Over time I've been asked questions about things not covered in the manual. So, for the benefit of all, I've written up a page covering some of the things I've discussed with people.
No, I'm not. If I were, I'd be advertising the fact quite obviously. And if I were about to buy another organ, I'd be making an explicit notice about it. Although I've mentioned regretting trading in my Yamaha D30 for the Technics, I'm not in the market to do anything about it at this time, and most definitely won't be buying a low-featured model of any organ. The next one would have to put this one to shame, in every respect (I want drawbars, a real Leslie, a double-octave pedal board, instant direct control of sounds without messing with computer menus, etc.), and I'd probably want to test drive it for an hour or more.
I don't know the current value of the organ. I've seen them go appalling cheap. On the other hand, mine cost me a few thousand (second hand) over ten years ago. According to a dealer that I spoke with, this model organ reached about $15,000 (Australian) before the model was discontinued. If you want a valuation, ask a dealer. Otherwise, all I can suggest is look on eBay or in your local newspaper. I'm just an enthusiastic organist, my interest is in playing them, not trading in them.
The SX-U40, SX-U50 & SX-U60 use a battery (in a plastic hatch, under the console) to hold some settings (the chord computer, custom rhythm patterns, personal registrations). The SX-U90 doesn't, it charges some large value capacitors on the mainboard to hold the settings. The organ needs the power turned on occasionally for this memory to be charged, else the settings will be lost. A normal playing session should be enough to recharge it for many days, perhaps some weeks. As the unit ages, you may find it loses its memory if turned off for just a few minutes, or even seconds. Mine had reached this stage of memory loss.
This is easy to repair by anybody with electronics repair skills, the parts are common and cheap, it's only the labour that might be expensive if you can't replace them yourself. The charge is held by three 3.3 farad, 1.8 volt, capacitors in series (which can be replaced with a single 1.3 farad, 5.4 volt capacitor) on the edge of the motherboard. The hardest part about servicing it is carefully removing all the daughter-boards, and getting them to go back into place again (the motherboard flexes too much, and you could break it if you're not careful). But any electronics serviceman worth their salt could replace them, even if they're completely unfamiliar with the organ circuitry.
The organ innards page has a picture that shows where the settings memory power capacitors are located, and that page details the complete procedure.
The organ does drive the speakers hard, and there's various reasons why you may hear non-musical tones coming from the cabinet, particularly when played loudly.
Bass notes can make everything vibrate, and parts of the organ can vibrate together. I've put felt in between the keyboards and where they rest against, and between the plastic panels, so that hard surfaces can't buzz against each other. I removed the plastic shroud over the expression pedal, glued felt along its edges, and refitted it into the cabinet with the felt between the plastic shroud and the wooden cabinet. I've put felt between two of the plug-in cards that had metal parts touching each other, and felt along the top of the plug-in cards so they're more firmly held down by the wooden brace along the top of them.
The various parts that are screwed together may work loose (speakers, wooden part of the cabinet), and slightly tightening the screws can stop them vibrating against each other, or slightly loosening some of them. Carefully repositioning the speakers can stop the speaker cones buzzing against the wooden front panels (the tweeters were bad for that, as the hole cut out of the wooden panel in front of them is the same diameter as the moving part of the speaker cone, and the speaker can slip down so the cone hits the wooden panel).
The cables can vibrate against each other, so tightly bundling them together, or widely separating them apart, can stop them buzzing.
Yes, the amplifier does make quite a bit of noise (hiss, etc.), it's deliberately muted a second or so after you stop playing to hide the noise.
There are two stereo amplifier blocks in the base of the unit, where the power supply is. It supplies the three-way speaker system, and the headphones. I've had both power amplifiers fail, over time (a few years apart). If you turn on and hear a loud hum, switch off immediately and leave it off, and call for a serviceman. At this point the fault will probably be contained to just replacing the amplifier blocks (a simple task, if you can get them, or some equivalent replacement). If you leave an amplifier running when it makes such a noise, you stand a good chance of ruining the speakers, too. I've lost speakers that way, before; it only takes a few seconds to burn them out. If servicing an organ, it may be worth adding some speaker fuses. Particularly to the woofer.
When I replaced the power amplifier blocks, I also replaced the small electrolytic capacitors on the same circuitboard. Some of them had dried out, and replacing them fixed up some faults. They're cheap, easy enough to replace, so you may as well replace them for the sake of completeness, and to avoid having to do another repair job in the near future.
As mentioned in the recording section, the organ will not sound the same if not using the built in speakers—the cabinet forms a large part of the sound the organ reproduces, as well as where the speakers are placed (bass is significantly enhanced when speaker systems are placed into the corner of a room, especially when they're in the longest diagonal space of the room). And the positioning and distance between the speakers are a part of how the fake Leslie and celeste effects work. Bear all of that in mind.
The in-built speaker system is a three-way sound system. There's a main speaker system (with a large woofer, smaller woofer, and a tweeter) in the centre, facing the organist's legs (where you sit on the organ stool). And there's two other speaker systems either side of it (each with a small woofer and tweeter), one on the right side facing the organist's legs, and one on the left side facing out the side of the organ. They get fed different signals depending on what effects are in use (fake Leslie, celeste, phaser), and different instrument banks go to different speakers.
Under the console there's connections for a “tone cabinet” which bring three feeds out to an external amplifier system. This duplicates what the internal speakers are fed with, including fake Leslie effects, etc. You'd use this to connect to a Technics tone cabinet, or to a real Leslie. However, if using a real Leslie, you'd have to turn off the internal organ effects and control effects separately from the console tabs (the organ will not remote control your Leslie tremolo/chorus/stop). Also, you'd end up with some of the drums going through a Leslie, so you might want to avoid the internal drum unit.
There's a stereo output pair which has some sort of mix-down of the above three signals. I haven't found it to be very well done, you'd probably be better off mixing the three tone cabinet signals, yourself (with one mostly left, one mostly right, the other fairly central).
Like most musical instruments, the organ is designed as a whole. The cabinet, itself, and where you place it in the room, also affects the sound that's reproduced. Although you can make a recording from the stereo line outputs, or remix the tone cabinet outputs, it won't sound the same as when you play the organ, normally. The cabinet resonates at certain frequencies, enhancing the lower frequencies. And the speakers are fed with phase modulated sound to simulate the Leslie and celeste effects, it doesn't sound the same if the speakers are a different distance apart (e.g. feeding the left and right audio signals directly to a recorder, with subsequent playback going directly to another set of speakers, etc.).
Neither does playing the organ wearing headphones sound the same, for the same reasons (the lack of resonance from the cabinet and the room, and the distance between the earphone speakers, etc.).
To record the organ so that it sounds natural, the best way would be with a good quality microphone (or more than one), placed somewhere where the organ sounds good to the ears, then start experimenting. This adds the ambience of the room to the recording, as well. Placing a microphone right in front of the speakers will not make a natural-sounding recording. After all, you don't listen to the organ in that way, and you can't place one microphone directly in front of several different speakers. Live bands do that sort of thing (close miking) to amplify an instrument without getting feedback, but recording an instrument takes a different technique. One microphone placed fairly close (microphones aren't as good as your ears, and do need a somewhat closer placement), and another further way, works fairly well for simple psuedo-stereo recordings. Alternatively, a stereo pair of microphones quite close to the console works very well, too.
No, it doesn't have MIDI, and I can't see any way that you could add it, the organ is not digital enough. I seem to recall reading there's some PCM sound (probably the drum sounds), but the organ is mostly analogue in its sound generation and control.
I finally got a service manual, in 2018. I bought a copy from W D Greenhill & Co, in England. I was quoted £35.00 GBP plus shipping (£7.95 GBP 1st Class UK, £10.95 GBP Europe Airmail, or £12.95 GBP Worldwide Airmail). I received a spiral-bound well-printed manual. I do not intend publishing a scan of the manual, but may be able to answer specific technical queries.