I have an Optiplex 790 DT, with a Core i3-2120. I would like to upgrade the CPU to an i7-2600. I have seen this combination advertised, and have come across postings which indicate that it is officially supported.

BIOS version A18, which I have, has notes indicating that support for Sandy Bridge is provided.

The owner's manual for the 790 DT indicates support for i7 Quad Core processors.

I attempted to install the i7-2600, and the machine powers on, but there is no video output, and no POST beep. The hard drive spins up, and the fans also do.

Replacing the Core i-3 processor allows the machine to run normally.

What else needs to be done in order to get the machine to recognize the CPU ?