Can I assume that you are not mixing ECC and NON-ECC RAM? Secondly if those RAM modules do not work in either system then it would appear the RAM are defective. It could also be RAM that is not compatible. The only way to know for sure is to install brand new RAM such as that from Crucial here (it is returnable if it proves to not work) and try that. I am not a big fan of purchasing used RAM. There is no way to tell if the RAM has been overclocked or has been bouncing around in a drawer for some time.

Hi JOcean

Yes, they are all ECC. To make sure it's not an incompatibility between modules, I plugged only one at a time. 

Those I bought second hand trigger the 2-7 amber light.

I've been buying second hand modules for a while now because the recycled ECC modules are way cheaper than normal ones here. But with this experience, I'm starting to wonder if it's a good idea indeed. 

The first set I bought were wrapped in bubble wrap and a thin envelope for letters. I wasn't too surprised to see them faulty. I returned them. 

However, for the second set, I'm getting suspicious...unless I'm just unlucky.

If any one can confirm that a 2Rx4 PC4-2133P module should work in the 3620, it'll great. 

It's not mentioned what brands of RAM have been working and not working for you.  KVR (Kingston Value RAM) is hit or miss in Dell's.  Regular Kingston is ok.

I've been using A-Tech for years, bought from Amazon, including in my 3620.  A-Tech is competitively priced with a lifetime warranty.

When searching for RAM, I have an easy time with using "Precision 3620 RAM" in search bars, whether it be Google or Amazon.  "Non-ECC" or "ECC" can also be added, including the brand you're looking for.

Hi bradthetechnut

The existing ones are SK Hynix, the set I returned were SK Hynix too and the second set are Samsung but they also have a lenovo label.  They are ex-server modules.  

Thanks for the tip with "Precision 3620 RAM".

Hi

This may answer my question:

https://www.dell.com/community/en/conversations/precision-fixed-workstations/rdimm-for-t3620/647f98dff4ccf8a8dec0ea2a

I bought Samsung modules as well...

If the thread you linked answered your question, could you please say how?  I couldn't figure it out just by reading the thread.

Dell uses SK Hynix for their OEM.  My 3620 has both Hynix and A-Tech in it.  It was Hynix I replaced with A-Tech when it failed in my previous Optiplex 755.

If it helps:  Precision 3620 Tech Guide

Samsung is one of the brands that has been working in the newer, more finicky Dell's.

Can also check out userbenchmark to see what users are running.

If you're mixing brands of RAM (some Dell's are finicky about that too) make sure they're all at least the same volts, besides speed.  Mixing volts will cause a failure.