Casio MT-600 Filter Mods (UNDER CONSTRUCTION...)

Robin Whittle states that Casio never made an analog synth and, from what I can tell, he's right. The closest they got was the MT-400V/CT410V, perhaps the DH horns, and the late 1980's HT series. These were all notable because, while their 'oscillators' were digitally generated waveforms, these were run through real analog filters.

The Tablehooters site reviews and investigates the CT-410V, big brother to the more common MT-400V. He's got incredible detail on the hardware, the nature of the sounds, as well as bends and mods. One of his mods tweaks the filter, getting it to be a bit more resonant.

That seems to be a theme with Casio: I guess they felt resonance would scare the home user, so all of these analog filters were kept very tame when they left the factory. The MT-600 is the preset version of the Casio HT series keyboards (and I think it's actually the very last Casiotone). The HT's all have custom analog filter IC's, similar to the CEMs and SSMs, but manufactured by New Japan Radio to Casio's spec's.

Interestingly, the wonderful custom VCF/VCA chip in the Korg DW, DSS, and Poly800 series was also manufactured by NJR. There is a well-known mod for the Poly800 called 'Moog Slayer' that brings the onboard cutoff and resonance trimmers on the PCB out to the front panel. On this page, I talk about modding the VCF in the MT-600 in a similar way (though it's a bit more involved). You might consider my mods as unleashing the beast in a very unlikely little Casio.

The Mods
Alright, the MT-600 is non-editable and it's somewhat roomy inside, so I thought I'd mod this guy for several things:
-filter cutoff control
-filter resonance control
-filter-in jack
-expand the range of the chorus.
-added LFO for filter cutoff

The first three mods are not too involved, but do require some trace cuts and some desoldering, as well as adding some pots and a rotary switch. If you want to do the LFO, that's quite a bit more involved, so I saved it for last.

Filter Cutoff
The Chip:
The NJM2090 is a four-pole voltage-controlled filter. What's unique about this chip is that it runs off +5/GND rails; I'm not aware of any other musical filter chips that run this low (CEMs and SSMs require at least a 10V swing). For that reason, I've already begun a separate project based on this filter chip where I plan on adding it to other low voltage keyboards. More to come on that later (and you'll see in the pictures that I’ve already desoldered the VCF chip for the accompaniment voice, which I never use in these Casios).
The usable cutoff swing of the filter is between 0V (filter open) and 2.5V (filter closed). The resonance runs between 0V (none) and >3.5V (self-oscillation).

The input to the filter cutoff frequency is an inverting op amp mixer, which uses an op amp that's on the filter chip itself. The stock design runs two signals from the main CPU into this mixer: cutoff control (a dc voltage) and an ADSR envelope (digitally generated by CPU). The value of these signals is dependent upon the selected voice.
The positive terminal of the input mixer amp is a fixed bias voltage. Since this sets the reference for the input CV signals to the amp, I chose to repurpose this input as my cutoff CV and to eliminate the microprocessor cutoff CV input by cutting traces. What this means is that you, the user, will now be manually setting the cutoff with your potentiometer, but that's what you do on a real analog synth anyway, right?

I used the circuit below for manual cutoff CV input.

I soldered the middle-pot-terminal-through-XXXk directly to pin 2, the mixer input on the chip. I grabbed +5V and GND from the empty analog power rails from the other filter (but there's many other places on the board to grab them).

Don't forget to connect the pot so that clockwise rotation gives you an increase in the cutoff frequency!

Filter Resonance
Now we have to deal with Casio's conservative design. First the uP drives the resonance directly from a pin on the micro itself. The max it goes is about 2.2V, which just doesn't self-oscillate (though it does get squelchy). I wanted full control over the resonance with my control and I wanted the filter to be able to self-resonate, so I decided to disconnect all uP control over the resonance. This involved cutting the trace from the uP to the filter chip (see pics). I also desoldered the 104 ceramic capacitor that was hanging off the pin to ground (you may not have to do that). Now I could completely control the resonance with a pot. Similar to the filter cutoff, I used a pot with a series resistor to +5V and the other terminal to ground. I used a 100k pot and the series resistor was about 5k (it seemed to be needed, because if the resonance was taken all the way to 5V, it actually reduced the resonance (must be an upper rail problem). The wiper connected directly to pin 9 of the VCF, which is the resonance CV. And finally, it seems you _have_ to change one of the resistors that's on the board. I believe it's a resistor that sets a bias current for the resonance CV. If you don't change it, you may not get the filter to self-oscillate. It's a 5.1k resistor (see pictures) and I changed it to 6.8k which did the trick.

Filter-In Jack
This one's nearly trivial. Just used a normally-closed ¼-inch phone jack mounted on the backside. Cut the trace of the melody signal coming from the CPU, ran it to the jack, then ran the jack back to the other side of the cut trace. When you plug in an external signal, that goes to the VCF through a 22uF capacitor. One note about this: since the MT-600 can be MIDI-triggered, you now can process your MIDI instruments through the MT-600's filter.

Chorus Expansion
I don't really like chorus. When its mono, it justs seem to thin out the sound. But in stereo, the effect can be nice, and since you've got two speakers on this MT, right in front of you, it can give the sound a real sense of space.
So off we go: I got this idea from some guitar stompbox site on the web. Basically, the chorus in the MT600 is implemented using the classic MN3207/MN3102 chip set. The MN3102 timing chip uses a 68pf resistor to set the baseline depth of the chorus. This website suggested changing that value to alter the chorus shifting from flange to very-spread-chorus effects.
All I did was desolder the 68pF cap from next to the 3102, and bring out the two lines to a 5-position rotary switch. At each position, you get a progressively larger capacitance. At setting zero, you have 1pF, which gives no detectable chorus. Rotating the switch gives you 22pF, 68pF, 150pF, and 470pF in succession.

See the schematic below and the pictures for my admittedly messy connections.

LFO to cutoff
Ok, you don't have to do this mod, but it really wilds this thing out if you do. There IS an LFO already running in the MT-600: it comes from the CPU and goes to the chorus. As far as I can tell, it's a triangle wave with maybe three settings of amplitude and depth. You could grab this and re-route it into the CV cutoff input of the filter. I actually hooked this up at one point, but it wasn't controllable enough. So I had to add my own.
I got the basic LFO design again from somewhere on the web (I gotta start bookmarking this things...). It's a simple dual opamp design that generates a nice triangle and square wave with a minimum of components. You can perfboard and point-to-point wire it, which is sorta the way I did it (I actually etched a tiny PCB). Then you just power the LFO circuit by tapping off the main PCB, bring your waveform, rate, and amount controls out to pots mounted on the chassis, and run the LFO into the filter cutoff through a summing resistor. I designed the LFO to run from ~ 0.4-90Hz. See the schematic below and the pictures.

How does it sound?
This is a changed instrument. Much more like an HT, but better in that there's more real time controls. I placed the filter cutoff and resonance pots right next to the pitch bend and with some deft manipulation you can run all three at once. The LFO and chorus stuff I placed at the top center of the board. It's a little more awkward to manipulate these while playing, but it's still very doable. The LFO, especially on squarewave, allows you to get rhythmic chording. The resonance can get seriously loud (my D-cells are a bit depleted right now and if I let the resonance fly, I start getting the 'batt low' blinking on the power LED!) as well as squelchy and screaming. The smoothness of the analog adjustment of the cutoff frequency really runs the sounds through some dramatic color changes. I like that I can pretty much kill the chorus on some sounds by using the minimal setting, but the maximal setting, while quite noisy, gives a terrific stereo effect.

Take a look:

What's next?
I treated this MT as a 'test board' for modifying my other HT's. I'm definitely going to do versions of these mods on my HT-3000. Plus, I'm going to leave the accompaniment VCF in the 3000, reroute the melody signal through it, and have it switchable to have individual filter outputs to the left and right channels; true stereo filtering with separate cutoff, ADSR, and LFO! That's kind of amazing for a Casio


				*Cheap Mods* *Casio MT-600 Filter Mods*



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		Background



		Robin Whittle states that Casio never made an analog synth and, from what I can tell, he's right. The closest they got was the MT-400V/CT410V, perhaps the DH horns, and the late 1980's HT series. These were all notable because, while their 'oscillators' were digitally generated waveforms, these were run through real analog filters. The Tablehooters site reviews and investigates the CT-410V, big brother to the more common MT-400V. He's got incredible detail on the hardware, the nature of the sounds, as well as bends and mods. One of his mods tweaks the filter, getting it to be a bit more resonant. That seems to be a theme with Casio: I guess they felt resonance would scare the home user, so all of these analog filters were kept very tame when they left the factory. The MT-600 is the preset version of the Casio HT series keyboards (and I think it's actually the very last Casiotone). The HT's all have custom analog filter IC's, similar to the CEMs and SSMs, but manufactured by New Japan Radio to Casio's spec's. Interestingly, the wonderful custom VCF/VCA chip in the Korg DW, DSS, and Poly800 series was also manufactured by NJR. There is a well-known mod for the Poly800 called 'Moog Slayer' that brings the onboard cutoff and resonance trimmers on the PCB out to the front panel. On this page, I talk about modding the VCF in the MT-600 in a similar way (though it's a bit more involved). You might consider my mods as unleashing the beast in a very unlikely little Casio. The Mods Alright, the MT-600 is non-editable and it's somewhat roomy inside, so I thought I'd mod this guy for several things: -filter cutoff control -filter resonance control -filter-in jack -expand the range of the chorus. -added LFO for filter cutoff The first three mods are not too involved, but do require some trace cuts and some desoldering, as well as adding some pots and a rotary switch. If you want to do the LFO, that's quite a bit more involved, so I saved it for last. Filter Cutoff The Chip: The NJM2090 is a four-pole voltage-controlled filter. What's unique about this chip is that it runs off +5/GND rails; I'm not aware of any other musical filter chips that run this low (CEMs and SSMs require at least a 10V swing). For that reason, I've already begun a separate project based on this filter chip where I plan on adding it to other low voltage keyboards. More to come on that later (and you'll see in the pictures that I’ve already desoldered the VCF chip for the accompaniment voice, which I never use in these Casios). The usable cutoff swing of the filter is between 0V (filter open) and 2.5V (filter closed). The resonance runs between 0V (none) and >3.5V (self-oscillation). The input to the filter cutoff frequency is an inverting op amp mixer, which uses an op amp that's on the filter chip itself. The stock design runs two signals from the main CPU into this mixer: cutoff control (a dc voltage) and an ADSR envelope (digitally generated by CPU). The value of these signals is dependent upon the selected voice. The positive terminal of the input mixer amp is a fixed bias voltage. Since this sets the reference for the input CV signals to the amp, I chose to repurpose this input as my cutoff CV and to eliminate the microprocessor cutoff CV input by cutting traces. What this means is that you, the user, will now be manually setting the cutoff with your potentiometer, but that's what you do on a real analog synth anyway, right? I used the circuit below for manual cutoff CV input. I soldered the middle-pot-terminal-through-XXXk directly to pin 2, the mixer input on the chip. I grabbed +5V and GND from the empty analog power rails from the other filter (but there's many other places on the board to grab them). Don't forget to connect the pot so that clockwise rotation gives you an increase in the cutoff frequency! Filter Resonance Now we have to deal with Casio's conservative design. First the uP drives the resonance directly from a pin on the micro itself. The max it goes is about 2.2V, which just doesn't self-oscillate (though it does get squelchy). I wanted full control over the resonance with my control and I wanted the filter to be able to self-resonate, so I decided to disconnect all uP control over the resonance. This involved cutting the trace from the uP to the filter chip (see pics). I also desoldered the 104 ceramic capacitor that was hanging off the pin to ground (you may not have to do that). Now I could completely control the resonance with a pot. Similar to the filter cutoff, I used a pot with a series resistor to +5V and the other terminal to ground. I used a 100k pot and the series resistor was about 5k (it seemed to be needed, because if the resonance was taken all the way to 5V, it actually reduced the resonance (must be an upper rail problem). The wiper connected directly to pin 9 of the VCF, which is the resonance CV. And finally, it seems you _have_ to change one of the resistors that's on the board. I believe it's a resistor that sets a bias current for the resonance CV. If you don't change it, you may not get the filter to self-oscillate. It's a 5.1k resistor (see pictures) and I changed it to 6.8k which did the trick. Filter-In Jack This one's nearly trivial. Just used a normally-closed ¼-inch phone jack mounted on the backside. Cut the trace of the melody signal coming from the CPU, ran it to the jack, then ran the jack back to the other side of the cut trace. When you plug in an external signal, that goes to the VCF through a 22uF capacitor. One note about this: since the MT-600 can be MIDI-triggered, you now can process your MIDI instruments through the MT-600's filter. Chorus Expansion I don't really like chorus. When its mono, it justs seem to thin out the sound. But in stereo, the effect can be nice, and since you've got two speakers on this MT, right in front of you, it can give the sound a real sense of space. So off we go: I got this idea from some guitar stompbox site on the web. Basically, the chorus in the MT600 is implemented using the classic MN3207/MN3102 chip set. The MN3102 timing chip uses a 68pf resistor to set the baseline depth of the chorus. This website suggested changing that value to alter the chorus shifting from flange to very-spread-chorus effects. All I did was desolder the 68pF cap from next to the 3102, and bring out the two lines to a 5-position rotary switch. At each position, you get a progressively larger capacitance. At setting zero, you have 1pF, which gives no detectable chorus. Rotating the switch gives you 22pF, 68pF, 150pF, and 470pF in succession. See the schematic below and the pictures for my admittedly messy connections. LFO to cutoff Ok, you don't have to do this mod, but it really wilds this thing out if you do. There IS an LFO already running in the MT-600: it comes from the CPU and goes to the chorus. As far as I can tell, it's a triangle wave with maybe three settings of amplitude and depth. You could grab this and re-route it into the CV cutoff input of the filter. I actually hooked this up at one point, but it wasn't controllable enough. So I had to add my own. I got the basic LFO design again from somewhere on the web (I gotta start bookmarking this things...). It's a simple dual opamp design that generates a nice triangle and square wave with a minimum of components. You can perfboard and point-to-point wire it, which is sorta the way I did it (I actually etched a tiny PCB). Then you just power the LFO circuit by tapping off the main PCB, bring your waveform, rate, and amount controls out to pots mounted on the chassis, and run the LFO into the filter cutoff through a summing resistor. I designed the LFO to run from ~ 0.4-90Hz. See the schematic below and the pictures. How does it sound? This is a changed instrument. Much more like an HT, but better in that there's more real time controls. I placed the filter cutoff and resonance pots right next to the pitch bend and with some deft manipulation you can run all three at once. The LFO and chorus stuff I placed at the top center of the board. It's a little more awkward to manipulate these while playing, but it's still very doable. The LFO, especially on squarewave, allows you to get rhythmic chording. The resonance can get seriously loud (my D-cells are a bit depleted right now and if I let the resonance fly, I start getting the 'batt low' blinking on the power LED!) as well as squelchy and screaming. The smoothness of the analog adjustment of the cutoff frequency really runs the sounds through some dramatic color changes. I like that I can pretty much kill the chorus on some sounds by using the minimal setting, but the maximal setting, while quite noisy, gives a terrific stereo effect. Take a look: What's next? I treated this MT as a 'test board' for modifying my other HT's. I'm definitely going to do versions of these mods on my HT-3000. Plus, I'm going to leave the accompaniment VCF in the 3000, reroute the melody signal through it, and have it switchable to have individual filter outputs to the left and right channels; true stereo filtering with separate cutoff, ADSR, and LFO! That's kind of amazing for a Casio



		Added 2/24/2007