In the 128 BASIC editor type "copy randomize" (without quotes). If you press enter and quickly hold the keys C, J and L before the error beep, you'll get a colourful screen displaying a hello message.
Reset your speccy and as soon as the screen goes blank, hold Q,A,Z,P,L & M together.
I can't remember if this enters the sound to light or the diag. screen.
If you get a black screen, try playing a tape (connected to the audio input) and you should... more