This was a quick 'buy it now' on eBay, described as untested which usually translates to tested and didn't work. The price was good though so worth a gamble. First thing I noticed after separating the two layers was that the battery had leaked and been removed by the previous owner.
After cleaning it all up and patching the three traces directly under where the battery used to be I powered it up. Slots 2, 3 and 4 worked fine apart from no sound and slot 1 worked but with vertical lines through the sprites.
This very helpful page told me I have an issue with a C rom data line. Then using the pinout found here I was able to find out which line exactly. I removed the case from Nam 1975 and used dip switch 8 to freeze the system on the image above, then using a wire connected to ground I touched it to each C rom pin at the cart edge connector. Thanks to the vias above each pin I was able to access all C rom data lines on one side of the pcb and didn't have to worry about the PRG pcb getting in my way.
Going through each pin I found touching each pin to ground made the image worse until I got to CR16 which made no difference to the image.
I then followed the CR16 trace to find the break, it was on the top pcb next to slot 4 just before a via.
That's slot 1 sorted now just the sound issue remained, bad caps are apparently a common issue on 4 and 6 slots for some reason so I started there. After recapping the entire sound section stereo sound returned, I checked the caps and a good 70% of them were way out of spec and had really high ESR.
The only problem now was I wasn't getting audio from the mono pin of the motherboard. This turned out to be an issue with the MVS to Jamma adapter I was using. It has the MVS mono pin connected to the JAMMA speaker + pin which would be correct if MVS motherboards output mono audio, from what I can tell most don't. I believe the only MVS pinout boards that do output mono have a stereo / mono switch on the motherboard which neither my 2 slot or this 4 slot do.
It's not a huge issue as I plan to use it in my JAMMA cabinet which has a stereo harness but for the supergun and testing I'd like to be able to test sound so I modded the adapter.
First I lifted and isolated MVS pin 11 (mono) because if I do ever connect it to a board that outputs mono I don't want it outputting mixed stereo and mono on the same pin.
Then I flipped it over and connected two low ohm resistors (10 ohms as it's what I had to hand) from stereo L and R to JAMMA pin 10.