Sounds like the tensioner was not installed properly. It is a ratcheting tensioner. Once extended it won't back up, it has to disassembled and re-set. If you install it without taking the spring tension out of it first the chain will be WAY TOO TIGHT! Too tight of a chain sounds like a sure fire recipie for a broken cam. BTW, as a 12yr M/B tech, I've never seen the cam break as you describe, though I do install the tensioners properly so that may be why.