No digital output

I have just installed an Nvidia 8500GT (to replace Radeon x300-working fine with DVI) in my ThinkCentre running XP Pro but cannot seem to get any digital output from the card. Here's what I have tried so far:

VGA cable is fine- good display,
DVI-D cable swapped with known working one
latest drivers have been installed
monitor has been changed to digital (but reverts to analogue)
with both DVI-D and VGA cables in place no second monitor is visible in XP display properties.
with only DVI-D cable monitor reports "Check signal cable"
have checked BIOS settings but nothing seems relevant

How can I check that the is a DVI signal coming from card?