OK so I saved my MIDI ch with S3 not S2 but after setting them up - now what? I send MIDI and NADA

I start up my WTPA using S5 and for sure the MIDI channels I set are showing correctly channels 14, 15 and 16. (Bnk 1 Bank 2 SD respectively )
I recorded a couple of samples and they are operating correctly. I paused both samples in RAM so they would stop looping 

Then I sent a MIDI sequence  out to each channel , nothing happens, even when I unpause the samples (of course then they start looping but not pitched)
I know MIDI is being sent because the MIDI box is flickering and I plugged the MIDI cable to a MIDI tester box and signal was coming out of the plug that connects to the WTPA
I swept the MIDI notes from c-1 to c-5 figuring maybe it was out of range, but still no output. I tried to send on all 16 channels too - nada 

DO I have to enable MIDI play some how????  If its not user error then how should  I go about diagnosing the problem? Everything else on the unit is working great 


  • edited January 2015
    Is MIDI out working? Plug a cable from WTPA MIDI out to your MIDI tester and see if the switches send anything.
  • will do MIDI out trick in a bit, gotta run right now, but I did check that MIDI in gets to pins 2 and 3 on the opto-isolator so its OK from the I/O board, I get 5 volts between pin 8(V+)  and pin 5(GRND) so it has power....a little later I will hook up the o'scope to see what comes out of Pin6 and report back.  thanks for the tip.
  • Let me start with this , I'm an o'scope doofus. I hooked it up to the MIDI Cable and was able to get a weak trace that obviously changed from MIDI being sent to stopping the sequence so I cold tell on the scope when MIDI was being sent. (It certainly didn;t look like that nice clean edge that Todd posted, but it was definitely a trace)  So I then plugged the cable in to the WTPA2 and then hooked the same scope to pins 2 and 3 of the opto-isolator and got the same trace, so up to that point I think MIDI signals are getting to the input side of the 6n138. 

    Where I get lost is that I should see a trace on the other side coming out of pin 6 but see nothing that I can discern , but to what do I hook up the ground of the probe too?  I used pin 5(GRND) .  Is that correct thinking?  There was definitely no change in the trace turning the MIDI stream on and off, so the MIDI voltage change is  either stuck there in the 6n138 or my use of the scope setting could be in error.....is the voltage rise/fall on both sides the same? weaker? my understanding is that the opto just reflects the voltage changes using an LED to change the circuit on the other side. 

    I ran out of time but will also try to probe PIN 14 of the the next IC down the circuit. 

    Ay hints on the o'scope settings to make that voltage change more apparent? 

    As for the MIDI out trick, I tried that but what should I see coming out??? i.e what MIDI out signals can be sent. I tried to use  S0 or S1 and I see a flash on the MIDI tester (its just an LED flash indicator on voltage passing thru)  but then nothing showls up the on MIDI monitor VST.  I will say that when i start MIDI clock nothing comes back, but when I stop MIDI clock, the WPTA2 sends OUT an ALL NOTES OFF string that is reported by MIDI Monitor 

    My current guess is that my 6n138 is bad???  The good news is that I socketed ALL  my chips so swapping it is easy  I hate desoldering chips, it makes a mess)  but the bad news is I don't have a spare to plug in and test that theory 
  • RE MIDI out: All the switches without a shift key pressed except bank change send a MIDI signal. Some of those are CC messages so restart loop is a good one, sends a MIDI note 48 with velocity 64 on the current bank's channel. Look at the codez to see them all.
  • Ok thanks for that so I would say no MIDI OUT
  • @bcomnes--
    Good work so far.
    If your MIDI tester reports some kind of MIDI or other from WTPA2 out, then I'd leave off on that for now and go back to looking at MIDI in.

    I'm assuming you have an analog scope.  Set it to 1 or 2v/div vertical, and 200uS per div.
    Connect the probe ground to the tab of the 7805 or any other easy-to-reach ground point.

    Adjust the vertical offset for your scope channel so that you will be able to see 5v of signal swing.

    Set trigger to AUTO, and trigger on the channel your probe is attached to.  Set coupling to DC.  Make sure input impedance is high-z.

    Look at pin 14 of the MCU.  See if it is idling HIGH (5v with no MIDI coming in, or MIDI disconnected).
    If it is, switch triggering to NORMAL.  Your trace should go away.

    Set your trigger point to about 2.5v.  You can use the clock signal to see if you are triggering right.  You should be able to see a stable clock.  You may need to adjust your time/div (horizontal settings).

    Once you're sure you're set up correctly, probe pin 14 (should show 5v DC on AUTO or nothing on NORMAL TRIGGER).

    Now, toggle a key on a MIDI keyboard plugged into MIDI IN on the WTPA.  You could send MIDI clock too, anything with a consistent input.

    You should see a sweep, regardless of your settings.  If you do, probably your MIDI hardware is OK.  Mess with your timing if you need to.  You should see a 5v signal swing which is pretty square.

    If you DON'T see this, try looking at pin 2 of the opto with the probe tip and pin 3 with scope ground.  You might see 0.5v-2v of signal, and it should look like MIDI info.  You will need to adjust your scope settings.
    If you don't see that, pull the 6n138 and tie a 10k resistor from pin 2 to 3 and try again.

    Still no signal, check your wiring and connectors, make sure your pin 4 and 5 polarities are correct on your MIDI jack.
    If you DO get signal with a 10k but not with the opto (even a small one) then maybe suspect the opto.

    WTPA counts midi channels starting at 0 for the first MIDI channel (like a programmer would), and some devices start at 1.
    SO -- if you have a device like this (most devices) and if you set your SEND channel on a keyboard to MIDI channel 2 and your WTPA2's input to MIDI channel 2, you WILL NOT get a reaction from WTPA2.  Make sure this isn't what's happening.

    WTPA2 needs a recorded sound in a bank to play a sound on that bank, otherwise it won't play anything (nothing to play).

    Remember while you're in there to solder a 10k or so from pin 7 to pin 5 of the opto.  You'll get better edges.

    Let me know how it goes!

  • I noticed a post elsewhere about using a <3K resistor across a couple pins of the 6n138.  Is the 10k or other resistor needed to get MIDI working properly in a normal setup?
  • edited February 2015

    post from Todd
  • I'm pretty well getting conviced that my 6n138 is fubar....I set my scope to see the MIDI pulses (thanks for that how to ) and I can trace them on the input side , but on the output side of the opto there is nothing going on....I socketed this IC  , so I pulled it and reseated just to be sure it but it made no difference ....I checked there is continuity to pin 14 from the opto pin 6 

    The good news is that I ordered a few 6n138's the other day so I will be able to swap it and report back. 
  • edited February 2015
    Thanks for the link rosch!

    I was wondering why this design used a 6n138 instead of the 137.  Any particular reason?  I  believe they are not interchangeable.
  • you can also quickly check continuity from each pin to the board itself, maybe you just have a bad joint somewhere.
    Or a bent leg.... yeah i know. But i like easy fixes hehe
  • Well la dee dah, I swapped out the 6n138 and MIDI now works just fine....I am sure glad I socketed that chip (actually I did all of them) so the swap was easy as can be. This habit comes from ruining several PCBs trying to desolder multi pin objects.  The only safe way is to cut all the legs and desolder them one at a time, blecch (solder wick is your friend!) 

    But a big thanks to all who chipped in on how to fix, at least I know how to get my o'scope to see MIDI signals.....Now all I have to do is add that on off switch, swap in the 1000uf cap and check my SD functionality when those 2GB SD cards arrive from China. 
  • I just hope none of my chips are bad, I've soldered them all.
  • @shimoda
    I believe I used the 6n138 because I saw it in the MIDI spec or the Anderton book a million years ago.  The '137 is more square, logic wise, and probably a better choice.  The MIDI circuit got carried over from WTPA1 and I was pretty much a baby then.
    That resistor I posted about helps edge time.  You'd still get signal without it.

    @bcomnes -- nice fix!
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