Green screens on Williams games indicate that the CPU chip is not executing the power on self test (POST). This can occur for a number of reasons, the primary ones are:
1) bad CPU chip
2) the watchdog circuit is pulsing the RESET line on the CPU
3) the ROM chip at $FF00 is bad or inaccessible (addressing errors)
4) the ribbon cable is bad between the ROM and MPU boards.
I'm sure if I thought about it, I could come up with a few more.
The easiest one to test for is #2. You need to pt a logic probe on Pin36 of the CPU chip. If it is low or pulsing then the CPU chip cannot run.
The next easiest is to replace the CPU with a known good CPU chip.
ken