Carnival on a Gremlin / Sega Vic Dual PCB.
Just had blue bars on screen, although probing the Z80 showed it was trying to run code.
Using a logic probe I discovered that the /WRITE signal of the ram chips was always high. This signal comes from a 74LS10 at U77. Checked this chip and one of the inputs – WG (pin 4) was always low, so the NAND output was always high. WG signal is generated from the 74LS175 at U19 and sure enough, Q and /Q were static.
Replacing this chip returned the write signal to normal, and fixed the game.