MIDI trouble while running first test

OK, here I am trying to final test the SHRUTI-1 I just finish building.

First of all, it uses a SMR4 board (so I rush to the “Troubleshooting and known issues” to check things).

  • Checking the power supply on the SMR 4 was OK while building

For the digital control board, switches and leds are OK, pot and encoders are OK, LCD was OK as soon as I correctly set the contrats trimmer.
I was starting the filter tune process but noticed some strange behaviour: playing notes on the midi keyboard just let the small corner icon appear but also kind of behave as a program change messsage (patch number and name when playing while being on the browse patch page or value change while playing while being on one of the edit page).

Checking the OSC out to the SMR4 (using the “Troubleshooting and known issues”) show me the 39 KHz signal but no variation while playing

My guess is there is something wrong in the MIDI part of my unit.

Any suggestion would be welcome

When you play a note, do you see the note icon or something else?

Do you also get spurious program changes when just touching the pitch bend lever or the modulation wheel?

Is there a way for you to try with another MIDI source?

Is the 6N137 correctly inserted and soldered? If you have a scope, check pin 6. It should be +5V, with dips to 0V when a bit of data is sent. You can also try unplugging the synth, removing the optocoupler, and check that pin 5 is continuous with ground.

Hum, I found the problem.

I used a 6n138 instead of the 6n137. I missed the 5th August Shruti kit rush so I had to source the components by myself.

I put the correct component and now it ROCKS !

I didn’t expect to have that behaviour but while thinking about it, I knew the trouble was around the MIDI part of the circuit and then I double checked the opto and noticed that you used a 6N137 rather than the 6N138 I had.

Thanks a lot for your suggestion.

Maybe this would avoid someone else to make the same mistake …

The 6N138 is a much slower opto, so you need to use a 220R pull-up with it. The 6N137 is fast, you need to use a high pull-up (10k) otherwise you get super sharp edges that the ATMega won’t like (it’s a not very well known factory defect they have - they don’t tolerate very sharp signals on the UART pins).

With a 6N138 + 10k pull-up, the edges of the MIDI signal get so soft that you can no longer distinguish individual bits.