i've had a tascam 788 portastudio for quite a few years now, and recently i've noticed a problem with recording my tracks. usually i will record 1-2 tracks no problem. save and shut the machine off i'll come back to it to record new tracks i'll plug something into input D and assign it to any track that has not already been used, 3-8. when i try to assign D to any track that does not already have material on it i get no signal or sound coming through. if i assign input D to a track that already has material on it, in this case 1-2 the signal comes through, the meters are moving and i hear sound. am i missing something here, please help.