I bought these off a friend, they're 8 yrs old but instantly became my favorite cans. They developed an issue where sound would cut or reduce volume in the left driver, this was temporarily fixed by jiggling the connector where it meets the driver until sound restored. I decided to fix the connector once and for all.
I searched the net and found many others with the same problem, a common flaw in an otherwise flawless and indestructible pair of headphones.
The common suggestion was to replace the cable (Min $50 for a Sennheiser 1.5m steel cable) or replace the driver (even more expensive and unnecessary for this issue).
My fix costs $0, works perfectly, and all you need is a soldering iron and some solder.
Step 1: What You Need.
To open the capsule and get to the internal connectors (diagnose only) :
-Small flat head screwdriver
For the cable connector fix:
Step 2: Diagnose
Easy to diagnose and repair these headphones since each component is user replaceable.
This is an extraneous step if you have the same loose connection issue, but I thought it may interest some people. If not, go straight to the fix.
1. Remove the problem headphone (in this case the left one)
2. Remove the foam & vinyl covering
3. Use a small flat head screwdriver to pop off the capsule cover
4. Tweezers or needle nose pliers to remove the small white plastic that shields the connectors
The internal connectors are two small springs, if there is no rust or otherwise visible damage to the internal connectors then the problem lies in the cable itself.
Step 3: Fix Em!
Each cable connector is made of two pieces of steel that tend to split if damaged. Use needle nose pliers to hold each connector together, this will also act as a heatsink so that you don't melt the plastic of the connector.
Get a small drop of solder on the end of your iron and apply it to the tip of each connector, sit the iron long enough that the connector takes it evenly
NOTE: Do this very carefully, if you add too much solder the connector ends may not fit.
Step 4: Enjoy
Carefully re connect the cable, observing the proper orientation (one connector is slightly larger than the other). It may be a little stiff at first but gently work it in and it should work great!
I've tugged and moved mine listening to music and can't make it cut or degrade the signal where it would have easily cut out sound before.
Hope this works for you as well as it did for me!