18 Apr 2018

MV1-F - SNK (repair log)

This one took me hours to fix...
When I got it it was working fine except newer games had sound corrupted (wrong or missing samples). However it didn't throw any sound related error (Z80 error) so I started to probe the sound RAM (6116 type), CPU (Z80), sound chip (YM2610) and sound ROM (M1 mask ROM) but didn't find anything weird. I also checked every single trace connecting the various chips in the sound section but they all turned out to be fine. This is where things got messy...

Problem being noticeable only with newer games I thought it could be related to higher address lines.
I pulled the audio RAM thinking the high part of its range was faulty, installed a socket and fitted a new chip: nothing changed.
Next probable culprit was the sound CPU as it's overclocked when running on a MVS motherboard (same mistake Capcom did with CPS1 motherboards). Same treatment, socket, new chip: no difference.
Ok, ok, could it be a partially corrupted M1 ROM? No it wasn't.
Humm and what about the sound chip (YM2610)? It has a shitty pin space (SDIP, 0.07" pitch instead of 0.1"), I had most difficulties finding an appropriate socket for a decent price. But nope, it wasn't faulty.
Last one in line was the custom NEO-D0 chip (SMD). I pulled one from a known working board and this is where I discovered a bad trace underneath it! It's the /NMI signal coming out of pin 29 of the custom chip. How could I have missed it!?

Anyway, I soldered the known good chip in place and patched the broken trace.
Board was fixed...

Out of curiosity I soldered the potentially faulty custom chip on the good donor and it worked just fine.
Conclusion: I've replaced all the chips in the sound section because of a single broken trace (ok, mostly because I missed it somehow).

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