Rainbow Islands repair log #2

Nearly 12 months ago my Rainbow Islands went a bit crazy and kept giving me random crashes like the following

I thought I had sorted it with a simple ROM change but after a lengthy gaming session recently I discovered that this was not the case.

I couldn’t find anything wrong with this board. Sometimes I could play it for hours, sometimes it crashed at startup, sometimes it wouldn’t even start up at all.

Whilst I never gave up hope that I would find the fault I just didn’t know where to start.
My goal here is to document all the code with comments, create a decent set of schematics for the PCB and basically note any points of interest within the code and hardware.

Tonight I feel confident in saying that I have fixed my board.
While I has adding to my schematic, trying to find where a trace ultimately ended up, I found that pin 9 of IC57 (74LS138) was tied to VCC (kind of anyway, it had a resistance of 38 ohms).
The pin does in fact go to pin 19 of a 74LS244 at location IC79.

I removed the chip and it failed all tests, the connection to VCC also disappeared. I’ve replaced this chip and all seems to be well in my world once again.

For all those people that have asked me in the past “how long does a repair normally take?”. Although this kind of fault is quite rare, it has literally been 12 months of chipping away at different things. Had this been any other game then I would likely have given up on it a long time ago.