I worked on this again for a few hours tonight (it's now 2:10AM here) and I just wanna punch some babies... 400 babies, to be exact. I cannot get the frickin' thing to work!!! Mario Kart still works just fine but I cannot get some other games to work. I just don't know what's so special about the Mario Kart cart. And again; the other carts work just fine on unmodified N64's....
I went through several steps tonight to troubleshoot:
Checked continuity and resistance of all the connections between the chips inside the carts and the RCP. Everything seems just fine.
Tried more grounding/more 3.3V power. Nothing.
Checked waveforms on all data lines with my oscilloscope and I noticed some slight differences between my modded board and a stock board, but I don't know what's causing the differences.
Out of desperation, and going on a hunch, I swapped out the RCP with the one that I removed from the first board I had removed parts from. Still same issue. Yes, you read right, I swapped it out... pics below.
Completely removed all kynar wiring and replaced with regular ATA type wire.... no dice, but Mario Kart still works.....
Thinking that maybe it was a power issue, I soldered the original power connector and power switch back on so that I could try it out with the original power supply instead of batteries. No difference.
In the process of doing all this, I managed to accidentally completely stick my soldering iron in my bottle of liquid flux.... and later I ended up accidentally knocking over my bottle of flux and getting it on my workbench and on some parts so now I have a really sticky mess to clean...
There's only a few possibilities left as far as I can think;
-My adapter has some issue with it that I haven't yet found (which is why I'm holding off on posting the file or pic for it until I can get it sorted out.)
-There's something wrong with one or more traces between the cart connector pads and the RCP on the board I'm using.
-There's a physical problem in the cart connector I'm using that's not readily apparent
-The N64 gods hate me and won't let me get anything working properly... They say; "Nay! Thou shall not make thy portable!"
When I get another chance, I'm going to try my adapter board on a completely unmodified n64 board and just solder it to the junctions without removing the lower connector of the board. If that works, I'll be able to completely rule out problems with my cart connector board.
Oh, and before anyone says anything about the video chip missing, that's not part of the issue, but more on that later.