Problem

Flickering/disappearing sprites:

Diagnosis

Out Zone is a game by Toaplan where you shoot things in what is presumably the Out Zone. I dunno…

The disappearing sprites fault was quite inconsistent; I had to wait for several minutes until it started to manifest. I probed around and found that when the sprites disappeared, the inputs to the 74LS244 at 18L would go inactive. I traced the origin of these inputs to some shift registers and drew a simple schematic:

Probing the common Shift/Load inputs of the 74LS166s revealed a signal where the low level was above V.IL, which is in the TTL indeterminate logic region:

When stable, the signal looked as so:

I decided to replace the 74LS04 at 20N which produced this signal. However, this did not fix the issue. Presumably one of the four 74LS166’s driven by that signal had a bad input pin that was affecting the signal. I removed and replaced B5 and B6, which resulted in discoloured sprites. I discovered that the output of E5 was now stuck low. Replacing E5 fixed the colours and appeared to stabilise the /LOAD signal. I left the game running for half an hour and played several games, during which the fault did not reappear.

Fix

Replace 74LS166 at E5.