Many thanks Adrian I tried all of the above. Connecting monitor to my old second PC confirmed the issue is not the monitor. Dell Tech confirmed it is the HDMI board on the Mother Board. Not repairable.  My Inspiron 7580 has been a good machine and is only 4 years old. Extended warrantly expired in April! I need the monitor due to vision issues. I'll give it to one of my children as everything else works fine.
New replacement Dell Monitor U2422H flickers to black on Dell Inspiron 15 -7580 just out of warranty. Tried my other Dell monitor thats OK on my very old HP laptop - same result - flickers to black. Two hours of Dell Tech concluded Inspiron HDMI board failure. Options? Thanks
Dell Monitor Flicker