Those old cables were going to short out at some point.
Just replace the cable, these are pretty easy to take apart.
Woah... faulty driver, that's terrible. Pretty sure it's impossible to repair these.
These are too rare to find spare drivers lying around.
You might have some luck posting a 'wanted' advert in the 'For Sale' forum for a spare driver, but I wouldn't have too much hope of ever finding one.
It's not quite impossible depending on the situation.
What he's got is likely a failed connection to the voice coil. The solder pads on this headphone are poorly designed in that the voice coil contacts them on the underside of the PCB, which is glued into place and can "shift" around due to the poor aging behavior of the adhesive. Simply moving the headphone around a lot (such as would happen from normal use or shipping) can cause the pads to loose contact. Cable tugs and jostles aren't particularly safe either due to how little space there is in the earcup.
This issue occurred on both my Z5 and Z6, but I was able to repair them with some extremely tricky soldering (though I only managed to fix the Z6 driver after parting out the rest of the headphone, bah).
The simple solution is to accept that these are a home headphone and never take them outside despite their small size, to keep them in a safe spot (headphone stand, etc) and to be as gentle as possible with them. Metal housing or not, they are almost 40 years old.
From my old observations of my working D7 I would never have never guessed it was so simple - especially since one would never under normal sane circumstances pry open a working D7 driver to see how it's put together
Sounds like an easy job if you've got patience and a good magnifying glass.
Getting these in my possession in NOS quality, I must say while the Z7 if great at detail and separation. The highs are just a tad too much, and I find the sound of the Z6 to be more balanced overall with more sub bass presence. Detail I would actually say is the same with either however, the Z7 is going to appear to retrieve and separate more with the way the highs shine a spotlight on the music. Just my two cents. If you can find them in this quality jump on them. I was quite fortunate to find them in IMMACULATE condition. Been a happy camper with my old school Sonys ever since.
So upon further listening I see that the detail retrieval, instrument separation still go to the Z7, which is fine if one takes into consideration the small soundstage ,paired with the right equipment and genre. It can really allow for intimate moments with your favorite pieces of music, timbre on string instruments really shine. Additionally the crunch of the guitar and decay of the bass are quite exceptional. However, again consideration needs to be taken when pairing equipment as well as quality of recordings. The Z7 benefits from a warm/dark sound signature in that it helps to bring a little balance and even out those highs. The Z6 highs I feel are more in line with the mid and lower frequencies, providing a more balanced even sound to my ears. I find the Z6 personally better because, it does not lack the detail retrieval the Z7 has, is better suited towards all types of genres, benefits from a bigger (not leaps and bounds bigger mind you but enough to hear a difference) soundstage and paired out of the Aune M2S with B1 amp is like electric crack to my ears and soul I am ashamed to say. Additionally while the decay of the Z6 is not on par with the Z7 the Z6 benefits from a little extra sub bass presence that again helps to round it out. If I were to say which was better it would come down to how you listen to music. Are you a critical listener? or more of a laid back duder who just wants good tunes and good vibes? Needless to say I'm in the second camp. Music to me has never been about critical listening and more about letting the music wash over me like an ethereal shower if you will :P
I will say though that I can appreciate the Z7 for all that it does and the fact both these headphones are going on 40+ years is insane to my mind.
In case anyone is looking, now is your chance.