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.
Step 2: Making room
I had to remove a post that was in the way and grind down a little plastic so there were enough threads sticking through my freshly drilled hole to enable me to thread on the panel nut.
Step 3: Wiring 101
I poked around a bit with some test wires and followed some traces to find audio ground, audio left, and audio right. I soldered a wire from the audio ground directly to the ground pin on the mini jack.
For audio left and right, I had to intercept the signal before it reaches the headphone speaker. Then I cut the trace so that the signal doesn't get through. Use several strokes of a sharp knife to cut through the track, but be careful you don't cut anything else.
Once the tracks were cut I tested the headphones to make sure nothing was getting through. Then I took some thin wire, and connected both sides of the cut tracks to the correct pins on the mini jack. The way you do this is important so be careful and take note of the diagram on the back of the mini jack box. Basically the signal side of the cut track needs to go to the side of the jack that will connect to the jack plug when it is plugged into the jack. The heaphone side of the cut track should go to the return side of the jack socket.
Step 4: 'Ears how you succeed
I routed the wiring around the pcb to keep it out of the way and make it look somewhat neat, and went for the full test.
Sure enough, the headphones worked fine as normal. Then once I plugged in the ear buds the headphones went silent and the sound came out of the ear buds instead.