Hi Colin, your posting is not the best composition I've seen! No grammar or punctuation at all making it difficult to follow. Not the best choice of type font either and why bold?
Notwithstanding all that, was your friend's unit connected with the same cable as tried with yours? I'm suspecting a cable issue, but if the other 3500 worked with the same cable then that can't be the source of the no sound problem.
When you say "everything works" but the sound ouput do you mean the the tape drive is ok and the cassette spools turn etc?