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I found in ebay an old fostex T20 it's an Isodynamic headphone (I bought it more by curiosity).

I received the headphone today as "working perfectly",  at first one driver didn't work

I checked the jack connector,  it was fine after that the cable with a voltmeter and finally I checked the driver, I thought a soldering default. .. I opened the driver and looks like this:

2 round magnet taking in sandwich the stator (kind of mylar + aluminum flat wire)

(there are not my photos!)

 

https://www.google.nl/search?q=fostex+t20+mod&client=tablet-android-samsung&prmd=ivsn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj-lYCqlZHTAhUDPRQKHZYMDmQQ_AUIBygB&biw=1280&bih=800#imgrc=ayIx0nUC6iTE1M:

 

https://www.google.nl/search?q=fostex+t20+mod&client=tablet-android-samsung&prmd=ivsn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj-lYCqlZHTAhUDPRQKHZYMDmQQ_AUIBygB&biw=1280&bih=800#imgrc=1MyzoklB0lz9cM:

 

The problem came from a caoutchouc join inside the driver having shrinked, I tightened the screws of both driver and voila both working now but the sound was so-so. 

I read some threads about moding the t50rp. I remouved the thick piece of foam at rear of the cup replaced by a piece of felt and tap partially the vent. I was astonished by the sound produced! I didn't expect that whaaou, they don't lake  of bass, the treble are extended the medium gorgeous. I don't understand why they have  abandoned this technology? 

 

no need any energyser, I can imagine using a bunch of little magnet but the wiring in the mylar

it's another story... if someone have any idea... I know it's not an electrostatic but a cousin