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  #1  
Old 04-04-2012, 01:03 AM
hippoBomb hippoBomb is offline
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Default MIDI Thu Circuit Difficulty

Hi Everyone,

I was wondering if anyone could take a look at the bread board layout I did for Highly Liquid's schematic of a 5 channel MIDI Thru box.

Here are the schematics:
http://forum.highlyliquid.com/showthread.php?t=1127

This is what I cooked up:


Am I doing something obviously wrong? I don't have a lot of exp with schematics.

It doesn't seem to be working. I'll plug in to the in and out to the out and nothing. Not exactly an electronics wiz so I'm not even sure how to go about testing in order to find the issue.

Thank you!
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  #2  
Old 04-05-2012, 01:15 AM
Jim McDougall Jim McDougall is offline
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If you pink/purple is positive midi then I think you have the output wires crossed. I believe the individual outputs should be the - midi line
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  #3  
Old 04-05-2012, 01:27 AM
hippoBomb hippoBomb is offline
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Hmm, I think I've got it wired correctly.
I wasn't paying attention to coloring when I made the image.

Good catch though!

Input
Pin 4 - Pink
Pin 5 - Blue

Output
Pin 2 - Black
Pin 4 - Blue
Pin 5 - Pink

Also, I hope I have the concept of a MIDI thru box correct. If I send MIDI information on Channel 1 then it will replicate that information across all 5 outputs? Pretty much works as a splitter, right?

Last edited by hippoBomb; 04-05-2012 at 02:38 AM.
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  #4  
Old 04-06-2012, 03:48 AM
Jim McDougall Jim McDougall is offline
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Nope -

Pin 4 should be positive which means it should be pink on both input and output

see here - this will help

http://pinout.net/cable_description....connector=MIDI

if the output is reversed then it will not light the led in the receiving 6N136 and you will not receive midi data

Yes it acts like a splitter --- any data received on the input will appear at all outputs
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  #5  
Old 04-07-2012, 05:57 PM
hippoBomb hippoBomb is offline
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So I did some fiddling and basically got it to work by connecting..
Pin 4 MIDI IN -> Pin 2 of 6n136
Pin 5 MIDI IN -> Pin 3 of 6n136

I can also get the LED to light by connecting...
Pin 4 MIDI IN -> Long Leg (Anode)
Pin 5 MIDI IN -> short leg (Cathode)

Basically any combination of what the schematic shows isn't giving me an LED that lights up and catches the MIDI IN information.

The resistor kills the MIDI IN Signal.

I'm wondering what the point of the diode resistor combination is?
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  #6  
Old 04-07-2012, 06:27 PM
hippoBomb hippoBomb is offline
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I'm basically confused.
I've built MIDI Ciruits with teh 6n136 before and they worked fine
http://www.instructables.com/id/Buil...the-schematic/

why does this one behave differently?
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  #7  
Old 04-07-2012, 08:45 PM
Jim McDougall Jim McDougall is offline
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On that diagram pin 4 is positive as well
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  #8  
Old 04-07-2012, 09:31 PM
hippoBomb hippoBomb is offline
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i don't really understand what that means.

i've built both. one works they way the schematic says it does.
the other doesn't.

same gear same midi cables same interface.
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  #9  
Old 04-07-2012, 10:57 PM
Jim McDougall Jim McDougall is offline
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The resistor is a dropping resistor -- leds normally operate at about 3.3V but midi sends a 5V signal. The diode is reverse polarity protection.

Here is another splitter circuit -- same input circuit

http://www.diystompboxes.com/smfforu...?topic=85425.0

https://ccrma.stanford.edu/~gary/controllers/midi.html


Try pulling the diode but leave the resistor in place and see if that works .

You might have an improperly banded diode which will create a short between the + and - inputs
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  #10  
Old 04-08-2012, 03:17 AM
hippoBomb hippoBomb is offline
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thanks for your help and input.
the thing works if i take out the resistor. i've tried lots of combinations of diode, LED, and resistor. seems to be the only way it works?

perhaps this is a case of testing homemade gear with other homemade gear!

Last edited by hippoBomb; 04-08-2012 at 03:46 AM.
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  #11  
Old 04-08-2012, 05:27 AM
Jim McDougall Jim McDougall is offline
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Just for fun --- check the resistor value with an ohm meter if you have one. The only other possibiliity I can think of is that the resistor is either open or it has a value that is way too high and there is not enough voltage/current coming through the resisitor to light the internal LED in the 6N126.
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  #12  
Old 04-09-2012, 12:58 AM
hippoBomb hippoBomb is offline
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tested a few 220 resistors that all checked out around 220 on the meter.

I tried using a 180 ohm dude and it worked alright
tried a 270 ohm and it didn't work

I also tried using various MIDI controllers and synthesizers so now I know it wasn't my homemade gear's problem or the MIDI interface I was using to pump info into it.
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  #13  
Old 04-09-2012, 01:20 AM
Jim McDougall Jim McDougall is offline
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would have to guess that there is a weak led in the 6N138 and it is not firing with the voltage drop across the 220 ohm resistor. The 220 should drop the Vin from pin 4 down to about 1.3 - 1.5V which is the std led forward voltage for the 6N138 according to the data sheet

So if it is not firing then two possibilities -- 1) another resistance leak between pin 2 and ground --

pull the 6n138 out of the breadboard \

Left the 220 resistor from pin 2 connection and measure the voltage at this end feed +3V to the input for the circuit. Note the value then re-insert the resistor in the path measure the voltage at the 6n138's pin 2 position on the breadboard. If it is not the same then there is a problem in the breadboarding.
or

2) Bad 6N138 Put a 5K pot in series with the + feed to the circuit and adjust the pot to get about 1.5V at the pin 2 position if you do not get that value trying above -- then try it with the 6N138 back in the circuit and measure the voltage at pin2 again . IF step 1 shows good voltage at the pin 2 position without the chip and then it drops with the chip in place -- I would suspect a bad chip.

The other thing you could try is a 280 ohm resistor between pin 6 and pin 4 as shown on the midi spec drawings. This is part of the standard spec and am not sure of its impact on this circuit. This may be a pullup and is already handled via the hex inverter.

Last edited by Jim McDougall; 04-09-2012 at 01:57 AM. Reason: corrections
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  #14  
Old 04-11-2012, 08:14 PM
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John John is offline
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Guys,

Here is the schematic for MIDI I/O hardware direct from the official MIDI spec:

http://home.roadrunner.com/~jgglatt/...c/hardware.htm



More MIDI technical info linked here:

http://forum.highlyliquid.com/showthread.php?t=95
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  #15  
Old 04-11-2012, 08:19 PM
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Here's a 10x MIDI thru circuit based on the HL 5x circuit. It might provide some extra insight:

http://blog.highlyliquid.com/2011/07...midi-thru-box/
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  #16  
Old 04-11-2012, 11:34 PM
Jim McDougall Jim McDougall is offline
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I think I now understand what the core problem is.

The designs, both the standard midi spec and the splitter circuit are based on using the PC-900 IC not the 6N136/138 series.

These two differ as follows -- the PC-900 has a typ input voltage of 1V for firing the LED whereas the 6N series are looking for 1.5 V That input resistor is to drop the voltage down into the range appropriate for the PC-900 and the net result is that this is insufficent voltage to fire the 6n series.
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  #17  
Old 04-12-2012, 12:30 AM
hippoBomb hippoBomb is offline
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I wonder if that is it.

I built this circuit a few months back and it functioned without issue. It uses a 220 ohm resistor and a 6n136.
http://www.flickr.com/photos/gameboygenius/6039056736/

It also looks like the 10x MIDI Thru guy used a 220 too? So it must be something I'm doing or wonky with my chip.

How bad of an idea is it if I just throw a 150 ohm resistor in there? Will the LED inside burn out quicker?
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  #18  
Old 04-13-2012, 11:44 PM
Jim McDougall Jim McDougall is offline
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As long as it isn't exceeding the max voltage of 1.75 it probably won't impact the life of the LED. Since we are dealing with short data pulses, the LED's on cycle is pretty low.
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  #19  
Old 01-20-2016, 05:24 AM
Prof H Prof H is offline
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Default 6N136 substitute

After building the 5X MIDI Thru board, I realized that it wasn't working because the 6N138 chip I used is not a direct substitute for the 6N136. I fortunately found this site with a nice schematic and explanation of the differences - mainly that the R2 resistor should be about 470 ohms rather than 10K. there is also a mention of a 1K resistor to ground from pin 7 that is not used on the 6N136. That I have not tried yet.

http://practicalusage.com/my-ultimat...#comment-21110

I was able to use a 74HC04N without any problems as well. Thanks to all for the 5X MIDI Thru schematic and comments. GCH
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