MIDI in not working, can someone verify my board photos?

Hi guys,

Can someone check I have the resistors correct for filter board and control board. Looking at the 6N137 layout it should be from left to right 220k,220k,10k but I think the last one looks suspciciously like a 220k which I assume would drop the current to low to pin 14.

Photos: https://www.flickr.com/photos/130401059@N08/

Status: Main board seems ok LCD and menu cycling. For completeness a summary of my mistakes during the build:

filter board: I lost a 1M ohm resistor or at least didn’t find it, so on the filter board you’ll see 5% 1M which I hope is ok for the
filter board: I sparked one of the end LM3700 by accident ( plugged in, trimming pins utterly stupid I know … ).
filter board: 104 101 caps I had reversed and had to desolder ,resolder on the L072 side.
control board: much more careful, no obvious mistakes other than the possible resistor mix up.
me: Need glasses I really had difficulty distinguishing the metal film resistors.

6N137 IC Testing:

Pin 2 fluctuating 0,0.2,0.15 up and down presumably my MM can’t keep up with the input. ( Alesis synth generating clock data )
Pin 6 4.97v seems to drop sometimes to 4.96 but nothing big and certainly not 2.4v range. ( LED light up not excessively bright )
Pin 5 0V
Pin 8 4.97v ( LED test very bright, high current would have burned it out if I left if for a few seconds )

MCU Pin 14 ok with connection to pin 6 of 6N137

When I was testing with LED a few days ago, if I tapped the ground on and off I could see a symbol show up on the LCD display I assume that means MCU connectivity is fine and the problem is the pin6 signal.

thanks for reading,


R18 and R19 should be 220 Ohm. You can measure them with your multimeter in resistance mode (without power on and without midi cables pluged in).
R14 should be 10k Ohm. Try to measure it with your multimeter, too (maybe you have to take out the 6N137 for properly measuring).

Thanks for the quick response. I will do the resistance check, is it OK to use 5% tolerant resistors here if I have too?

Yes, 5% is fine.

I also suspect that these are the wrong resistors.

Yep that was it thanks! works now but no resonance, cutoff. I also notice there is a space invader shaped symbol constantly on looks like a square with a line in middle. Is this midi out playback of the sequence mode or do I need to debug more before moving into checking audio path?

Thanks again,

Can you send a picture of the space invader?

Maybe the pitch bend lever icon if your sequence has pitch bend modulations?

Aah that’s it, the alesis micron has that non returning pitch slider…
I’ll send a pic later.