No MIDI - 6N137 looks good... [SOLVED]

I hate to do this but I’ve hit a wall.

Menus are all looking fine, I’ve got the test tone but I can’t get MIDI in to my Shruthi. I’m using a M-Audio Axiom. I have everything set to the same channel. I’ve opened the Axiom and verified continuity between the board-side of the MIDI port and the other end of the cable. I don’t have a scope so I can’t directly check for MIDI data. All of the connections around the 6N137 optocoupler look good:
Pin 2 -> R19/D1
Pin 3 -> MIDI Pin 5
Pin 5 -> GND
Pin 6 -> Pin 14 of the Atmega
Pin 8 -> VCC
MIDI Pin 4 -> R19

These are from the chip itself, not the socket so I’m sure it’s in the right way. I’ve tried sending MIDI from Ableton through my audio interface (Edirol UA-25) but no joy. I’ve trawled these forums, tried everything I could find but no. Nothing. I give up. Can anyone suggest something?

What is the voltage on pin 6 of the optocoupler? It should +5V at rest and should drop down when there is heavy MIDI traffic.

Using the right resistors? if the resistor value is wrong the LED inside the optocoupler won’t be bright enough.

It’s at about 4.96V normally but only drops to around 4.81V when I start hitting keys. There is definitely a response. I thought it should drop lower though right?

Your multimeter smoothes the values with a low-pass filter, so even if the voltage drops to zero as it should, you’ll read a value close to +5V. It doesn’t look like there is a problem with the optocoupler from your measurements.

Check if pin 14 of the Atmega has not been bent when inserted into the socket.

And are you sure that the data is sent on the right channel?

Pin 14 looks good. Continuous through to the pin of the socket on the other side of the board. Everything is on channel 1.

Hmmm sorry I don’t have any idea…


Checked another MIDI cable ?

I only have one, but I checked it for breaks and it seems ok. It’s been sitting in a drawer for a few years but it’s never been used.

Oxidation ?

Well I got the Axiom sending MIDI into Ableton through the interface, so I can rule out the keyboard and the cable… Moving the cable and every thing looks fine, no breaks. Ableton is behaving as expected to channel changes on the keyboard.

Hm, true…forget about that.

You can test the midi cable on Pin 5 and Pin 4 with an LED.

Do you have access to a keyboard with regular DIN MIDI Out you can test with instead? Or maybe just another MIDI interface?

Is ableton midi in/out correctly set up ?
In ableton’s preferences? Does your edirol interface shows midi out?

I believe I may have found the problem. It looks like I got resistors R15 and R16 mixed up with R18 and R19. I have the 220ohm resistors where the 2.2kohm resistors should be and vice versa.

red red black brown brown <=> red red black black brown

I’ll report back to confirm when I get this fixed.

Welcome to the world of bullshit color coding. Is it red or brown? Is it yellow or orange? under what lighting conditions was this all determined? It’s not a stupid error, It’s a stupid system and why they just print the value on parts these days if they can. Get a DVM and always double check if there is any question… If you plan on doing more of this, find a line of resistors from 1 manufacture and stick with them, at least you will get use to the colors so you can identify what is what quickly

Haha yeah I started measuring the resistance just to be doubly sure during the build, but this mistake was literally on step 1! It’s all working just fine now. Very happy. I checked pin 6 again to see what was happening there and the voltage is dropping a little bit more than previously under MIDI load.

Thanks for all the help and suggestions guys. I can eat my victory sweet now!

wait is this reading 104 or 103… damn its so small i can’t read it, under what lighting conditions its meant to be deciphered? :wink: