I´m starting the restoration today. Mice had been gnawing the cable, you could see their little teeth marks, and some wires had been cut, so I need to replace the wire. After searching Ebay, and Mouser, I could not find curly stereo cable, much less in blue like the original, so I have ordered 3,5mm minijack socket to mount into the volume box. For the connection between headphones and volume box I have blue Cat5 ethernet cable the fits perfectly. The darker blue in the misscolored original in the picture below.
Opening the headphones involves pulling the pads of a flange, and unscrewing 4 screws from brittle plastic - take care. Someone had already broken on screw point on mine.
The inside reveals a second order filter for the tweeter, which is probably coaxial. The speakers are glued in to the front of the headphone, and I was happy to see that they seemed untampered.
To open the volume box you have to bend back the beltclip to get at the screw behind it. The clip is a snap fit and was probably mounted after the box was opened. This had kept pryning fingers from the inside of that box as well.
The dial wheels on the sides are pull of from splines on proper potensiometers You do not need to do this to open the box though..
Tone at the top volume at the bottom.
I do not have the experience to see how the tone filter is made.
For today I have cut 104 cm of the ethernet cable. I with drew the brown wire pair to make the cable even more pliable, and used orange for the old red, blue for old blue and green for old black. I carefully plied of the old retaining clip from the old cable and mounted it on the new cable. Now I have to wait a few days for my mini jack sockets to arrive, before I can continue. The last picture is of the new ethernet cables connecting the headphone to the volume box.