It could be the board, but it could also just be a physical problem on one of the PL sockets - which means you can splice the cables to the one good socket, and it will be fine, as both left and right signals are present on each socket.
To test this, swap the switches on the (currently OK) left speaker, so that it becomes the right speaker electrically.
As the cables and speakers won't change, you will be fault finding just the PL socket on the board used for the r/h speaker.
Assuming you get an output, then you need to get a PL splitter, and run both speakers from the one PL socket. Or you can chop into the cable and splice them, if youre happy doing so.
If this doesn't work, its an engineer job.
Regards
David
Bang & Olufsen of King Street - Manchester,UK. SKYPE - beokingstreet