IBM RT PC Problem Determination Manual page 83

S t e p 111. I B M 5151 P e r s o n a l C o m p u t e r D i s p l a y
Check that both the display power cable and signal cable are
plugged into the system securely.
See
YES Go to "Step 115. IBM 5151 Personal Computer Display"
on page 2-59.
NO STOP. You have found the problem.
for cover removal.
User S e t u p G u id e ,
Section 2. Hardware Problems 2-55