Midimuso optocoupler trouble

Hey there,

So I’m working on a polysynth, and a key factor to get it running is the midimuso chip in 4PV mode. I’m using the schematic available on the LMNC site, and it works great on every mode other than the 4pv. At this point I’ve isolated the issue to be around the optocoupler, where it seems to get overloaded and send out garbage to the midimuso, but I was curious if anyone had any insight or suggestions to tweak it and get it running. Let me know, thank you!

Hard to believe the optocoupler’s at fault; it’s rated for 10 MBit/s. Granted, that apparently is with a load resistor of 350R and per the datasheet it’s around a factor of 2 or so worse with RL=4k — and Sam’s design uses 10k. (Other MIDI circuits that use the 6N137 I’ve seen online use RL=1k to 10k.) And the datasheet shows a 100 nF bypass capacitor from pin 8 to ground which Sam did not include. You could try lowering R19 and adding the cap to see if that helps.

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Hey thank you for the insight! The reason I’m looking critically at the optocoupler is based on some help from Robert from Midimuso, I’ll give it some tweaking per your recommendations and see how that goes.


Tried your recommendations, again thank you for the advice, unfortunately the same problem persists. I added the cap and tried a few resistances for R19 and its the same issue.
To elaborate on the specifics of the problem: It’ll receive the first few midi inputs, but once a key is released it just gets a garbled mess of data, only after tweaking the mod/pitch wheels it’ll cool down to a baseline where I’m able to send it more key press data, but the problem repeats whenever I let up on a key. Let me know if you have any more tips or ideas, in the meantime I also have some 4n35 chips on their way, per the midimuso proprietary schematic.

If it is only on a key-up, and every-key up, it’s 98% a software problem…
(The other 1% is some “kind of interference” when a gate output goes down at key-up.)
I don’t know midimuso, is it possible to reflash the chip, or do you have to buy it preprogrammed ?

(You see it doesn’t add up to 100%… :slight_smile: )


Thats what I think too! But according to the midimuso man himself, it’s not a problem with the chip. They come preprogrammed, you can update the settings via midi, but I dont think a reflash is an option, worth mentioning, but I’ve swapped midimuso’s a couple times to test. But thank you for the corroboration.

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Update: new optocoupler, same problem. A garbage signal after key release, it can be brought back with the mod/pitch wheel but obviously that’s not really workable. Swapped in the 4n35 and yeah, same thing. Let me know if you guys have any ideas!