I have a Ronda 753 I've been trying to fix for a couple of weeks.  I cannot get the rotor to behave while putting on the train bridge.  I've tried putting small rotor magnets beneath the jewel and over the train pivot in various combinations.  The problem is that everything is so close together and the train bridge is magnetic.  Also, the rotor magnet is a strong little bugger.  Just can't do it.  So in assembly I'd be particularly interested in how you get the train bridge back on for the 715.