I added a Famicom cart slot to my NES, but it has FDS issues

As the title says, I've added a Famicom cart slot to my front loader NES. I used a copy of Gyromite with the Famicom converter inside and soldered it up the NES 72 pin connector. It works fine with actual Famicom carts, but when playing FDS games I get sprite graphical corruption. Here's some images of the issue and how the cart slot is soldered in https://imgur.com/a/eIUmV. Here's a short video of the issue in action too



https://youtu.be/sqoBnq3YnKU. Now I'll go into detail on how I did the mod and what troubleshooting I performed so maybe someone can give me some helpful suggestions.

Since I used the Famicom converter from a Gyromite cart, the wiring was straight pin to pin from the NES 72pin connector to the NES side of the Famicom converter. The only exceptions are I left out a few uneeded pins such as the CIC lockout chip pins (pins 34, 35, 70, 71) since I disabled the lockout chip on the NES mainboard. I also left all the pins disconnect that go to the NES expansion connector, except for one needed for Famicom carts with extra audio chips. For half of the connections i soldered to the back of the NES 72-in connector, and for the other half I ran IDE wires between the pins to get to the "inside" of the 72pin connector to solder there. All the wires are IDE wires except for a little bit thicker wire on 5V. The image link above also shows pics to give you a better idea. I have not finished getting this mounted inside the case and getting the cart slot hole completely cut yet because I wanted to make sure the mod is working fine before finishing the physical details.

Now for the troubleshooting. Famicom carts work fine with no graphical corruption. I can even move the cart slot and wires around during operation with no issues. I've checked and gently but firmly tugged on all solder joints to make sure they are connected solidly. The Everdrive N8 Famicom flashcart also works fine, and when playing FDS games from it there is no sprite corruption. However, when playing disks on an actual FDS there is sprite graphical corruption as shown in the pics and video above. The FDS works fine on my AV Famicom, and it works fine on a NES 72pin connector with a regular Famicom converter (no wires) once you have things taken apart enough to plug it in that way. The only thing I can think of that's "wrong" is the IDE wire length used to connect everything, but I kept those as short as I reasonably could and it seems odd that Famicom carts are fine.

Does anyone have any suggestions or insight?

Hmm, could you try a gyromite converter with the built in zif socket of the system and see if it works that way? I'm curious what would be causing this. I'd check the gyromite converter for traces connecting to any pins you didn't bother wiring up, I know it sounds silly but it can't hurt.

There are no traces on the gyromite converter for the NES Expansions pins I didn't wire up. There are traces for the CIC lockout chip pins, but those are unneeded since I disabled the CIC on the NES mainboard. The FDS does work correctly when using a regular gyromite converter plugged into the NES 72pin connector, though getting it actually plugged in that way is tricky. You either need a Game Genie or the ZIF connector has to be modified to a regular cart slot. Basically it boils down to no wires, no issues. Wires? Issues!

Well, nevermind now. After researching around I fixed the issue. The solution came from this specific post http://www.famicomworld.com/forum/index.php?topic=786.msg36470#msg36470. I didn't have the exact value resistor listed in the post, but I read pull down resistors are usually between 1k and 5k. I had some 2.2k resistors laying around so I soldered one each to cpu pins 21 through 25, soldered those to ground, and now everything works with no sprite corruption on my NES.

cr4zymanz0r wrote:

Well, nevermind now. After researching around I fixed the issue. The solution came from this specific post http://www.famicomworld.com/forum/index.php?topic=786.msg36470#msg36470. I didn't have the exact value resistor listed in the post, but I read pull down resistors are usually between 1k and 5k. I had some 2.2k resistors laying around so I soldered one each to cpu pins 21 through 25, soldered those to ground, and now everything works with no sprite corruption on my NES.

I'm looking at my original famicom schematic and there's no pulldown resistors on cpu pins 21-25.  He figured this out by licking his fingers and touching the cpu pins while the system was running, I wonder if a multimeter would have showed something.  I'm really glad this fix was discovered but like anything else I like to know why this fix works and why there was an issue to begin with.

I need to research what pull down resistors do. To make the original famicom / nes controller work with longer cords I had to add pull up resistors to a few lines, so I'm curious why this fix is the opposite.