Malfunctioning VT240 - help please

Until a few minutes ago, my VT240 was operating normally, but now it's unresponsive (fails during power-on self test). Normal behavior was: display a checkerboard, then two different intensity all-white bands growing slowly up from the bottom of the screen, then a beep and the expected "VT240 Monitor Error 9" (because I'm using an old B&W composite monitor instead of the DEC VR201 with special cable). Thereafter, normal operation. Now, it briefly displays the checkerboard (and all four keyboard lights turn on, then off); then the Lock and Wait lights come on and nothing else happens. Blank screen. Power-OK light on the back is illuminated and 5.19 volts measured on the board. Haven't checked +12 (or the internally derived keyboard +5) yet. Another possibly useful observation: I can press the Setup (or any other) key about four times and hear a keyclick sound each time. But then it stops playing the click sound if I keep pressing keys. This suggests that the interrupt on the CPU (a T11) is not being responded to. The technical manual is very detailed but does not describe the specifics of the POST, which could be useful in locating the failed circuit (or firmware). Can anyone with experience in debugging these terminals lend a hand? Should I even be looking at the main board, or the keyboard which also has an 8051 CPU?? thanks.