Update (March 11, 2009): This modification is also needed for Sonic 3 and the combination Sonic 3 + Knuckles. Thanks [ColdFace] for pointing out the issue.
The SEGA Mega Drive / Genesis cartridge for the game “Sonic & Knuckles” has slot on top that other carts can be plugged into. There are three special cases which unlock new games:
| Combination | Game variation | Extra ROM chip active | S2K Mod needed |
| S&K + Sonic the Hedgehog | Full version of Blue Sphere game | No | No |
| S&K + Sonic 2 | “Knuckles in Sonic 2″ | Yes | Yes |
| S&K + Sonic 3 | “Sonic & Knuckles 3″ | No | Yes |
| S&K + any other game | Single level of Blue Sphere game | No | No |
When combined with Sonic 2, the S&K cartridge uses an otherwise hidden ROM chip. Since this requires two extra control lines that the Retrode doesn’t use by default, we have to add two wires if we want to play “Knuckles in Sonic 2″.
Here’s how it works (click on images to magnify):
| 0. | Open the case and take out the PCB. | |
| 1. | Locate the pins A and B on the top side of the circuit board. |  |
| 2. | This is the trickiest part: you have to solder a wire to each of the two. Make sure you don’t spread the solder to the neighbouring pads. Also, you may want to use two different colours in order to avoid confusion. |  |
| 3. | Put the wires through any hole in the PCB to get them to the bottom side. Remember which wire is which. |  |
| 4. | Connect the wires A and B with the SEGA connector as shown. |  |