location of purple is top right corner, this photo is rotated CCW 90degree's I did this to match MOLEX documents.
The above is old colors and below is new with the adapter. I am using molex Pin number spec.'s only.  The Motherboard has mark for P1 too, top left in photo above.
P1 Purple
P2 black
P3 black
P4  gray
P5 yellow
P6 yellow
P7  black
P8 green
------------------------^lockside^------------------------- (mobo jack viewed)  
There are 2 views , mobo socket view and end of  PSU plug view. This is mobo socket view.
Molex view. and #  Seems Dell uses the same pin # names.

Note the small lie here, ATX this is NOT ATX spec. of ANY KIND. and for sure not EPS-12v
Data (collected by me using meters) Careful testing or risk blowing up good parts. (SOLVED)
MY NEW raw data (live testing) for the above jack:
P1 Standby power is 11.3vdc "ON"(and I measured the trip point threshold on this pin, using variable power supply, and was  at 4.95vdc exactly (that will be a problem)
P2 and P3 black, are 0.0v ground.  as is pin 7.For sure GROUND and confirmed.
Pin  4 gray is 4.9v "Power on pin" status. (this voltage shows me the onboard VRM 5vdc is working) The PC is on now in this test.
Pin 5 and 6 , Yellow  is 12.170vdc  exactly on both pins and tells me there is only one 12vdc VRM inside the PSU. Raw power from PSU. ( I think proves it is a Virtual RAIL PSU) Same exact voltage hints to that.
Pin 8 is 0.06vdc, the  power on pin. "power GOOD"  (PC powered on) If the PSU or Mobo sets the pin to logical high  ,or near 5v the PSU shuts off.

Do not let the smoke out wiring this up wrong. (that be NO JOY):
The big blunder here is 2 fold,  using the word ATX above markings, and even vastly worse, not using  custom MOLEX jack with custom keys ! no ATX keys !!!
The best fix would have been  to make the 2 square keys not SQUARE ending blowing up new PSU, but alas we can't put the tooth paste back into the tube. (but I never just guess, when I see non spec parts I read, or use my meter)
Later:
 
NOTE carefully the unique  MOLEX  keyed SHAPES on the plug below, same as wrong using EPS above, but  dead to rights wrong (boom smoked if wrong) Luke "Avoid the dark side" do not guess.

Below are real ATX v:2.2 or v2.3 modern PSU, that use ATX12v spec connectors and correct colors. the yellow (ATX spec) means 12vdc rails, black is ground.  The PCI-e work on  Video cards (hogs)


 The 4 pin above works on our 3020 mobo VRM jack, do not use the 6 pin unless inserted into a high powered, GPU card, or let the smoke out.
The 4 Pin powers the CPUs VRM regulators modules and is NOT OPTIONAL
One serious issue using old PSU's  is the minimum current spec, sees 3.3vdc rails/ 5vdc rails with no current  allowed and self shuts down.  (yah that) < mostly RARE to see today.
Buy a PSU with spec version 2.3 no less, and no minimum current limits.
Watch out for any PSU  with  minimum 3.3vdc load of 3amperes  or on the 5vdc rail, and if this trips the whole PSU goes off line (power_OK false)
The PSU should run ok at 0 amps load on any pin, per ATX spec; but  some do not. (avoid cheap PSU? or relics)
One way to avoid this is run the HDD/SSD/DVD drives power directly to the PSU, not using this mobo power feeds for those. Leaving only un-used 3.3vdc to worry)

The rules of this game are simple buy new PSU, not 10years old, and from top brands not "
XinTongYe or the like"
I like what is seen on Newegg, and one is Thermaltake brand., or Corsair.  (buy PSU that has no 1x4 molex power plugs for year 2000 HDD or DVD , buy only SATA grade PSU, for sure if used. (nor floppy power)
There is no need for 2 decade old device power plugs today, only SATA, mobo and PCIE power.
One way to avoid tons of useless cables inside any PC and better air flows., is this choice, modular design. 
Only connect up cable 1 to 4 below top down list right side.  (note how the   avoids ATX true wire color spec)

See my page on bench testing any PSU

MY PC is now fully modified, and works perfectly. 
MORE TEST DATA BY ME:

The AC power line current on my PC draws 0.2 amps,  that is only 24 watts (VA) to run this PC.  (no GPU card , Windows 1 idle) NO LIE my PC before hacking is A GREEN PC at 24 tiny watts.

 With an old  test video card GT640 the current is .3amps and is 36watts total. (flea power this is, tiny) No where near 300watts (possible)
Then I remove the 640- and put in a GTX650 power hog card and the PSU and the adapter.
I  ran  the   full benchmark PASSMARK(tm) and it all runs perfectly.  my i5-4670 runs PM of  CPU mark=
8140 (65%), the old  G3320 lame Pentium, runs a low 1700.
With correct fan (32mm deep) CPU 4 cores never cross 50C ever using Furmark or Passmark GPU full testing. (at idle CPU temp is 32C)
The new PSU below is good for 20a +20Amps , 40 Amps  , 480watts to the 3020 rails.  ( the new PSU burns 192watts, total with my GTX650 card (full tilt 3D)tested.


We now know the mobo, 12vSB input has  LDO regulator that works at 4.95vdc, (on the cusps of working) This is a fact, that I discovered. (tested  and has
Hysteresis  a bit lower)
DONE:

FAQ:
  1. Can I run a SSD DRIVE , yes ,even run 3 SSD drives. or  a mix of HDD/SSD and kick the DVD drive out and replace with a HDD/SSD caddy drive using  this DVD space. (yes it can even boot the PC) 
  2. Can I clone the old HDD to SSD sure you can (I did) and it will run, but the old Dell recovery partitions will now be useless. (best is to upgrade to W10-64bit  and own your own paid for ISO  and COA)
  3. Can I replace the PSU with standard PSU ATX12v , yes  as seen above (buy the adapter ,plug in new PSU ,done)
  4. Can I upgrade the CPU, for sure yes, up to i7- 4790. ($130 used)  i5-45xx and 46xxx
  5. Can I run Window 10 64bit, yes. (fresh loaded, and boot to DVD or USB stick installer , with Microsoft ISO built. (using MS tools to that ) (in BIOS turn  on all advance CPU features, first, NX/ND/TXT/VT(x) etc.)
  6. Will W10-64bit run on that dirt cheap Pentium gen 4? answer , yes ! even the $20 CPU sold on Ebay! G3320 !
  7. Can I load 16GB of DDR ram, yes, buy it from Crucial.com and sure. 1600MHz rated, the vendor will show you what works.
  8. Can I upgrade the the GPU ? Yes, with  a modern powerful GPU card, sure. The  GTX750 with no PSU swap,  or better a GTX1050-TI or any card that fits and with a PSU upgrade.
  9. Will my new GPU card fit the case?  not hit the HDD cage? ( the space is 10.25 inches or (260mm) there are 100s of cards, sold, each as a data sheet and size stated , check first card sizes).
  10. What are the Nvidia card wattage spec.'s that might exceed 75watts, the full Nvidia list and  best here, but I made a counter list of power hogs that all need a larger PSU.
  11. The GTX750 is an old but  fastest card old card , at 75watts max !  + cheaper  and best of all no PSU upgrade needed.
  12. My PC has amber LED blinking error codes, what do they mean? , well the manual tells you that, if you read that. (yes, do read the manual ;RTM)
  13. What heat sink grease is best, easy,  Shin Etsu !  G751 or better, ZERO HYPE !
  14. My fan(s) run  too fast, but PC is not overheating , answer turn on the autofan mode in BIOS, so it can regulate better.   For real overheating see here.
  15. What CPU processor do  I have?, Answer:  in the windows run box type, "system information" with no quotes,  and bingo, there it tells you. (OR IN BIOS F2 page click information, bingo)
  16. Why does my heat-sink over heat,  you may have the wrong one 20mm , only the DELL 06GCNY (deeper fins 32mmm), rated at 95watts,  see both HS together here.
  17. Where is Dell parts lists,  Dell thinks you don't need one, so here is mine,,,,or the a good parts picker is here, 
  18. Why a GTX690 GPU will not run with the factory PSU, because that GPU uses 300watts all by itself, so only  a PSU upgrade works here, a 600Watts ATX12v PSU, with adapter cable   $4-8 
  19. What AC power "watts" does my PC  use, with no modifications at windows desktop, idling, answer 24watts, (yah the whole PC) how much max load on CPU/GPU , answer 36watts. (Pentium CPU version)
  20. How much power does my newr GTX650 power hogging GPU card use. (110watts itself and the whole PC 192watts running the FURmark GPU oveloader)
  21. Answer: GTX650 and running  PASSMARK 3D burn-in test, scored 2576 -26% DX12.
  22. My PC is dead, why? (generic answers long)
  23. What are PSA diagnostics, see below answer.


This PC has firmware embedded DIAGNOSTICS called "ePSA" , Enhanced Pre-Boot System Assessment.   Dell Optiplex PC's are great PC's !
This
tool runs even with the HDD removed ! (it resides in FIRMWARE is why.)
IT IS THE BEE's Knee's.
See beeps here
The top 3  errors on any PC and sure same here is bad laptop battery death (or shorted worse) but not applicable to any desktop.
The RTC coin cell dead at 7 year mark, and last HDD death at 5 years (common) below 2.9vdc is no good and doomed if close. (3.3v is normal )
Next is HDD failures (5years old or  more)
Before doing all the HDD tests, turn on S.M.A.R.T hard drive testing in the main boot F2 menu's so the HDD tested properly
Power on PC and hammer the  F12 key gently, tap tap tap F12 then pick Diagnostics from the boot F12 boot menu list.
The HDD test is very important to run and will run on encrypted drives and Bitlocker drives (it does not touch your data, is how this works) the HDD under test  can be blank or a non Dell HDD of any kind (SATA).
To abort tests , hit ESC key, then click check box "Thorough Tests" mode (full), and use the space bar (help = F1) and de-select all tests but Hard Drive 0 (boot drive) and then  click run tests for a better HDD tests

2000-0151 (this error code flags any HDD that has failed S.M.A.R.T tests 142 ! or any HDD errors, not just lost boot files or just can't boot) 
If the HDD is missing or not connected , the 151 pops up fast! (and begging for F1, (no boot found)
If the HDD fails, we run a boot Linux "Disk Test" in demo mode.
If  any HDD fails  S.M.A.R.T tests. You can install Windows fresh 10 times and will never install or fails fast. (buy a new drive now)
 

The other PSA codes are here.! at DELL.com easy as eating pie to find.

Here is the HDD failing, (the drives are bad if S.M.A.RT or ePSA or PSA  , learn to believe PSA it will not lie to you and the HDD actually has a brain inside that does all this magic, 2001 to now.
The below laptop battery, ignore that for any desktop PC,  there is no such part;
The below errors first a new main battery is to be installed  then charge it and then see if the HDD wakes up from the dead?,  as many  know, low power  can make the HDD self shut down to protect itself. (a FACT)
  PSA

The power on button LED has blink codes too and can beep, all this is in the manual to read.
The top of the SATA cage has  beeper that beeps on errors. (It first plays a melody like song first,  then uses beep codes and the power button flashes yellow (amber) same code)
The codes are in the manual on page. 39/of 47, code 2,4  is dead coin cell, 3,2 (a USB device shorted) 3,3 memory failures, I pulled the case fan got code 5,1,1. (newer BIOS, more codes, that I do not know yet)

Best links:  
Youtube secrets. on PSU and power switch .

INTEL (tm)   best processor. Gen 4 , CORE:

My parts list with Dell P/N (DP/n)

This PC does have standard devices. (my list of yes/no standards)

The ATX motherboard holes are and industry standard, per this drawing.  and this too.

ATX  PSU RAIL, facts best I've ever read, and is true.

I marked up a REAL RAIL supply here.

More photos:
My i5-4670 is much better than the Pentium.
Here is mine.


The FAN deep is Dell P./N  06gcny (with or with out that 0)  one on the left is 20mm , and right is 32mm.


Look Ma... no Dell PSU !


Micro_ATX (m_ATX)  is a SUB SET OF ATX HOLE SPACING,  ( The 2 key documents are Here and  Here)


Lets talk RAILS,
(PSU topic) , no each seen single yellow wire is not a rail , sorry , no way Jose'.
WHAT ARE RAILS:


New PSU, GPU card ,heatsinkfan,SSD, HDD 1TB, best of class processor i5,

Not covered here: is me reverse engineering the front panel USB jacks or HDD activity blink LED , just buy the panel for $10 if lost or missing and avoid that pain.
This page I made helps others mix parts up and get a working PC.

version 1.  4 -23-2019 . No association with DELL company, am a Retired USER only now.  and this page is only my Opinion....