There's a service manual link on rigpix.com, unfortunately it seems you are right, I don't see a battery.
Was mostly guessing.
It works fine otherwise?
If there is a yahoo group
Yahoo! Groups - Join or create groups, clubs, forums & communities for the 706, which I bet there is, I'd join and search the archives and files section. If you don't find an answer, just post an email there, I bet other owners have had a similar issue over the years.