IBM PCjr Manual page 247

Press and hold the J key. The "J"
appears at the top of the screen and
continues to repeat while you press
it.
Did the "J" continue to repeat while
you pressed the J key?
YES - Press
NO-
Then press B
Note: You may need to
press the Fn and Break
keys a second time.
Go to Step 4.
Have your IBM PCyr
serviced. See "Service
Information" in this
section.
IBM PCjr Keyboard Test 6-105