mb33 Mk1 DIY mods

I haven't found the schematic of the mb33 Mk1 on the internet. But the schematic of the mb33 retro is available, which is quite similar and helpful. The main difference is that the Mk1 does not have a negative power supply, i.e. there is no -12 V. Have a look at the great Maffez Synth Mods for details on how to modify the mb33 retro.

A good entry point for modulating the cutoff frequency is the interconnection of the three resistors behind the cutoff and resonance pot pointing to the front panel side. This point is marked with "1" in the picture below. I use a modulation voltage of 0 to 5 V and a series resistor of 47 kΩ to modulate the cutoff frequency. In addition, I connected 5 V via a series resistor of 100 kΩ there to compensate the cutoff drop caused by connecting an external voltage to this point. 5 V can be taken from the resistor to the right of and directly next to the VCF input jack at its pin pointing to the back side.

A decreased slew rate for the envelope modulation is realized by connecting the center pin of the Env Mod pot and GND via a capacitor of up to 10 µF. A switch could be used for fixed slew rate values. Instead, I soldered a little transistor circuit to be able to modulate the slew rate. Please find a schematic below. I took a 2SC458, which is actually outdated, but was the only npn transistor that I still had at home at that time, and a 33 kΩ base resistor. Pay attention to ensure that the maximum base emitter voltage is not exceeded, which is 5 V with a 2SC458. And the modulation only takes effect when a certain voltage is reached that makes the transistor become conductive, which is about 2.5 V in my case. A good point to reach the center pin of the Env Mod pot from the component side of the PCB is marked with "2" in the picture below. And GND can be taken from the VCF input or audio output jack.

A 50 kΩ pot can be added to the front panel to mix an external audio signal to the VCF input. Therefore, the right pin of the pot is connected to the audio signal, the left one to the GND of the audio signal and to the GND of the mb33, and the center pin of the pot is connected to the tip of the VCF input jack. The connection of the tip of the VCF input jack to GND must be cut if no plug is inserted into the jack, i.e. cut off the metal contact of the tip of the jack, that bridges the tip to GND, or cut one of the respective PCB traces.

If needed, the gate and accent signal can be taken from the 80C31 microcontroller: Pin 4 jumps to 5 V if gate gets active, and pin 2 drops to 0 V with accented notes.