Just a quickie, as all these decks suffer so - make sure the bracket holding the idler assembly is free to move back and forth. B&O used a particular grease that turned into vaseline very quickly (ten years or less) and then siezed altogether. Somehow, you'll have to free this pivot off so that the belt alone pulls it into position. I was able to do this on mine while keeping the idler clean and it now works just fine.
By the way and regarding comments on the other thread too. Even when new, the belt always came off if fitted and the platter spun without the top platter in place.