Section 3--307. Burden of Establishing Signatures, Defenses and Due Course. (1) Unless specifically denied in the pleadings each signature on an instrument is admitted. When the effectiveness of a signature is put in issue (a) the burden of establishing it is on the party claiming under the signature; but (b) the signature is presumed to be genuine or authorized except where the action is to enforce the obligation of a purported signer who has died or become incompetent before proof is required. (2) When signatures are admitted or established, production of the instrument entitles a holder to recover on it unless the defendant establishes a defense. (3) After it is shown that a defense exists a person claiming the rights of a holder in due course has the burden of establishing that he or some person under whom he claims is in all respects a holder in due course.