Dell Precision 690 Lost service tag.

Hello
i have just brought a Dell Precision 690 from eBay and i am having problems with it.
It is flashing an amber light at the power button and lights 1 and 3 are lite up after looking at the Dell website i saw that it was a power issue.

I have a look at the power supply and found that it was making a long high pitch noise. I think i may need a new PSU.

But anyway back to my point i have been looking at it and it does not appear to have a service tag number so i cannot contact dell support with my problems and it got me thinking is there any way of recovering an original service tag after it has been deleted?

Tim