Originally Posted by Paul_Blackburn:
“My wife has a similar pair so I swapped the leads around my pair has still got the problem even with her lead which we know is fine and her pair works with my lead so the the problem must be with the headphones themselves”
There are any number of possible causes for such a fault. The two "silly" ones that come to mind first are...
If the headphones have volume controls for each ear could one be turned down to zero? Or could it be the plug on the lead is not being pushed home fully in the socket on the headphones?
Those "faults" are the easiest to fix. If it isn't one of those then you might need some tools and test gear to figure out what's broken. Likely suspects are the socket on the headphones having one channel connection faulty. I assume as the headphones need power that they must have on board amplifiers, especially if they can be used wire free in Bluetooth mode. So if the amplifier is still used wired then one of the amplifier stages could be faulty.
The other possibilities are, the wire to the affected earpiece is broken or the driver in the affected earpiece has failed.
Even a simple multimeter could help narrow down which part is faulty if you have the right skills and it is possible to get at the innards of the headphones without breaking them even more than they are.