new pc with radeon 6670hd GPU.. cant install ubuntu

[R Wooster]

Hi.
I recently bought  an Advent DT2111  PC with a AMD Radeon  6670 HD GPU.
When trying to run Ubuntu off the disk, the screen goes all lines and then switches to standby. Obviously Ubuntu starts with lower graphic setup. I uncovered the motherboard D plug socket and plugged my monitor in there, to try running Ubuntu and then hoping to download a driver to work the 6670HD card, but there was no signal from motherboard on that socket. 

    The PC is only weeks old and I don’t want to open it, and void the guarantee.
Can someone suggest how I can get the system to work on this PC.
Bob
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