I solved the problem (new one). What happened was there was a central hub that the outer arm rotated on and it evidently sheared right off, allowing the arm and pulley to come free from the main hub that is bolted to the block. The pulley/arm assembly is held in place by a plate which also went missing when this piece sheared off. I thought that by replacing the plate I could bolt it back on, but it didn't work. Until I bought a new one could I get a proper idea of how this assembly works. Thanx for all your input though, greatly appreciated.
To clarify things further, none of my pulleys were binding either, it was just a matter of this belt tensioner under too much vibration I guess.