CT2K51264BD160B

Crucial memory is suggested as it is guaranteed to work in a Dell.  Dell's have a history of memory compatibility with non-Dell memory.  Also replace memory with all the same type, do not mix as that frequently causes problems.

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-780-small-form-factor

For future posts, always include your specific model.  I had to look up what the motherboard was to find you have a Small Form Factor version.  

Dell Optiplex 780 Tech Guide.pdf - if it helps.

Adding to what fireberd said, max ram for the 780SFF is 16GB.

Hi @bradthetechnut,

Not sure why I cannot reply your private message and thus reply you here.

And yes, you're right on the information you found about mix use of memory in different frequency. 

In some cases of other PC brands/models, DIMM chip density and memory slots in single or dual channel may make a difference too.

I discovered there's at least 2 different manuals for the 780.  One manual states it can only take 1066MHz RAM while the other states that it can take both 1066MHz RAM and 1333MHz.  This has caused debate in other posts.  (I have one manual (stating the 1333MHz RAM) and bmcowboy has the other.)

However, it's in the fine print:  "This system supports both 1066MHz and 1333MHz DDR3 memory, however the memory will operate at 1066MHz speed due to chipset specifications."

That is the correct memory according to Crucial.

Assuming the PC worked OK with original memory, the new memory is installed correctly, and you didn't damage it by handling, contact Crucial as they guarantee memory to work in a Dell.

However, one thing to try.  Reset the CMOS memory.  Power off the PC, disconnect the AC power cord to the power supply.  Open the case and locate the CR2032 battery on the motherboard.  Remove the battery for at least 10 minutes.  Reinstall, reconnect the AC power cord and try.  If it still has problems with the new memory.  Power off and reinstall the original memory and if it works OK, contact Crucial.

I've installed 4x4GB RAM in my Optiplex 780. The spec is:

Crucial CT2K51264BD160B 8 GB Kit (4 GB x 2) (DDR3L, 1600 MT/s, PC3L-12800, DIMM, 240-Pin) Memory

Despite many attempts it does not work. 

PC makes 6 buzzing noises. 

Nothing appears on the screen.

I'm ignorant when it comes to computers so any advice woukd be much appreciated.

Thanks. 

The "1600" in the RAM model # = 1,600MHz.  As I posted above, the 780 takes either 1,066MHz or 1,333MHz RAM.  Optiplex 780 Tech Guide 

I got after Crucial twice this year for having the wrong RAM on their website.  Twice for each time a Community Member had a problem with it.  Now this is the 3rd time.

I sent Crucial a link to the Tech Guide + a screengrab.  I see they still have the wrong RAM on their site.  By the way, when selling RAM, they don't refer to something like a Dell Tech Guide; they only refer to their website.

A couple of other tips:  When upgrading hardware, make sure you have the latest BIOS that's for your 780.

If your size 2032 CMOS battery hasn't been replaced within the last five years, upgrading hardware is a good time to replace it, not just reset CMOS with the same ol' battery.  A low battery can cause all sorts of bizarre problems.

Hi and thanks. 

Only just realised this morning that your comments were for me. 

You said:

"When upgrading hardware, make sure you have the latest BIOS that's for your 780." 

I've no idea what that means, let alone how to go about upgrading the BIOS. 

Help! 

Andrzej 

I may look into getting anew battery after our holiday booked from tomorrow. 

I'm confused re. how the emails I receive integrate with this site and how I find whoever asked me for security tag. It was Delcares, I think.