Optiplex computers are known to have proprietary psu's. Better to replace the original one with the same model psu. This is from a post from 5 years ago--

The  OptiPlex 9020 MT supports/requires a Dell proprietary 290w power supply, a standard ATX power with 20/24-pin main motherboard power connectors are not compatible, there no compatible retail power supplies available.

Hi Mary, 

Thank you for your answer! There exist a thread (also here, in the Dell Communities) and a YouTube clip that claim it is easy to replace the PSU using said adapter. The links are: 

https://www.dell.com/community/Desktops-General-Read-Only/PSU-upgrade-for-Dell-Optiplex-3020-Mini-Tower/m-p/4633951#M989905

https://youtu.be/v7k0ZfT9n7A

Do you (or anyone else) know whether these users are in error? Thanks! 

JW 

next time tell what PSU you bought ,exactly, the 600 or 700w is my wild guess but what.

and the the lid off tell the helper in this MT what is inside, mine has a full full load of HDD and SSD and power eating GPU , 190watts GPU alone.

see.  , we can't see inside any Desktop and  lots can be there.....

my guess is you did not do the strip down testing with old PSU

BeQuiet has products from 300w up 1000w, and to be sure the 700w will work or the 600, but no PSU stated

at all , sadly.

here is how to post, what you bought an example

https://www.newegg.com/be-quiet-pure-power-11-700w/p/1HU-004H-000F8?Description=be%20quiet%20power%20supply&cm_re=be_quiet_power_supply-_-9SIA68V8UH3859-_-Product

one of  these, i bet but which one, in price order.

https://www.newegg.com/p/pl?N=100007657%2050085693&d=be%20quiet%20power%20supply&Order=PRICE

be quiet! Pure Power 11 700W CM, 80 PLUS Gold efficiency power supply, silence-optimized 120mm be quiet! fan, user-friendly cable management  as you can see the 700 is goodchoice.

700w

First off I want to give a BIG thank you for all your help. It was extensive and in depth - great!!! 

I realize that I should have given more information along with my question, so here goes... My PC is 1.5 year old since I bought it refurbished. The model is an "Optiplex 9020 Tower QuadCore i5 4570 3,2 GHz" which I use(d) as my home office workhorse.

Everything worked just fine until a couple of weeks ago, out of the blue the PC went **bleep** up: Nil reaction on pushing the startup button. The only reaction I could evoke is by pressing the push button next to the "BIST LEDs" that results in the LED lighting up green, and the fans of the CPU und PSU starting up, nothing more. After releasing the button the fans stop turning, the LED is extinguished, and that's it. One after another i removed all peropherals while repeating the test until mothing but the mainboard and the CPU were Be Quiet! Pure Power 11 400W cm". After connecting it to mainboard and CPU I switched it on and...  nothing happened; quiet as the grave. I borrowed another PSU from a friend, an "Antec VP Power Unit, 450 Watt ATX" - same negative result. 

I am now at a loss, how to proceed from here. I'm starting to suspect that the real culprit is the mainboard, and I'm thinking about throwing away the whole proprietary Dell caboodle and installing standard components instead. What do you think? 

Thx agin, yours truly, JW 

Fake, under powered, counterfeit power supply and moddiy adapter could have fried everything including the adapter.

There are no issues with proper known working power supply and moddiy adapter or the optional 365W dell power supply without any adapter.

There is no free lunch or fix for physically bad power supply etc.

You have to find out what is not working before you can find out what is working.

This is why I recommend the DELL 365W power supply.

Why, for goodness sake, do you suppose I bought fake products? Furthermore, my new power unit has 400 Watt; you call it underpowered, and recommend a PSU with less power. Not helpful. 

Recommended optional 365W unit is from Dell.  Dell does not over state the capability of their power supplies.  Choosing a power supply based on a single spec of watts and a price is a problem.  You chose poorly and may have fried everything.  You now have a dead machine that will not power on.

DELL 460W units are 650w if you add up all the amps on all the rails.

DELL 365W units are 550W if you add up all the amps on all the rails.

The catch is that you cannot use the MAX power on all the rails all at the same time. The issue with a counterfeit knockoff power supply is that they do not have over voltage and over current protection AND they DESTROY things they are attached to.

You have to have the correct amount of power available on multiple rails.

This is why if you use moddiy adapter that an EVGA 700BR power supply is minimum recommended.

700 B2 BQ G2 W1 N1 versions from the same vendor DO NOT WORK.

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

There is a reason why the Bequiet power supplies are $30 to $60 and the working EVGA versions are $74 to $110.

https://www.bestbuy.com/site/evga-700w-atx12v-eps12v-80-plus-bronze-power-supply-dc-dc-technology-black/6346160.p?skuId=6346160

Hi there I'm curious why is the minimum a evga 700br. Cant I use a 500br instead?