- Skeleton Frame
- ROM Logic Bay
- Timbre Controller
- Synth Body w. Auxiliary Bends
- uC Interface
- 5 Octave Keyboard
The Frame recieved some major updates in the last month. It is now as deep (20") and as tall (32", folded up) as it can be, while still fitting in my friend's bus. The legs are reattached, now with hinges that allow it to fold in, like a camel sitting down. 12 latches hold the 4 legs tightly in place when standing up. Additionally, the rig for the ROM Logic Bay, attached to the top, also folds over.
The ROM Patch Bay/Logic Controller. Each ROM pin is broken out to a column on the controller. Each column has six components. The top three are the ROM Patch Bay. Banana jacks let ROM pins get patched together using banana cables. The bottom three are the Logic Controller. An on-off-on switch lets one connect a ROM pin to either V+ or Gnd, forcing it to a logic 1 or 0. The 200 ohm 10 turn Pot allows for analog tweening between the digital logic levels, as well as allow for "tuning" of patches on the Patch Bay (some patches need between 0-400 ohms resistance). The bottom switch is a bypass of the Pot for quick switching.
todo:
- add LEDs for logic indicators
- make it look nicer
I've had this idea in my head for a while, and after a lot of experiments it's ready to be built. The idea is that grounding address lines 14-17 on the ROM only affect the timbre of the sound- it leaves all other aspects (adsr, glitches from bends) the same. The other address lines can be used to gate MOSFETS with the source as A14-17 and drain as Gnd. Each of the pots on the spiral turns a MOSFET on and changes the timbre in some way. One of the spirals is complete, and I hope & imagine that using all of them together will give me control over some pesky bends that would otherwise sound amazing.
todo:
- change pots out for 2000 Ohms
- uC pulse switchable grounding, with LED indicator
This is the Synth Body, with the original main MT-240 circuit board. I've removed the back and made the board vertical so I can access it easier. The body is also home to all auxiliary bends. The controls for these are where the original keyboard used to be, and the circuits are mounted on the bottom.
The aux. bends circuit can be broken down into a couple of smaller circuits. Read more about them below.
- Clock circuit
- LC pitch circuit
- Hidden functions
- ROM dysregulate and disengage
- DAC overdrive & squealers
A menu interface with a rotary dial, select, return buttons to choose a uC function. The idea is, for example: 5 => LC pitch circuit; 5.7 => LC tremolo circuit; 5.7.9 => fastest tremolo speed, send. I'm very happy with how this turned out. I think it'll also be a good framework for programming the microcontroller.
There's also an ignition key now.. vroom vroom!!!!!!!!
to do:
- add indicator LEDs (indicates when critical things are functioning)
A five octave keyboard torn from a different Casio. I added switches to every single key for individual note sustains. It's good and reliable for now, but I think I will replace this in the future with a more advanced keyboard.
todo:
- add pressure sensitivity to volume control
- add simpler uC sequencer interface, perhaps combine with individual note sustain
- add LED ROM-Patch range indicators
- reconfigurable keys for different ROM-patches? would have to have keyboard piped to uC before synth
The clock is powered by an external ltc1799 squarewave oscillator chip. The speed of the oscillator is currently controlled by a resistance up to 1M. A rotary dial is used to select between modes: potentiometer, ldr, or steps (controlled by another rotary dial). A switch controls the range of the oscillator. I believe I've covered the entire range of clock speeds possible with this chip and this Casio, so I'm satisfied with this.
todo:
- Try uC pulsing an LED to control the LDR
- once the second MT-240 is up, chain this oscillator to that clock circuit to sync both synth's clocks. May need to "duplicate" this wave, maybe use an op amp unity gain follower?
- far future: divide the frequency to have clk speed ratios between synths. Also, maybe the uC can then interpret this to do things based on the synth clock speed
The pitch of the synth is based on the frequency of a resonating LC circuit. I've added two variable capacitors to greatly vary the pitch, one as trimmer, other as main pitch control. I'm really happy with the range, almost a P5, and just at the upper limit of before the synth begins to crash. The lower limit can still be explored. I did some experiments and found a weird way to use a 10 turn 100ohm pot (I think as a variable center-tapped inductor, since the 10 turns are coiled) to reach this super low, awesomely unstable pitch range. This really is one of the best effects, but this isn't quite as reliable as I'd like, and it does not work with the pitch varicap. I am considering using a rotary dial to choose between these two modes, but that wouldn't be ideal. I need to do a lot more research, or get lucky.
todo:
- find a way to access that super duper low range reliably
- stepped switches for pitches, maybe also uC pulsed reed option
- touch node
- uC controller motor to turn trimmer varicap to make tremolo effect
There's a bunch of useful hidden functions that are on other Casio synths but no hardware for on the MT-240. These are simple switches & diodes between points on the circuit, but connecting them efficiently was a challenge, since many of them share points. One of the more interesting things is that there are two additional super secret TONEs in addition to the known 10 hidden TONEs. They are glitchy and not very pleasant, but they act as a preset TONE. I think this is because there are 2^5 = 32 Address "chapters" in the ROM that correspond to TONEs, but only 30 that are actually programmed. I think I can make buttons for those extra two with the help of the uC.
todo:
- add two glitch TONEs
- add 8 'atmosphere' buttons & control buttons (part of MT-540 ROM- maybe also add optional 540 ROM??)
- add tritone spam button (a special effect coming from the way the keyboard matrix is layed out)
These controls affect the BHE, OE, Vss, Gnd pins on the ROM chip. The BHE and OE pins have stable digital signals that enable the ROM, but using a pot like this causes the signals to enter a scary disregulated mode which may glitch out or disable the ROM. Some improvements can be made so that they aren't just leaking current when not being used. The Vss & Gnd disengage switches disengage the V & G signals on the Logic Patch Bay from the ROM chip, useful for a type of quick reset with help from the uC.
todo:
- optimize BHE, OE pot circuits
These circuits affect the DAC. I've experimented, but haven't built them yet. The Squealer scream and have all sorts of oscillating effects by creating feedback paths to the DAC inputs. These will probably affect the functionality of some or all of the other bends, when enabled, because they sort of take precedence over everything else being at the end of the audio chain. The Overdrive will probably be the most stable, and can be adjusted for intensity, it can get LOUD. Excited to work on these. They can be controlled by LDRs and uC by pulsing LEDs.
- Switchboard (to connect to patch bay)
- Gliss Controller
- Power distribution
- uC keyboard sequencer
- 4x4 active matrix mixer
- pedal rack
- foot pedals
- Second MT-240 with a few aux bends, small patch bay, and logic array controller
After the timbre controller, things need to be finished and ready for a June tour. I'll be going over each of the modules and doing a fine-tuning/cosmetic upgrade, and add a few small but major things like mounting the power and stereo audio jacks. When I return in July, I'll finish the uC sequencer next.