I have no clue if this'll work or not but there's a good chance it will. The japanese cart of gimmick! comes with a chip in the cart that adds extra audio to the famicom. The problem is that this chip was only ever used in gimmick! famicom carts and nothing else. A gimmick! famicom cart costs a small fortune to get your hands on due to rarity, the game isn't fun enough to justify the cost. As good as the nes powerpak may be, it doesn't recreate custom famicom audio chips 100% like the real thing. I've always wanted to experience gimmick! using the actual audio chip.
The audio chip in gimmick is called the sunsoft 5b, the 5b is part of a series of compatible chips which are the fme 7, the 5a, and the 5b. All three of these chips are compatible with each other, the fme 7 has no audio capability and the 5a nobody knows if it can produce audio or not (probably not). I looked up the sunsoft 5b, on the nes cart database they have a picture of a gremlins 2 famicom cartridge that came with a sunsoft 5b inside of it. I read up on this, a lot of people posted that nesdev that they bought gremlins 2 famicom carts but found they only got the sunsoft 5a, so it seems that finding a gremlins 2 with a sunsoft 5b is very rare. I'm guessing sunsoft only stuck the 5b chip into a gremlins 2 cart when they ran out of 5a chips.
I found a cheap gremlins 2 famicom cart on ebay and bought it, if it didn't have the 5b in it that wouldn't be a huge deal the game is still pretty fun. I opened up the cart annnnddddd.....
Apparently it was my lucky day.
I did also have a backup plan, it's possible to take a cart with a fme7 / sunsoft 5a and wire up a stand alone version of the audio chip that gimmick uses, but it's 10x the amount of work and nobody knows how accurate it sounds compared to the sunsoft 5b. The whole idea is being able to build something that sounds 100% identical to the real gimmick! famicom cartridge.
You can see in this above picture that I socketed the maskroms, that's because I'm going to be burning eproms of gimmick (when my ebay eproms show up) and wiring them in. Here's some pictures showing that everything still works after socketing:
Once I get the eproms wired up and working I'll probably remove the sockets and hard solder in the eproms since the pcb won't fit in the case with sockets installed. The eprom wiring is going to be a bit of a b#tch to do. After desoldering the maskroms I studied the wiring of the eproms with the help of the very useful nesdev documents.
Using a combination of this maskrom / eprom pinout:
This maskrom pinout:
This sunsoft 5b / fme7 pinout:
And this nes / famicom cartridge slot pinout:
I was able to rework the pinouts of the maskroms used on this gremlins 2 pcb.....yes this part was a massive amount of work where I learned a lot. I also took my batman return of the joker fme 7 nes cart, desoldered the maskroms and the fme 7, and took a picture:
Then I installed sockets for the maskroms and re-soldered on the fme7 and everything works fine. Studying the batman return of the joker pcb both maskroms use standard eprom wiring. I tested this by taking the maskroms from the return of the joker pcb, sticking them into my eprom burner / dumper, dumping the roms, taking the return of the joker rom I downloaded from the internet, splitting that downloaded rom into chr and prg, and comparing my maskrom dumps to the split rom from the internet. The crc's of my dumps are 100% identical to the split internet rom meaning my files are 100% identical so the dumps are good. Since I stuck the maskroms into my eprom dumper without rewiring this confirms that the maskrom spots on the return of the joker pcb use standard eprom wiring and not the funky nes wiring.
Studying the gremlins 2 pcb had interesting results. The gremlins 2 pcb the chr maskrom uses standard eprom wiring but the 128k prg rom uses custom nes wiring. I did the same rom dumping test with my gremlins 2 chr maskrom and that matched the chr rom I downloaded from the internet as well. The prg maskrom spot for gremlins 2 uses the custom 28 pin 128k nintendo wiring, so wiring up a 32 pin 256k eprom into that spot will require a little rewiring magic to get it going. Also there's an a17 pin on the sunsoft 5b that should get connected to the eprom, the a17 pin wasn't connected before because 128k roms don't use a17 but 256k roms do.
The other thing to worry about is the audio circuit. Me and Ace looked at the gimmick! famicom pcb pictures on the nes cart database and reworked as much of the audio mixing circuit as we could:
*deleted I found the schematic*
We managed to rework almost the entire circuit. The two capacitors we don't know the strengths of but most audio stuff uses 10 uf caps. The only other unknown thing is which end of the 100k ohm resistor connects to which pin of the sunsoft 5b chip. One end connects to the 5b pin 3, the other to 5b pin 18. There's only two possible ways to wire this so a little trial and error should get that sorted out.
I ordered the proper eproms and resistors on ebay last night. So I won't be able to work on this until the hardware shows up. The first challenge will be converting the cart into gimmick! I have the wiring worked out but you never know until you actually wire it up and see if it works. Once the game conversion gets done then I can wire up the audio circuit and see how that sounds.