I saw a Neo Geo MVS system board for pretty cheap on eBay. It is the MV-1C, the really tiny one with the vertical cartridge slot. It would certainly be easier to use than my hulking MV-1 full-sized PCB. This MV-1C was listed "For parts or not working". That means "It's cheap". So, I got it for the fun sum of $35 USD.
The description was that it powered on, but would do nothing but just show the grid screen, as if no cartridge was inserted. The auction pictures confirmed this:
When it arrived, I took off the black cover and gave it a little visual inspection before I even powered it on. It looks fine, at first...
Of course, after catching this in the right light, it became clear why it would not read any cartridges:
The PCB was cracked a tiny bit in that corner, and those 12 traces were all severed on the top half. I first tried to scrape off the coating and bridge the breaks with some solder:
Unfortunately it appeared to only link a few and this did not seem like the path to pursue. I followed the traces and found they went over here.
Those twelve resistors correspond to the twelve lines, and you can see where the lines all curve and then go under the board in a staggered pattern. Meanwhile, the other end was around here near the cart slot:
It was trivial to match those pins to ones on the cartridge slot and solder directly to that. I then used some trusty old IDE wire and fashioned a fix:
The remaining four lines were fixed in a similar fashion. I did all of this before I even tested this thing out, so I was relieved when I plugged it into my JAMMA adapter and it worked the first time:
I had a few graphical bugs but those cleared up when I cleaned out the filthy cart slot. Now it runs great and I've pulled stereo line-out from before the power amplifier. I reinforced the area near the cartridge slot to prevent further damage.
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