First remove new ram, test pc with old ram, verify it posts.

then remove old ram, install only one new ram in DIMM 1 slot, make sure new ram snaps into slot using correct technique, test post.  It should post if everything done right. The spec of new ram is correct.  DDR3, 4GB, 1333Mhz, unbuffered, desktop only.

if successful add the second new ram in DIMM 3 (so you can have dual channel) using same technique.  Test again.  If good, add old ram one at a time to DIMM 2, 4.  this is assuming you had identical pair of old ram.

Whenever new stick causes error or beep, remove and try again (of course every time power off and drain power from system by pressing power button first). Wear ESD band or ground fingers.

I presume PC posts with old ram now.

try clear cmos:

If still no change, and unless you have another working PC that accepts DDR3 to test new ram, the working diagnosis is that these new RAM are bad.

Thanks @redxps630 

My BIOS is the A22 (newest available). I changed the CMOS battery, just in case.

I took out the working 2 sticks out, then tried to put 1 stick and DIMM1, then DIMM2, then DIMM3, then DIMM4, then did the same with the second RAM stick.

I always get the same behavior:
* screen is black (DELL logo does not appear)
* indicator leds 1 + 3 are shown and power button is blue

So I cannot get any of the ADATA sticks work in single mode at all.

Is there anything else I could try, before concluding, that both RAM sticks are corrupt in some way.

And thanks for the help.

AD3U1333C4G9-S ADATA 4GB PC3-10600 DDR3-1333MHz non-ECC Unbuffered CL9 240-Pin DIMM Dual Rank Memory

Newer JEDEC spec specifies 1.35v low density CL11 Single rank ram.

This ram doesnt work and will never work especially mixed with other ram that is wrong density, speed, rank, voltage etc.

https://www.amazon.com/dp/B00W7DUSPA

https://www.amazon.com/dp/B005LDLUW8/

• CT51264BD160B
• 4GB PC3-12800 DDR3-1600MHz non-ECC Unbuffered CL11 240-Pin DIMM 1.35V Low Voltage Memory Module

Mixing rank ,speed both wait states CL9/CL11 as well as DDR3 1600 or slower, density, voltage, vendor is not supported.

Dell is VERY Picky about multiple parameters including the NEW Jedec 1.35/1.5v spec. DDR3 Low Voltage, devices are a new standard that operate from a single 1.35V power supply voltage compared to 1.5V in older devices. Under the new standard, DDR3L memory devices will be functionally compatible to DDR3 memory devices, but not all devices will be interoperable at both voltage ranges. The other issue is that 1.5v uses more power than 1.35v and 4 modules load down the bus more than 2.

Thats why some dell models only have 2 ram slots instead of 4.

DDR3-800, DDR3-1066, DDR3-1333 1.5v is too slow for 4 sticks and uses too much power.

https://web.archive.org/web/20100905022734/https://www.jedec.org/standards-documents/docs/jesd79-3-1

Addendum No. 1 to JESD79-3 - 1.35 V DDR3L-800, DDR3L-1066, DDR3L-1333, DDR3L-1600, and DDR3L-1866

@redxps630  Thanks I tried and unfortunately that did not help.

@speedstepThank you for your comment, but do I understand it right, these RAM modules ONLY do not work, because they are CL9 and my motherboard requires CL11? But how come my PC runs right now with two of these

* 2X Nanya 4GB NT4GC64B8HG0NF-CG
* PC3-10600
* DDR3 1333MHz
* non-ECC Unbuffered
* CL9
* 1.5 Volt
* 240-Pin DIMM Dual Rank Desktop Memory
https://www.cpumedics.com/desktop-240-pin-dimm-nanya-nt4gc64b8hg0nf-cg-4gb-1333mhz-pc3-10600u-ddr3-1333-240-pin-dimm-desktop-memory-ram/

Or what details am I missing in the description of the RAM modules? Or is it, that there are only limited selection of RAM modules, that work with Optiplex, although all details appear to be identical?

"If you dont have an answer, so please, be so kind and do not post anything."

All of my answers are true.  They are not what you want to hear.

You didn't choose compatible ram and you chose ram that is too slow.

This usually happens because its lower priced "high density" and is not intel chipset compatible and its not 1.35v compatible.

The vendor you posted links to also lists specific ram

Hynix HMT451U6AFR8C-PB Computer Memory 4GB 1Rx8 PC3-12800U

I do know what works and so does the vendor

3D-14D330N643053-4G 4GB Module DDR3 PC3-12800 CL=11 non-ECC Unbuffered DDR3-1600 Single Rank, x8 1.5V 512Meg x 64 for Dell Optiplex 790 MiniTower

https://www.memory4less.com/3d-memory-4gb-ddr3-pc12800-3d-14d330n643053-4g

This is 12800 ram not 10600 ram

its single rank and mixing speed and rank and vendors is not supported.

Your ram doesn't work and it wont.

"SINGLE  ADATA module" which did not work which means it won't ever work because one or more specifications are incorrect.  I do not support Adata.  I'm not confused at all. Your Adata ram is incompatible and it always will be.

Memory for less also recommends single rank dual voltage DDR3 1600 ram

https://www.memory4less.com/3d-memory-4gb-ddr3-pc12800-3d-14d330n643053-4g

Hynix HMT451U6AFR8C-PB Computer Memory 4GB 1Rx8 PC3-12800U

This too is CL11 and VERY specific

Mixing ranks and speed and voltage and vendors is never supported.

Using anything other than what vendors say is compatible with your model is not supported.

I'm not posting details for specific vendors as well as JEDEC timing and speed and voltage etc.

I'm not supporting this and I never will.

I do not work for those vendors.

You can only get support for what was shipped with your system.

If you buy from 3rd party then you must get support from them.

@speedsteplook, I asked a question, this is what the forum is about. I asked something about a 3rd party product.
If you dont have an answer, so please, be so kind and do not post anything.

Because essentially you are spamming here the following message: "I dont know anything. If it does not work, it does not work". Maybe its funny to you, but it is time consuming and not helping.  If you have insights, WHY something is not working,  so that it improves the knowledge of the readers - then post, else better not.

not sure if you have said it or implied, are you able to bring pc back to normal POST (Dell logo on monitor) now using either 1 or 2 of your old ram?

He already tried using 10600 single stick of ram and it does not work.

Its not going to work,

Its dual rank high density 1.5v only CAS9. DDR3 10600.

Crucial and the other vendor specify single rank low density CAS11 DDR3 12800

3D-14D330N643053-4G 4GB Module

DDR3 PC3-12800 CL=11 non-ECC Unbuffered DDR3-1600

Single Rank, x8 1.5V 512Meg x 64 for

Dell Optiplex 790 MiniTower

https://www.memory4less.com/3d-memory-4gb-ddr3-pc12800-3d-14d330n643053-4g