Incoming MIDI stops audio

I reviewed the existing MIDI problem posts and didn’t see anything quite like this, so if this has been addressed and is redundant, please let me me know!

I have a Shruthi-1 with a SMR4 mk2 filter that is partially functional. Audio output works, i’m able to play the demo tone, all the encoders seem to adjust parameters, and all buttons are functional. However, upon playing a sequence from my Octatrack into the Shruthi, all audio abruptly stops, and Random characters that I’ve never seen before show up on the edges of the screen intermittently.

I’ve seen a few posts about probing the pins of the 6N137, but admittedly am not sure how and what I should check in this case. In order to power the module and probe the pins. I’m assuming I just have to probe the underside of the digital board (since the power comes from the analog voice board).

Also, for IC4, I have a 24LC512 rather than a 128 (the BOM specified “128 or higher” and I saw the 512 in an existing Mouser cart)… I assume this should be acceptable, but am wondering if it’s contributing to my problems.

I’m a troubleshooting novice but really want to better my skills, so any guidance will be greatly appreciated. Thanks!

Try swapping the 6N137 for a 6N138

@pichenettes Thanks for the suggestion — with the 6N137 it no longer stops with receipt of incoming MIDI, but it seems to ignore the messages and MIDI events have no discernible effect.

I’ve already removed the LCD to reflow the ATMega pins in an earlier troubleshooting attempt, so cant check for visual confirmation of the abnormal screen characters. Any other troubleshooting advice before I re-solder the screen (that is, any test points to check while they’re easily accessible)?

UPDATE: completed more troubleshooting and measurements; issue still not resolved.

Had some time over the weekend to troubleshoot some more.

  • Voltages on the 6N138 are as follows: Pins 1-4: 0. Pin 5: 0 (continuous with ground), Pin 6: 4.76 (and seemingly unaffected by incoming MIDI note messages… I saw mention of a voltage drop from 5V in another thread regarding incoming MIDI), Pin 7: 0.5, Pin 8: 5.0.
  • When attempting to measure the voltage at D1, both with incoming MIDI and without, the audio of the demo sound stops. Audio after power-cycling resumes (upon activation of the demo sound).
    -Are there specific pins on the ATMega you can point me toward to measure with my multimeter? I’d like to complete any troubleshooting necessary before replacing the LCD?