I have a pair of Beyerdynamic DT 770s from a few years back. The left side no longer produces any sound. I opened them up to see if there was a loose wire. They're wired just like the ones in this guide (the left wiring connects to a tab on the right driver before splitting off). Here's what I found:
Tip to right (working side) positive: zero resistance
Ring to left (non working side) positive: zero resistance
Sleeve to right (working side) ground: zero resistance
Sleeve to left (non working side) ground: zero resistance
Sleeve to right (working side) positive: zero resistance, slight pop in speaker [this part confuses me]. The same happens if I connect the left ground to the right positive.
The Y where the wiring splits off to the left (non working side) seems fine, too.
As far as I can see, the tiny wires under the driver tabs have not become detached.
tl;dr - One side of my headphones doesn't work, but the drivers on both sides seem to have a solid connection to the plug. The + terminal on the side that does work seems to have a connection with the sleeve, though ... is that normal?