My trusty and ancient hd545 didn’t produce sound in the right cup when I tried to use it today.
The cable is ok as I used it with another Sennheiser to be sure.
I suspect (and hope) it is just the spring in the connector.
I did find this old thread describing disassembly:
I also found this which should help once I succeed in disassembling the thing:
Any other hints would be appreciated.
Sending it in for repair probably won’t be possible/worth it 