ProtoTypo PT7 – CMY

Here’s a small mod for Fonitronik Cascade, a 4HP eurorack module that allows mixing, attenuating / inverting and offsetting of three separate input signals (later “channels” or “ch.”). I totally love it for the size, features and price.

The mod brings on-board jumper terminal JP1 to front panel, so that the signal routing behavior of Cascade can be changed without removing the module from eurorack case.

Fonitronik Cascade, JP1 (with blue jumper) next to the board interconnect. Photo by Thonk.

The way JP1 works is, if its pins 1&2 are bridged then output of ch.1 is normaled to ch.2 input. If pins 2&3 are bridged then output of ch.1 is normaled to ch.3 input. On either setting ch.2 output is normaled to ch.3 input. This allows the module to be run (normaled) either in cascade mode (1&2 bridged) where output on ch.1 will flow through ch.2 to ch.3, or in parallel mode (2&3 bridged) where ch.1 & ch.2 outputs will flow to ch.3 input, effectively creating a 3-channel mixer.

The design

Looking at the schematic & module (photo above), the easy way of making this mod would be to solder a 3-pin cable to JP1 and hook that up to a SPDT On-On toggle switch.

Fonitronik Cascade V1 schematic

However I wanted to go about it the hard(er) way, and modded the circuit so that signals are routed through the board interconnect with no dangling cables. Doing it this way involves relocating the input resistors of each channel from the main PCB to the panel PCB. So modding the above schematic we end up with something like this..:

Fonitronik Cascade switch mod, main PCB.

Fonitronik Cascade switch mod, panel PCB.

On main PCB resistors R7W and R13W are wire links.

The build

Main PCB is easy, just remove parts JP1, R7, R13 R20, R21 and R28 and then bridge R7 and R13..:

Modded Cascade main PCB.

On panel PCB things get a bit more messy though, as there isn’t much free space to accommodate the relocated resistors nor the toggle switch. The latter needs to be some kind of mini-sub-mini-mini-subminiature type, which will just barely fit between ch.3 potentiometer and ch.1 polarity LED.

A really nice, clean way to go about the whole thing would be to redesign panel PCB to accept 1206 or 0805 size surface mount (SMD) resistors, tighten up the jack & potentiometer spacing and produce a set of boards some place like OSH Park. But it would’ve been a overkill for a one-off project like this.. And besides, figuring out mechanical aspects of a hack can be quite the fun puzzle!

Using through-hole parts, the resistors should be installable by trimming the each leg to correct length. In my case R20 & R28 needed a short extension. I also covered each resistor leg with heat-shrink tubing just to steer well clear of any short-circuits. Chose yellow color just so that it would stand out well in the photos ;)

First up, the toggle switch will have resistors R28 and R20 connected to pins 1 & 3 and a wire to pin 2. The latter gives a initial mounting point to fine-tune the install location of the switch. Pins 1 & 3 are soldered to the panel PCB once the front panel has been drilled & switch test-fitted.

Toggle switch resistors & wire

R7 & R13 go under respective channel pots, R21 over board interconnect.

Switch location adjusted, R28 sits on top of R21.

R20 is installed next to right edge of panel PCB.

Modded panel PCB

For the front panel it was once again my (now tried & tested) “paper stickers + packing tape” route. Instead of funnay cat stickers I chose to design custom graphics with a CMYK theme and laser print it. Using colors to represent each channel left the text design more spacious and allowed using larger font size to slightly improve readability.

Modified Cascade

Letters C, M & Y aren’t all too nice in terms of abbreviations, so instead of brainfarting something like “CMon Yeeahh!!” I’ll opt for sth more more technical like “Channel Mode Yoke”. The switch is joining channels anyways….

In case you want to use my panel design on your Cascade please check the section below for the pdf. It doesn’t have any drill holes marked and should thus fit any panel, whether yours is a stock Thonk kit or a hand-drilled one like mine.

Resources

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