An organ from the DX7 times. Inside we mainly have Frequency Modulation technology, and Yamaha did not reuse the DX7 6-operator chips! Instead this seems to be a separate development, the sound engine has 8 operators and much more algorithms. The mono section even has 16 operators, but they have obviously got two engines in parallel.
I must say, I have never opened this organ for repairs or improvements. She is perfect :-)
Ok - one drawback: I also own a SY99 and I like the FM programming. The FM sounds are in principal user-editable, if you have the appropriate PC-editor software. I assume that Yamaha assumed, the buyers of organs are the people who do not go into the deep of FM programming. I have not found any SysEx-documentation to write my own editor. The only possibility is an Atari program for editing HX1 sounds, this program can also run under Windows using an Atari Emulator. The User Interface is a night mare under this emulator (if you want to change a parameter from 0 to 100, you have to click left mouse 100 times...
However, the playing is very pleasing, a wonderful keyboard and pedal action. The "Hammond" section sounds surprisingly authentically. And you can change most registrations on the fly without using sub menus.
Yamaha HX Fan site: http://members.lycos.co.uk/yamahahx/whatisthehx.html