display flashing on and off?

hp 1502 lcd ,on starting computer monitor led turns green display appears for a split second then monitor led goes out then turns on again ,display appears again for split second. This on/off loop is continuious.I purchaced 4 of these monitors at auction and all have the same fault.Have tried them on a different computer ,same result,if I turn screen on without computer on i get widow saying no input signal, this stays on without flashing. model is hp 1502(PE1233)

ray
January 2008