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I can only release the shutter on brand new 501cm when the mirror is locked up. I am familiar with 'jams' and the camera has none of these symptoms; lens comes off, back comes off, winder turns, etc...any thoughts?
I don't know what's causing your problem. Sorry.
But i do know that, your camera being brand new, so you should be able to take it back to the shop and get another one that does work the way it should, it would be time and effort wasted trying to figure the nature, cause and solution of this problem out yourself, wouldn't it?