PCB Repair: Out Zone

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…

Outzone PCB.jpg

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:
OutZone_Schem.png

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:

Outzone_ShiftRegisters_15_Bad.png

When stable, the signal looked as so:

Outzone_ShiftRegisters_15_Good.png

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.

2 thoughts on “PCB Repair: Out Zone”

    1. Hahaha, wow. I never even thought to check! Thanks; maybe they’ll be useful for when it eventually breaks again 🙂

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