Holy cow on a popsicle stick I had a tough time making this thing! I kind of regret digging out my old N64 and thinking "Hm, should take a look inside this puppy" Had I but known! Oh well. Here now are the facts of the amazing tale, along with photos. Enjoy!
Once I took apart the N64's case (not an easy task in itself, with those pesky Nintendo screws) I removed all the metal shields and took a look at the motherboard and the cartridge slot. Strangely the expansion slot (on the bottom of the N64) is connected directly to the cartridge slot. Once you pry up the normal cartridge slot you can desolder the expansion cartridge slot.
I then made some aluminum brackets and mounted the cartridge slot to the bottom of the motherboard as shown below.
Little did I know that this would be the EASIEST part of wiring the N64 portable!
This is (what's left) of an N64 controller. After tracing which buttons went to which pins on the black IC (center of board piece) I chopped most of the board off and did a little rewiring (mostly ground/VCC) to make it work again. I then wired an extension plug for the analog stick (left thing with electric tape)
I then designed the portable using trusty old AI (the program, not the Steven Spielberg movie) I had a battery pack laying around that was first intended for the DreamcastAway, then the PS2p, then... the closet. I figured the thing cost me $40 so I should use it for SOMETHING!
the batteries packed into the rear half of the unit, which is basically
all batteries and the cartridge. The pack was really a bunch of Ni-MH
"C" sized cells, but that allowed me to chop it up and move
them around in the case as I saw fit. (and so they would fit)