A friend just got one of these, but belt slips off all the time.

Hi Vidar, I struggled to find a user manual in English for this projector let alone a service manual. I still have the projector but mine does not have this problem, however I can have a
look at the setup and try to figure out why it might slip the belt off or how its tensioned. I thought it was a round shaped belt on this which most newer belt replacements are, but suspect it may have been a "V" shaped belt originally.