DIY Mutable Instruments

Tides 2 LEDs

I’ve put together a few Mutable Tides 2 modules, and I think just about all of them have LED problems. Today we’re going to be looking at LED6 and LED7 which are among the audio jacks on the lower right of the board. As always it’s a good idea to look at the schematic.

Not much going on here so it should be easy to troubleshoot. We’ve got a buffer amp driving the LED and a current limiting resistor. Circuits for LED6 and LED7 are duplicates which is nice since they’re probably suffering from the same problem. The first thing to do in these situations is to check the signal. I like to start with the op amp to make sure it’s outputting something. Next making sure Pin 1 on the LED matches the op amp output will give you an idea of trace integrity. Since the signal to pin one of the LEDs looked good I assumed the LEDs must be bad and removed them. And this was wrong. With the LEDs out it became apparent that the LED Pin 2 net was floating. And connecting the LEDs to my bench supply showed they worked fine. So the actual culprits here were the R66 and R67 resistors which had bad solder joints. Visual inspection showed that the resistors were slightly misplaced and only one solder joint had formed properly. After removing the J11 and J12 jacks this was easily fixed by adding a little more solder.

Tides 2 R66 and R67 on the PCB

If I could remember how soldering this board went a few months back I suspect these resistors tombstoned. Tombstoning is when one terminal of a two terminal part lifts into the air during solder reflow. In the classic example the part stands perpendicular to the face of the PCB and looks like a tombstone.

Tombstoned SMD resistors, Courtesy of O-fey and Wikimedia commons

Primarily tombstoning has to do with asymmetrical heating of the pads for small surface mount components. Since Tides 2 uses almost entirely 0402 sized passives and I reflow solder the boards on a hotplate with a noticeable temperature gradient between the center and the edge, I had quite a few tombstoned parts.

One of the things I like about reflowing on a hot plate is it’s easy to adjust parts while the solder is melted. So I can just tilt tombstoned parts back into position with some tweezers and forget about them. Unfortunately with parts this small it’s easy to knock the parts out of alignment and miss one of the solder pads. The lesson here is I should be better about using a microscope during reflow.

DIY Mutable Instruments

Ambiguous STM32 part marking

So you want to DIY a Mutable Instruments Module

A few months ago, I was assembling a Mutable Instruments module (let’s assume it was Rings), and I was about to place the microcontroller. We all know that Pin 1 IC packages are marked with some sort of dimple, dot, or indentation. For example

Pin 1 designator

However, this microcontroller had two two dimples.

So which one marks Pin 1? If you’re literate you read the datasheet, and note that Pin 1 is denoted by the dimple at the bottom left of the text marking.

Mechanical drawing by STM

If you’re illiterate, like me, you ignore the datasheet, decide that pin one must be the big dimple near the top of the text, install the part backwards, rage at the world when the module doesn’t work., heat gun the micro controller off, replace it with a new IC oriented correctly, and then wonder why you weren’t smart enough to read the documentation in the first place.

The moral of todays story is when in doubt read the documentation. In fact just read the documentation, always.

DIY Mutable Instruments

Mutable Instruments Rings: Should the SJ1 pads be bridged?

So you want to DIY a Mutable Instruments Module

This is going to be the first in what I anticipate will be a series of posts I’m tentatively titling ‘So you want to DIY a Mutable Instruments Module’. When I started designing Eurorack modules I decided to put together a few DIY modules to learn about common components and design patterns. The modules I picked to DIY were designed by Mutable Instruments. Mutable Instruments has designed a line of top quality Eurorack modules, and because they’re better people than I am, all of their designs are open source. That’s right schematics, bills of materials, PCB layouts, cover panel designs, and software source files are all easily downloadable from GitHub. I generally consider the documentation to be quite good. However there always seems to be something fiddly that doesn’t work quite right for the first one you assemble. And I’m going to discuss the solutions to the questions and problems I had while I was assembling the modules.

Back to the actual topic, the solder jumper on Rings

On the Rings module there is a solder jumper, SJ1, with no clear indication if it is supposed to be shorted or left open. Since the pad is wedged between two mono jacks it is best to pick its state during assembly. What should we pick?

Short version: short the jumper

The longer version:

To answer this question we need to know two things: What does SJ1 do? How is this module expected to function when it passes QC at Mutable? The first question should be pretty easy to answer; we’ll just take a quick look at the circuit schematic.

This section of the schematic covers SJ1, IC8 (output opamp) J9 (Odd output), and J10 (Even output). SJ1 connects the switched pin on Odd output to the amplifier mix for the Even output. Interestingly there is no solder jumper in the connection between the switched pin on the Even output and the Odd amplifier mix. So SJ1 causes the Odd output to be mixed with the even output if no cable is connected to the Odd jack.

Is this desired behavior? Fortunately we’re in luck and this is covered in the official manual for Rings no digging required.

Mutable Instruments rings manual

5. Odd and even audio outputs. In monophonic mode, these two outputs carries two complementary components of the signal (odd and even numbered partials with the modal resonator, dephased components due to picking position and pickup placement with the string resonators). In polyphonic mode, splits the signal into odd and even numbered strings/plates. Note that you need to insert a jack into each output to split the signals: when only one jack is inserted, both signals are mixed together.

The final sentence tells us what we need to know. “Note that you need to insert a jack into each output to split the signals: when only one jack is inserted both signals are mixed together.” So the factory default is both signals are mixed when only one output jack is used therefore we need to bridge the solder jumper SJ1.