Modular for the Masses 909CRC Eurorack Module

You've managed to become the owner of a Modular for the Masses 909CRC kit or PCB. Well done. Let's build it!

You will need:

• The PCB set with most of the SMD parts already installed
• Tools for building up PCB projects, like a soldering iron, solder, pliers, cutters, a file, a little screwdriver, you know the drill (you won't need a drill)

And these parts:

• 1 x AVR128DA28-I/SO
• 4 x Thonkiconn-style 3.5mm mono audio jacks with nuts
• 2 x 100K linear Song Huei "tall trimmer" style potentiometers
• 1 x 100K box-style multiturn trimmer
• 1 x pushbutton switch
• 1 x tall pushbutton switch -- 13mm tall
• 6 x LEDs, 3mm, any color, with two of them filed flat if you want
• 1 x 10-pin 2x5 IDC boxed header (standard Eurorack power header, I prefer boxed)
• 22 x DuPont pin headers broken into various lengths...
• if you don't want to bend six of the DuPont headers into 90 degree pin headers, use some of these

Let's get started!

Snap apart the three parts of the PCB. You'll need to file the mousebite leftovers, and snip off the ears next to the potentiometer footprints. I didn't do this right away on this build, but it's easier to get the boards all cut and smooth ahead of time.

Installing the microprocessor! Explaining how to do this is beyond this tutorial -- SMD soldering can be such a fun thing to do! Pay attention to the orientation of the chip. The circular indent in the chip (and the beveled edge of the chip) indicates pin number 1 of the chip, and goes near the white dot on the PCB. Use flux to help the solder flow where it needs to, and check for bad connections or bridges between pins.

The trimmer goes in the little indent area, bent over like you see. The Eurorack header attaches with the notch in the box facing "down" from where most of the components are. The pin header to program the AVR128DA chip should be bent after installing (or use right-angle headers).

The short switch goes on the processor board.

Not shown is a 5-pin header you may install right next to the Eurorack header. It's possible to use that spot to link two of these side-by-side and have them share one Eurorack power cable.

Okay, install the pairs of pins (or the 2x2 pins if you're fancy) in the control board.

This is tricky -- you'll need to have the pins installed and held straight by letting the control board rest in place on the processor board. Please don't solder the header pins to the long pads on the processor board yet.

When the pin headers are all in place, install the Thonkiconn-style mono jacks, the tall trimmers, and the two ground-flat LEDs. You don't need to grind the LEDs flat, I just like the way they look. Mind the polarity of the LEDs -- the long leg of the LEDs go in the rectangular pads.

Put all the parts where they need to go. Don't solder them in place yet -- first, put the panel in place over the controls and jacks. Wiggle the potentiometers around until you're happy with how straight they are and make sure they turn nicely. The tall switch can be crooked -- make sure it's in the hole where it needs to go. The LEDs should fit snugly in their panel holes.

Once you're happy with all that, solder the potentiometers, jacks, LEDs, and switch. Looks great, doesn't it!?

Put the control and panel assembly onto the processor board. Make sure the pins match up with the long pads you'll solder them to. Solder them together -- it may be challenging to get the joints strong and secure, so I recommend using flux, and experiment with holding the soldering iron tip at an angle to touch the pin and the pad at the same time.

When all 16 pad/pin joints are made, put the optional jack LEDs in. Remember, long leg goes into the rectangular pad, and put them in and bend them over so they point out the hole beneath each 3.5mm jack. These are totally optional -- if your modular has a weaker power supply, you may leave these off to reduce the current draw of this module.

Programming this chip is weird, and I'll show you how I do it, and leave it up to you to really sort out the details. There's many *many* ways to get this done, and I only know this one way.

First, you'll need a UPDI programmer. You can build the one I have, based on links you can find at this Hackaday article.

You'll also need an Arduino IDE on your computer, with jag2updi configured correctly, and the AVR128Dx board support included. Get this zip file, unzip it somewhere, and open the AVR909RCC.ino file with your Arduino IDE. UPDI only uses 3 pins, so connect +5, GND, and the UPDI pins to your programmer, load up the sketch, and click "Sketch -> Upload using programmer".

This is a reliable way to get your sketch uploaded, but if you're more comfortable working with binary files that are already properly complied for some particular bit of hardware...... I'll figure out how to get a binary file up on here and you can use some other tool to get this firmware onto your module.

Here's a video of my module working:

This should do it!

The trimmer can be adjusted to set the maximum output volume of the voice. Turn it clockwise until you're happy with the signal level. Eurorack standard is 10V peak-to-peak, so if you've got a scope, you can watch the signal to get it to comply with that range with the Level potentiometer turned up all the way.

Clockwise makes it louder.

The top jack labeled Pitch is for a control voltage that sets the pitch of the voice. This control conforms to the 1V/octave standard, so you can play tunes with your 909CRC.

The top potentiometer sets the pitch as well. If you turn the knob lower than 12 o'clock, the sample will play backwards. It's possible to scrub back and forth through the sample if you mess with this knob correctly.

The two LEDs show which sample is being selected, and playing.

The button below selects between the three samples, sample one is crash, sample two is ride, sample three is 808 cowbell. Hold this button for two seconds, and it becomes a way to trigger the module without an external trigger signal.

The jack labeled Accent allows you to control the volume of the voice with a control voltage.

The Level pot controls the volume of the voice.

The Trig jack is for the trigger. Send this a pulse of voltage, the module will trigger!

The Out jack is, well... obvious.