Mega STE problem - black screen in low/med res but high ok

[MR*Atari*]

i have a Mega STE which has a prob, basically it works fine in high res mode with an SM 124

 

BUT not in colour low or medium resolution or with a TV either, it's just a dark screen with some randon vertical lines BUT i can hear sound in these modes (key click)

 

someone told me the U501 chip which is surface mounted and custom to the mega STE

 

can anyone advise what else i can look at?

 

thanks

 

 

and on another faulty machine i'm looking at i get 4 bombs just after the memory tests as it's about to go to the desktop

 

4 bombs = illegal instruction, processor fetched an instruction from ROM or RAM which was not a legal instruction

 

what does this mean?