If you're technically competent, or have someone else who is, and can refer them to this page, it lists a few changes that you can make to the organ, if you feel the need.
Tab voice effects
The upper and lower manual tab voices (either or both) can be piped through two effects devices: The artificial Leslie effect, and a celeste (detuning chorus) effect, or be unmodified by any effect. In fact, you can use the fake Leslie and celeste effects on top of each other.
When fake Leslie and celeste are switched off, the tab voices are fed straight through to the amplifiers. When the fake Leslie is switched on, the straight sound is cut off, and the tab voices are fed to fake Leslie effect circuit. When the celeste effect is switched on, straight audio is cut off, and the tab voices are fed through to the celeste effect circuit. And when the fake Leslie and celeste effect are both switched on, the straight sound is cut off, and the tab voices are fed to the fake Leslie effect circuits and the celeste effect circuits.
These two effects are created by running the sound through bucket brigade delay (BBD) ICs which introduce varying amounts of delay (low frequency oscillators wobble the clock frequencies to the BBDs, and therefore the amount of delay), creating pitch wobble and doppler kinds of effects. Audio also passes through voltage controlled amplifier (VCAs) ICs to vary the audio amplitude, for the Leslie simulator.
The Leslie effect is simulated by trying to produce a circular kind of rotating sound effect, getting quieter and louder, and the doppler effect of the sound waves bunching together and spreading apart, trying to emulate the sound of rotating Leslie speaker horns spinning away from and towards you (but unable to emulate the room effects of audio reflections coming from all directions, though some of this effect is sent to the reverb tank to help with that effect). It is a two-channel stereo effect, as one channel moves towards you, the other channel backs away, and vice versa. It's fed to two of the main amplifier channels
The celeste effect goes through three varying delays, individually fed to the three main speaker amplifiers (the organ uses triphonic stereo), trying to emulate the celeste effect of a pipe organ where two sets of slightly detuned pipes are played together (and not succeeding at sounding like it–it really ought to be one straight sound with the other pitch-wavering).
The fourth amplifier is a subwoofer feed, it's a low frequency mix of the three main audio signals.
Vibrato on this organ is done by applying a low-frequency oscillator into the tone generator board, to wobble the tone generator's master frequency, which will effect all the divided down frequencies (actually pitch-bending the tones being generated). Essentially the same way a violinist would vary the frequency of a vibrating string by wobbling their finger on the instrument's neck. Vibrato is independent of the Leslie and celeste effects, all of them can be on simultaneously (not that it sounds good like that). Vibrato will be forceably disabled with some combinations of voices where it would cause unpleasant effects.
Bass pedals don't get vibrato (they're from a separate tone generator), they can be put through the celeste effect, but there's no option to put them through the Leslie emulator (though I did experimentally try feeding them through it, and it worked quite well). Our old Hammond did apply vibrato to the bass pedals, the effect can be nice, but it's even better when it's optional.
Signal switching
The directing of audio through one route or another is done using 4066 CMOS switches, controlled through digital signals, which come from a microprocessor, which reads the tab switches through a multiplexer. Between the microprocessor and the CMOS switches are latches, so the microprocessor sends change of status commands, and the latches hold the settings until the next change (acting as demultiplexers, and hold switches). None of the tab switches actually have audio signals going through them, nor do they directly control the audio signals.
Because there is a microprocessor in the middle, tab switches can be read, stored, and retrieved as registration presets. There are two fixed presets, and four that you can set yourself on the green registration buttons (all the stop rail switches are read and stored), and four more fixed organ tab presets on the white buttons. When it's using stored settings, all the stoprail switches are ignored. You can change them and not affect the sound, but when you cancel the presets, you'll switch over to the stop rail switches being in control.
Faulty switching
My organ had a fault where whenever the bright/mellow timbre switch is set to mellow, the lower manual tab voices were always piped through the straight sound path, even if you had the fake Leslie or celeste effects switched on (the sound would go through the straight and effect signal paths, when it should only be going through effects signal paths). This produced some unpleasant effects, especially when emulating the fast Leslie speed.
Fault finding didn't find any electronic faults with the multiplexing ICs involved in reading the tab switches, nor the demultiplexing ICs controlling the CMOS switches, so I've come to the conclusion that there's a microprocessor programming fault. Many other things share the multiplexed data and address buses, but this was the only control fault noticed.
The original circuit had three data latches (in ICs 27 & 28) controlling the CMOS analogue switches (in ICs 18 & 22) feeding audio straight through, to the Leslie emulator, and to the celeste effects. My solution was to fit a NOR gate (soldered above the latch IC) that had the Leslie and celeste switching signals also going to its inputs, removed the original straight sound latch signal going to the straight audio CMOS switch, and feeding the output of the NOR gate to the straight sound CMOS switch. Using hardware to do the job that software was failing to do correctly.
On the SEL (select) PCB, cut off pin 1 on IC 27, solder a NOR gate above IC 27, using pins 14 and 7 (supply and ground). Tack onto pin 13 (Leslie on) of IC 27 to one of the NOR gate input pins (e.g. its pin 13). Tack onto pin 15 (Celeste on) of IC 28 to the other input of the NOR gate (e.g. its pin 12). Feed the output of the NOR gate (e.g. its pin 11) to where IC 27's pin 1 used to go IC 22 pin 12. Ground the other NOR gate input pins (e.g. 1, 2, 5, 6, 8, 9). Leave the other NOR gate output pins unconnected.
Feeding an external Leslie
If you wanted to take the straight unaffected audio to an external device to fake a Leslie, or to feed a real Leslie, it's possible to do so. You can take a feed that will be sent to the internal Leslie emulator and send it out of the organ, instead. You'd also want to interrupt the internal Leslie emulator, so it's muted.
This signal is before the expression pedal, so if you're feeding an external effects box, you'd want to bring its signal back into the organ, so the organ can control its volume in the normal way. If you're feeding a real Leslie speaker, you either want to insert a VCA circuit controlled by the expression pedal, or have an extra expression pedal between the organ and the Leslie.
While you could feed the entire organ sound through a Leslie, there are things that you wouldn't normally want going through a Leslie (the drum kit, and some of the synthesised voices, such as the piano). So the internal feed to the Leslie emulator is only the stoprail tabs.
On the CEL (celeste and tremolo) board pin 5 has the incoming tab voices signal to this board (only present when the Leslie emulator is engaged), and pins 22 and 23 are the stereo outputs from the Leslie emulator to the organ mixing and amplifier circuits (on the RME board) which is always listening to these pins (no matter what the Leslie emulator tab switch position). You can tap into the signals here (as outputs and inputs). I used switched 1/4″ jacks for break in points with signal re-routing through their switches, and audio coupling transformers (this gives me balanced audio connections, and signals that are floating from ground).