Sometimes, one half will work when plugged in by itself, and the other half will work when plugged in by itself (probably with its keys mirrored), but they won't work when connected together with a TRRS cable between them.
Make sure you are using a TRRS cable. It has four contacts: the tip, two rings and the sleeve, which is the ring furthest from the tip.
Most audio cables are TRS cables, which means they only have three contacts. Most split keyboards will only work with a TRRS cable.
First of all, you'll want to make sure the solder joints on your TRRS jacks and microcontroller are good. Adafruit has an excellent overview of common soldering problems, of which the image below is taken. Compare your solder joints with those in the guide, and if any have problems, rework them to be alright again.
You may also want to check the orientation of your RGB LEDs. The LEDs are square, with one corner being cut into a triangle. This triangle should align with the dot that's printed next to the LED on your PCB. Are they not aligned, then you should desolder the LED and reinstall it in the correct orientation.
If you socketed your microcontroller, and have not found the issue after following the steps in this article, detach your microcontroller from the sockets. There's a slight chance solder flowed into the sockets, possibly joining them together. It's a very slight chance though, so check the other steps first.
Often, the cause of only one half working has to do with continuity: the ability for electricity to flow between two points. You'll want to make sure there is continuity between the right points, to ensure you have no solder bridges or defective components.
To check for continuity, you can use a multimeter that has a continuity check mode. Often, the multimeter can emit a tone so you can hear it when there's a connection between two points.
Check for continuity on the following pins:
- When connected with a TRRS cable, each pin of the TRRS jack should connect with its corresponding pin on the other half of the keyboard and to no other pin on the same TRRS jack.
- The pins on the TRRS jack should connect with the appropriate pins on the microcontroller. You can find the schematics for the Kyria at Which microcontroller pins does the Kyria use?
If you still weren't able to find the problem after taking these steps, please get in touch: send an email to firstname.lastname@example.org and I'll help find a solution.