Music and Synth DIY

DIY tips, Eurorack, Synth DIY

Troubleshooting Eurorack DIY builds – the basics

As nice as it is when a freshly built module works perfectly first time, troubleshooting is unavoidable.

Part of my goal with building this stuff was to teach myself synth electronics – so although it’s frustrating when things don’t work immediately, I’ve learnt a lot more from the mistakes I’ve made than I would have otherwise. So at least there’s that I can tell myself after I’ve spent six hours building something that doesn’t work to stop myself throwing it at the wall.

Stuff you’ll need

Without the minimum amount of tools, troubleshooting is limited to visual checks, so this is what I use:

  • Multimeter (you won’t get far without this one)
  • Desoldering braid or desoldering gun/pump, and flux
  • Seperate Euro power supply (I use a FC power board as a bench supply)

Very nice to have’s:

  • Oscilloscope (more than a nice to have, really, even a cheap £15 DSO kit is better than nothing)
  • Audio probe
  • Loupe (for checking fine pitch soldering)
  • Hot air gun (or Chipquick – for removing fine pitch ICs, etc. ) and Kapton tape to prevent heat damage / displacement of nearby components
  • Function generator (to give you a CV/audio input etc to the module on test) with BNC to 3.5mm cable, or spare oscillator module, etc.
  • Mutable Instruments Module Tester (if you plan on building many modules)
  • Schematic for the module

Disclosure – I’m not an expert by any stretch of the imagination. In my experience of building four large cases of modules so far, the most common problem with a DIY build is either a single misplaced component (incorrect value, reversed polarity, etc) with through hole PCBs and a microscopic solder bridge on fine pitch ICs on surface mount PCBs.

The challenge is finding it, and it really helps to know where to look and what you’re looking for. Based on the problem with the module, you can narrow it down to a section of the circuit, and then to a component (or components).

I’m going to assume that your soldering skills and your iron are up to par, and you’re not lifting pads and damaging components left, right, and centre. If not, stop here and get that right first – there are thousands of free Youtube tutorials on through-hole and surface mount soldering, hot air reworking, and desoldering. I’m also assuming you’ve grasped the basics of reading schematics – if not, read this,

Module is dead – no audio, no LEDs etc

The absolute basics

So, firstly – is the power connector definitely in right way round? Red stripe = -12v. Sounds daft but we’ve all still done it, or are destined to at some point. If you plugged your module in backwards, and the module doesn’t have reverse polarity protection, you may have fried some of the ICs (check the op-amps first) and/or polarised components such as electrolytics.

Is the power cable itself OK? Is the power supply on?

Is everything electrical in your house also off because you have a power cut? Is the module sat in a bowl of salt water? No? OK, that’s the totally obvious stuff, then.

Is the module supposed to do anything at all without patching in a clock, or whatever? If your answer here is “dunno”, go and download the manual first. For example, you’d expect an analog oscillator to continually produce a tone, if the frequency is in the audio range – but digital oscillators such as Neutron Sound’s Dust of Time have a built in VCA, and might expect a gate signal before you hear anything.

Power section: Check

Without circuit powered up, check continuity between +12v and ground and -12v and ground with your multimeter. A short tells you there’s probably a solder bridge somewhere, or something’s touching something it shouldn’t be (eg. heatsinks of positive and negative voltage regulators). It isn’t necessarily in the power section, though.

Did a fuse blow, or did a very low value resistor in the power section go up in magic smoke? In modules like the uO_c, a 4.7 ohm resistor or similar behaves like a fuse. You’ve probably got a short somewhere. If it was a polarised electrolytic that spontaneously combusted – it’s probably in the wrong way round.

Check orientation of diodes and voltage regulators and any polarised components.

Measure +12v/-12v input at the power header with voltmeter. Is voltage present past the protection diodes and power filtering caps?

Is the correct voltage present on the power pins of the ICs? Check the schematic/board file, datasheets, or google “<IC name> pinout”. For example, there should (generally) be +12v on pin 8 and -12v on pin 4 of a TL072.

If the board has voltage regulators like a 7l809 or LM1117, check they’re outputting the correct voltage, and that voltage is present on the power pins of the ICs expecting it. Again, check the datasheets for the pinouts.

Check your soldering again – look for cold solder joints as well as bridges.

Check components / values carefully. Is your 78l05 actually a 79l05? A BAT54 is not compatible with a BAT54S…etc.

Builds with microprocessors : check –

Solder bridges – reflow fine pitch microprocessor IC, using lots of flux. Check visually with a loupe. Does the firmware load? If not, check the orientation of the IC (especially on the confusing ones with two “pin one” dots where the datasheet shows one…). Check the clock from the crystal is OK with a scope (using the “10x” setting on the probe) or a frequency counter. ON MI builds: have you flashed the bootloader before the firmware?

Is there any output from the data lines? Check the datasheet and scope for input/output from the MCU, check the RESET line isn’t held low/high, etc.

If you’re using a switching Euro power supply, try a non-switching one. Some digital modules (Ornament and Crime, etc) can have issues with power supplies that bring one line up slightly slower than the other, or are particularly noisy.

Module outputs distorted/nonsense audio/no audio

Check power pins on all semiconductors with a multimeter (look up the pinout on the datasheets). Is everything getting what it should?

Check orientation of op amps, transistors, diodes.

On digital modules:

  • Trace output from microprocessor to DAC on a scope. Reflow MCU / check for bridges. Check orientation of MCU.
  • Check and/or reflow DAC
  • Check orientation of DAC

On all modules:

  • Trace the audio path to the output, either with a scope or an audio probe (link to making audio probe) – really helps to have a schematic here. If you have decent audio from – say – the DAC, and it goes from there into an op amp mixer, but there’s no output – you’ve found your problem.

Don’t be afraid to ask the designer for a schematic or part schematic if they don’t supply it with the kit/PCB, you’ll often get lucky.

Module works but one section is dead or misbehaving

Firstly – have you calibrated the module, if it needs calibrating?

Check power pins on all semiconductors with multimeter. Check outputs of any voltage references (such as the LM4040) and pins of ICs expecting the reference voltage (check datasheets, schematics or PCB files for pinouts)

Check the value and placement of all passives against the BOM/iBOM/schematic/Eagle file (resistors, caps etc). Laborious, but remember – this circuit works if you build it correctly, and this is one of the most common errors. Have you used a 1% tolerance resistor where 0.1% precision resistors / matched resistors were required, or non-matched transistors in your ladder filter? Have you missed soldering one side of an 0603 resistor?

Locate the section on the schematic. Check all the component values and orientations. Check the whole section for cold solder joints or bridges. Check continuity between components connected in this section on the schematic, one by one.

For non-functioning control pots:

Is the knob a potentiometer, set up as a voltage divider? Check the output from the wiper (middle pin) and follow the CV back through the path with a multimeter. Is the range correct? Are any other resistors in the circuit definitely the correct values? Is the relevant ADC pin on the MCU getting the control voltage the pot is putting out?

Is the knob an encoder? Worn encoders will give you randomly jumping values. The wrong spec encoder can give you reversed values, or values that jump two at a time, etc.

RTFM. You can’t troubleshoot a module unless you know what it is supposed to be doing.

It may be that you have the module in calibration mode, or some other menu item is interfering with what you expect the module to be doing, or it needs restarting twice after flashing, or it needs an external clock source to function – etc. EG – say you’re building MI Braids, and your 1v/oct control voltage is just playing four notes when you sweep the octave. You may have a QUANTIZE option – which is in a sub-menu – turned on, and the module may be functioning completely correctly.

General

Is anything getting too hot to the touch?

Are you sure about the provenance of your semiconductors? There are many fake ICs on the market, especially of older/discontinued ICs – if you’ve ordered from Mouser, RS Online or any of the reputable suppliers, this shouldn’t be an issue. Ali Express or random ebay seller, not so much – IME it’s just not worth the bother.

Take a break. Sometimes the problem is pretty obvious when you look with a fresh pair of eyes.

Stuck? Google it first. Ask for some advice on one of the many forums – Wigglers, FB groups (Synth DIY for non-engineers, Eurorack DIY, Pusherman’s Eurorack DIY noobs, Eurorack SMD DIY, etc) -post a decent detailed photo of the board/s. If it’s a commercial kit/PCB, rather than an open-sourced build, try asking the designer (AFTER you’ve checked your component placements, etc!).

If all else fails, don’t give up – put it to one side and come back to it when you’ve built a few more modules.

Like I said before, I’m not an expert and I’m not claiming to be. This is just somewhere to start. If you want to see how I’ve stumbled around finding a problem in my DIY builds, try reading this.

Good luck!