A digital synthesizer which utilizes special processing chips designed to support Yamaha's FM synthesis architecture. This, the most popular rendition, used 6 'operators' in the model with 32 algorithms (ie. arrangements of the operators in which they could modulate one another or just act as signal sources in the mix.) The DX7IID and FD units double patch memory plus have setups for laying and stereo settings, and more MIDI features, two assignable sliders and less noisy DAC. TX7 is the table top keyless version.
I've found most of these units to be fairly rock solid circuitry wise. Abused units sometimes have solder problems. I've fixed the few I've had in by just going over suspected areas of the circuit boards soldering questionable joints! Later DX models have Local off option remember. Diagnostics are accessed by precisely holding "Function" while holding the 16 and 32 buttons. It's tough sometimes. Keep trying! hehe. Also supposedly EDIT and 16 and 32 will do a hard reset destroying all data but helping with problems like random key to note assignments.
Other than non-black keys I don't have any particular parts for these now.
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Supermax Upgrade Info
I've scavenged the web for what I can find on this, having finally run across a unit that has this upgrade. Apparently two French developers, Dan Armandy and Alain Seghir, under the name "Group Center" created the Supermax which went beyond just upping the memory to 8 banks of 32 patches, but it also offers layering, enhanced MIDI controller features, MIDI echo/delay, velocity cross-switch and arpeggiation! I'm trying to secure a user manual that I can post on this. -Bob