Step 1: Preparation
3 things bugged me about these headphones. 1, they were leaky, everyone could hear what I was listening to. 2, prolonged listening made my ears hurt from the headphones pushing against my ears. 3, they make my ears sweat - blergh!
After doing some searching for wireless ear buds, and being shocked with how expensive they are, I decided to modify my Sennheisers to allow me to plug in my comfy Sony earbuds. I figured I could just wear the heaphones around my neck but have the comfortable earbuds in my ears.
Okay: Lets get started.
You can open these headphones by gently prying open the cover near the battery (see pic).
The very first thing I needed to do was find a low profile stereo mini jack. Be careful though, you need one that has 2 normally closed circuits that are cut off when the mini jack plug is inserted. Fortunately Radio Shack had exactly what I needed (cat # 274-246), it was $2.99, that's why I don't like shopping there :) But hey, it was a Sunday afternoon, and this project just couldn't wait.
Now that I knew the size and shape of the jack socket, I could try to find somewhere to mount it inside the headphones. I opened the side with all the electronics figuring that it'll be easier to make connections on there, and looked for a suitable space.