Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I have searched extensively and thoroughly and have found no source whatever for such an instruction manual or user's guide for your model of Yamaha Portatone.
I suggest that you contact Yamaha directly, in whatever nation/division the Poratone originated (probably US) and ask specifically for such instruction guide(s).
This link is supposed to be to a guide (or at least a preview of it) but does not work for me: