Some weeks ago I've bought an old NES gamepad. The cable was broken and it was not possible to repair it. So I've modified the NES gamepad into a retro style game controller for my PC with USB port.
Step 1: Preparation of the NES Gamepad
Open the enclosure of the NES gamepad and cut the old cable with 1 inch distance to the old pcb. Do not resolder the cables from the pcb - we need these cables later! It is also the right time to clean all plastic parts with water and soap.
Step 2: USB Converter PCB
The original electronic inside the NES gamepad can not drive an USB port. We need an additional pcb with this function. You can buy an assembled pcb called NES, SNES, DB9 (Atari/SMS/Genesis) or TGFX16/PC-Engine to USB conversion circuit with the required function from the Canadian Website Raphnet.net
Step 3: Prepare the USB PCB
The USB pcb supports different operation modes for SNES and NES. We have to set the rigt mode by soldering a bridge to JP1.
Step 4: Solder the Cables to the USB Pcb
Solder now the cables from the old pcb to the usb pcb. Furthermore you need an standard USB cable. Cut the USB-B plug and dismantle this cable and solder the 4 wires inside also to the USB pcb.
Step 5: Mount the Pcbs and the USB Cable
Fix the USB pcb with self adhesive tape to the old pcb of the NES controller. Mount the USB cable as in the picture and close the enlosure.
Now your new retro game controller is ready for gaming.