Malfunctioning VT240 - help please

[Charles]

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.