Try to get an original DX7 user manual and/or "Yamaha DX7 digital
synthesizer - Yasuhiko Fukuda", Amsco Publications, Order No. AM 39371,
ISBN 0.711.0653.X. It's from 1984 and in English. Unfortunately I
don't own a scanner. Or fortunately, because copying is illegal?
However, I can't email it to you, because I can't scan it. The original
manual anyway is in German only. IIRC most you need to learn has to be
done by trial and error. But I guess I also learned from Yasuhiko
Fukuda's book. I don't remember correctly, it's around 30 years ago :).