How'd you go , have you sorted it out ?
Uh oh 
Just found this at RGP - could this be me now 
> I have a '92 Bally Creature from the Black Lagoon and Column 4 on the switch
> matrix is dead. I've done some basic checking and I'm thinking that I may have
> blown a line on the LN2803 IC. I was changing a right slingshot GI bulb near
> one of the switches on Column 4 (the right slingshot switch!) of the matrix and
> think I may have shorted the switch and blown a line on the 2803.
Yep ... common problem.
If your U20 (the ULN 2803) isn't already in a socket, make sure you put
one in prior to replacement -- those chips fail often. Newer CPU's
(the security and WPC 95 cpu) put 'em in sockets at the factory.
