Eurorack module testing tool (DIY)


The tester is really easy to build and calibrate. Works great. Just don’t let yourself get scammed into getting a plexi case for it.


Or drill some holes in a bit of plexi, and add a ready-made wooden box …


Finished my module tester last week and it works great. Thanks for developing this!


I don’t know if I mentioned this… but we’ve updated the module tester case files on github, in case somebody wanted to get those lasered.


@rumpelfilter have a link for this github repo of case files? I just sent a crappy/quick layout I whipped up to get laser cut last week, unfortunately.


It’s all in the official testing tool repository on github:
which is linked on the testing tool page on the MI website:


Anyone happen to have made a Farnell or Mouser saved basket/BOM for this project?



There is an official Mouser BOM on the same page I have linked above.
THough there are some parts missing, so double check it.


MI newb here, this will be my first project in tandem with an Anushri, thanks to rumpelfilter for posting the case file to Ponoko, I was even able to save $20 since it was my first purchase from them! Looking forward to the build. Finally will force me to look at embedded programming.


Got it all built. On power up the backlight for the display comes on as do all the leds but no gibberish on screen. Loaded the hex file onto the blank chip and the display turns on to show…

“>tempo 120
resol. 24ppqn”

I get clk pulses, a gate out that toggles between a high and ground every few seconds, cv out that does similar but going from negative to positive, and a squarish signal on the audio output.

I randomly pressed some things and the encoder push toggles the “>” but rotation does nothing and the leds are all still turned on. Button presses other than the encoder do nothing.

At one point my rough handling somehow got to this screen, I don’t know if a button actually did something or static charge hit a flip flop…

“pulse 5ms
>midi on”

On this screen the audio out seems to be providing a weird tone (I guess it’s a 5ms pulse). CV out is toggling between a positive and negative value every few seconds. Clock out has some hash at extremely low levels but no real signal.

Seems to me that the chip is working but my button interface isn’t.

Any ideas?

Also, the display seems a little dim. If I take/short out the 6.8ohm res will it brighten up a bit?

PS. More futzing with the encoder knob and I’m able to change some values but it’s hard to make work, maybe I got a defective encoder? LED’s still are constantly on though and pressing the buttons does nada.


If the buttons don’t work well, you might have put the resistor network (RN1, just on the right hand side of the buttons) the wrong way around - do check that the dot is at pin 1.

As for the encoder, just reflow the solder on its legs, and the corresponding legs on the microcontroller. Just briefly


“If I take/short out the 6.8ohm res will it brighten up a bit?”

First try to change the contrast with the trimmer about the display. Always needs to be done.


Thanks for the tips, the resistor network is fine and looking at the scheme I’m not sure what else other than a faulty chip I can see. Resistor values and cap values all check out. I guess I’ll start swapping chips. I reflowed the encoder but nothing changed. Maybe I should reflow the board, I don’t see any bridges but maybe I’ve got some cold solders.


@bkbirge – check if the fuses are programmed correctly: it sounds like your MCU runs at speed other than 20MHz


Thanks kvitekp! That was the problem, it all works great now! What a cool build, thanks!


Is there a way to trim the supply voltages? I’m getting +/- 11.85 and 4.95 on the connector output and I’d like to dial in the/- 12 rails a little better.


Why? The voltages are all good and within the specs of the regulators. And aside from that, what are the tolerances of your multimeter?


@bkbirge You can’t do that without redesigning the power supplies to use adjustable regs like the 317/337-and if you want to do that you might be better off building something external like the CGS66 power supply, which I can thoroughly recommend. In any case your oscillators should be using either an internal reference for V/Oct span (digital) or the inherent Vbe/Ic characteristic (analogue) neither of which should be thrown out by a small deviation from ideal on the power rails-indeed any problems due to not tolerating this should count as a design flaw ( cough Anti-Oscillator Mark 1 cough)


Thanks both of you, makes sense. I can’t seem to power any modules and I’m trying to track the issue down, I think it may be down to some bad ribbon cable. I thought maybe the regs were suspect but they are steady with load.


No problems! Check the soldering on your power headers, and the orientation of your cable stripe…