Dang nice lookin' unit! I actually have one now in about as good condition. I had gotten it, along with its speakers, on ebay late last year from its original owner. She told me that it was her high school graduation present from 1974. I think she said her dad was stationed in Japan at the time.
Only hang-up it had was the head mechanism was stuck. If yours hasn't had that issue, I'm surprised, as it seems that is a common ailment amongst these systems that use this mechanism.
I do have the service manual as a pdf file. It includes the schematics. I had purchased it on line a couple of years ago. I'd like to get the original paper manual, but that will be hard to find due to these being 'overseas' units. Just send me an e-mail address and I'll send it your way.