looking for a yamaha chip datasheet, YM3807

[12afael]

The YM3807 was used as modulation signal source for many yamaha products Rev7, SPX90, DSP1, etc. I´m interested in the symphonic effect. I measured the modulation for the low speed settings but for the fast stuff gets really difficult so I wanted to check how that chip works but the datasheet is very difficult to find.
I would appreciate if someone have a clue about it.

best regards
Rafael