Power transmission cables
The Armdroid works by using pulleys and string. This was definitively the weak point in the design as, over time, the string deteriorated and snapped. Most Armdroids that are resurrected nowadays have to be restrung. I have done this to mine, and have had sucess using thin flexible metal wire. I bought a roll at model shop (usually used in model aeroplanes to connect servo motors to control surfaces).
To help you to restring your Armdroid, here are a few schematics, followed by some closeup photos of the joints.
As with all images on this site, click on them to get a full size picture.