I think everyone had that uncomfortable feeling when "borrowing" something from a working device for another project - later you start to feel regrets about breaking the working device. I had the same problem when I "borrowed" the Konami SCC chip and its resistor matrix from the original Nemesis 3 cartridge for the MegaRAM SCC project:
I was hoping to easily get a replacement, but after searching for a week for the necessary parts I realized that in order to repair the vandalized cartridge I will have to vandalize another one. And that's not the way! So, I've got an idea of restoring the vandalized cartridge to at least the working state.
The normal Konami SCC cartridge has 2212P003 (the SCC sound chip + mapper) and a ROM with a game, that is usually 128 or 256kb in size. Some cartridges, for example Metal Gear 2, had 512kb ROM. In order to emulate the SCC sound card, the DAC and the K4 mapper one should get a more or less large FPGA that is usually expensive and big in size. Besides, someone would need to write the firmware for this FPGA and I don't have such skills. Soon enough I realized that I could only create the K4 mapper circuit and use another SCC cartridge (for example Carnivore2) in a different slot. For my project I took the already available schematics created by Jipe and improved by Eric Boez and Eugene Brychkov. The schematics only has 3 logic ICs, a few resistors and capacitors - nothing complex. As a result, I've come up with the small circuit board design that has the same size as SCC chip and that can be soldered at the SCC chip's placeholder.
But then there appeared another problem. The game that I was using refused to use SCC sound card located in another slot because it assumed that it was still running on the original SCC chip in its own slot. So, there was a need to patch the game so that it would not only find and use SCC in another slot, but would also fit into the EPROM of the same size. I contacted BiFi, who is known for his excellent patches for various games and asked for his help to add the necessary functionality into the game. And he created the patch that just in a few hours (respect, man!). The original Konami's ROM was removed from the board and the UV-erasable 27C020 EPROM chip with the modified game was installed in its place. I had to perform a few minor modifications on the cartridge board in order to make things work - 4 cuts and 3 wires. And in the end I've got the properly working Nemesis 3 cartridge!
Here's a few pictures of the repaired cartridge:
Note that the resistor and the wire were only needed for the initial board's version. In the repository the board is already modified to have that resistor and its connection to the ground.
The project (including the patch for Nemesis 3) has been released in the RBSC's repository:
https://github.com/RBSC/Konami-SCC-Cartridge-Repair
It should be noted, that this mapper circuit board not only allows to fix the original game, but also to replace the game in the cartridge with a different one, even of a larger size (however, not all games will work in this cartridge without necessary modification). But such task wasn't on my list though I made a few successful experiments. The repository has a few pictures with those experiments with the 512kb EPROM as well as the photos of the bare cartridge board with the locations for the necessary cuts.
I hope that this project will help anyone to restore his broken/vandalized Konami SCC cartridge. I still have 4 boards from the initial batch for sale if anyone needs them. I don't have plans to produce more boards or assemble them.
