Problem solved.Although Windows [10] update said "you are up to date", a visit to the Dell website revealed that 14 drivers needed updating, including the video graphics driver, and once that was done, HDMI video worked fine.
Dell Precision 7550, running Windows 10, does not drive monitor on HDMI port.Same monitor, same cable, displays without problem plugged into Toshiba Tecra.Different monitor, different cable, no problem with ThinkPad, will not work with Dell Precision 7550.It would appear the Dell Precision 7550 HDMI port is DEAD.What to do?
Dell Precision 7550 HDMI Problem