Problem

Incorrect colours.

Diagnosis

Here we have a Gradius II Dark Adventure boardset in fairly decent shape:

To trace our colour issue, let’s start from the RGB outputs and work backwards. The large, 007327 hybrid module contains the palette RAM, output latches and RGB DACs:

Here it is on the schematics, with inputs from the colour bus and the main 68000 (for palette RAM R/W access):

I began probing the CB[0-6] inputs until I found this utterly lousy signal on pin 28:

Said signal originates from a D-type flip-flop:

Replacing 7U restored the full range of colours. Sadly, fun is absent from the game by design…

Fix

Replace 74LS174 at 7U.

Gonna Make You Zzzzz…