[jisu]

The left ear in my AD900's stopped working a while ago. 

Rather than replace them I thought I would at least try figure out what's wrong.

Having never soldered in my life, I am a complete beginner so I thought I would post here for advice.

This is a photo of the left driver:
 

 
 
I have no idea what I'm looking at but it doesn't look like any wires detached.
 
Does anyone have any advice on where I could start?

 

[jisu]

Bump!

 

[jisu]

Halp!

 

[arteom]

My guess is the problem is at the connector. 
 
That being said. There are a few things you can do to try and figure out where exactly the problem is. As the left driver stopped working, did it just one day not work at all or was it gradual? Can you get it working by moving wires in any/all directiosn near connectors (the headphone connector, where wires go into earcups)? Any physical damage such as cuts or kinks on the wire itself? 
 
So a new connector will set you back just a few bucks. You can get one from your local electronics supply store, if there is one, or order one online. You can search online for a "switchcraft headphone 3.5mm" (or 6.3mm if you want a bigger connector), or go for neutrik, any brand will do . You will need a soldering iron to replace the connector, but it's one of the easiest things you can do with a soldering iron so don't be afraid of it if you haven't replaced one in the past. 
 
Best of luck!