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.
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.
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.