Thanks so much for the feedback! I only realized at the end of the job that the reason I was having so much trouble was that I was using a large size soldering iron, which made it so difficult to be accurate. It is also my first time doing anything more than a single solder joint.
Now the extra board makes sense! Yes, midi was installed, it’s the P6-KBD midi mod.
If I remember the LEDs on the front panel all light up and seem responsive. I only changed preset buttons and this didn’t dramatically alter the sound issues, so I didn’t try more buttons or knobs.
As soon as I get a minute (my job just got busy again) I will check soon all my connections are wired correctly, double check continuity, and rule out continuity between pins due to spilled solder.
Worst case scenario, I will have to get a professional to fix it up and just hope it doesn’t cost too much. Will post more soon.