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

[cr4zymanz0r]

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 http://imgur.com/a/eIUmV. Here's a short video of the issue in action too http://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?

[80sFREAK]

CHR RAM in FDS adaptor. If it was made in mid 86, especially by Sony, just replace it with 100ns or better SRAM.

[cr4zymanz0r]

Can you help clarify? Here's some pics I just took of the board in my FDS adapter http://imgur.com/a/0Rz6V.
I'm assuming it's one (or more) of the 5 chips at the bottom. I see 4 labeled as D-RAM and some BU4069UB chip labeled as U5, but that appears to be some sort of hex inverter. If you can tell me which chip or chips need to be replaced and could link to a suitable replacement (even better if it's on digikey's website) that would be a great help.
Post Merge: October 24, 2013, 09:15:16 pm
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.

[jpx72]

Congrats on the fix, posts like the one from 133MHz should be gathered somewhere on the main Famicomworld page under technical section.

[80sFREAK]

Do you think it's correct idea to pull down 5 out of 8 data lines?

[cr4zymanz0r]

Well, I've got a glorious update. After adding the pulldown resistors I fixed probably 98% of the problem. Occasionally I would see a graphical glitch for less than a second. It was slightly annoying, but tolerable. Now on another NES motherboard I tried FDS with the same Famicom cart connector but still had the sprite corruption when using no pulldown resistors.

I got viletim's new NESRGB board that takes the digital signals from a stock composite PPU and converts it to RGB (then encodes that into s-video and composite) http://shmups.system11.org/viewtopic.php?f=6&t=47617. I was waiting until after that was installed before installing pulldown resistors on this NES motherboard because I was curious of the NESRGB board might have any effect on the FDS sprite corruption. To my surprise, it completely eradicated the sprite corruption and I saw no graphical glitches at all on FDS.

[80sFREAK]

Could you please upload photo of your FDS's PCB? It's for statistics.