| Music Gear Repair and Hacking ^ | ||
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I got this organ in the fall of 2009, mainly so I could practice on organ pedals to perform on the Virginia Theater's Wurlitzer theater organ. It is solid state, but does a pretty good imitation of an older tonewheel organ like a B3. It has a little built-in Leslie speaker and all kinds of weird gadgets like an analog drum machine, automatic chording, Fascinating Fingers II (an arpeggiator), etc.
I decided to add a MIDI Out interface to it so I could use it to control the Miditzer software. (Try the Miditzer...it's free and fun to play with!)
I am designing the boards myself and programming a PIC 16F877 chip to generate the MIDI. The key contacts on the Commodore output about -20V when the key is pressed. I wanted to use CD4067 16-to-1 analog multiplexer chips, which cannot handle a voltage range wider than about 20 volts, to scan the keys into the PIC. So I am using voltage dividers to reduce each key signal to about -2V before it goes through the multiplexers. The multiplexer chips are running on +/- 5V power supplies generated by 7805 and 7905 voltage regulators from the Commodore's +/- 24V supplies.
Each manual has one of these multiplexer boards mounted underneath and wired to all the keys on the manual. Each board contains 64 voltage dividers (only 61 are used currently, 1 for each key) to reduce the -20V key signals down to about -2V. They are then multiplexed down to 4 data lines using four CD4067 chips. A ribbon cable connects the Mux board to a central board with the PIC (currently a breadboard). It carries +5V, -5V, and ground pins to the Mux board as well as the 4 address lines to the multiplexers, and the 4 key voltage outputs from the multiplexers.
The Commodore conveniently has an empty circuit board mounting area on the left side under each manual. It appears that this is where the white-on-black drawbar preset keys board would go if the same frame hardware was used on a higher-end Hammond from the same era, like the Concorde. This was the perfect place to mount my multiplexer board on each manual.
I designed the boards using Eagle and used an LPKF milling machine to fabricate them. Here is the second Mux board before adding the parts.
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Here is what the Mux board looks like installed to the left of the upper manual. I originally wanted to use screws and spacers to hold the board in, but it proved so difficult to reach the area with a drill that I resorted to sticking it on with double-stick tape.
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Since the key signals are mounted on 3 separate boards within the Commodore, I soldered a separate ribbon cable to each one so the boards can still be removed separately. The ribbon cables run to three 26-pin connectors on the Mux board. I intend to tie the wiring down a little more neatly.
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 This picture shows how the individual wires from the ribbon cables are soldered onto the traces of the key boards. |
This board, which is still a prototype at this stage, contains the PIC 16F877 processor and interface circuitry.
The PIC outputs its logic signals at 0-5V. But I need to use it to send address bits to the analog multiplexer chips, which are running on +/- 5V power so they can pass the -2V key signals through. So I used LM339 comparator circuits to convert the PICs 0-5V signals into -5V-to-+5V signals before they go over the ribbon cables to control the CD4067 chips on the Mux boards. Each PIC output is compared to 2.5V obtained from a simple voltage divider on the 5V supply.
The PIC also needs its logic inputs to swing between 0 and 5V. When the multiplexed key signals come from the Mux boards, they are about -2V when the key is down and 0V when the key is up. So I use an inverting amplifier circuit on each key signal to turn it from -2V to about +4.3 volts before it goes to the PIC to be read.
When you power down the Commodore, its -24V power supply dies down to zero a long time before the +24V power dies, causing my -5V power to die long before my +5V power. When the -5V has died but the +5V is still on, my inverting amplifiers output +5V to the PIC constantly during multiple whole scans, making the PIC think that ALL the keys are down. So every time you switch the organ off, my interface sends out a MIDI Note On message for every single note! Whatever synth you are controlling with the MIDI generates a whole lot of stuck notes. Not pretty. So I added a circuit which detects when the +24 volt power has started to drop slightly after power is switched off (this fortunately happens before my -5V power has died) and grounds the reset line of the PIC, holding it in reset until the power dies out and preventing the spurious notes.
Here is the organ with the prototype master board sitting on top. The ribbon cable is coming from the Mux board mounted under the upper manual on the left side.
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Here is what it sounds like when the organ is driving a synth in Reaktor: (1.4 MB mp3). The Hammond is doing the drawbar sound, strings, arpeggiator (sorry, FASCINATING FINGERS II), and drum machine. Reaktor is being driven by the top manual and is doing the analog-synth-like notes.
The old Wurlitzer theater organs have a feature called "second touch" that is a lot like "polyphonic aftertouch" on more modern synthesizers. When you push a key down until you hit the first resistance point, the organ sounds whichever ranks are selected for normal operation, i.e. "first touch". But if you continue pressing the key until you hit the bottom point below that, you get another rank of pipes or percussion in addition to the first one. Using this feature, certain notes can be made to stand out from other notes on the same manual.
Here is a video of Jim Riggs doing really cool second touch work on Jesse Crawford's "I Love to Hear You Singing". The notes he presses harder are weaving in between the other chord tones and making melodies and countermelodies! Notice how at 2:40, the notes he presses harder with the left hand (using his thumb and pinky, then index finger) trigger the glockenspiel counter melody. That rules.
I am thinking it might be possible to add second touch to my Hammond by mounting additional boards full of pushbutton switches on top of the existing contact boards. Well, ON TOP when you are looking at the manual when raised, but BELOW when it is in the regular playing position. There is enough room in there to mount additional stuff, and the pushbuttons can simply be pressed against the point where the black plastic tabs from the keys press against the metal contact springs. That way, when you play, you will be able to feel when the spring touches the added pushbutton so you know that is the threshold point. If I get this to work I will post more details.