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midiDAC midi in problem
MUFF WIGGLER Forum Index -> Oakley Sound Systems Goto page 1, 2  Next [all]
Author midiDAC midi in problem
catatemycaps
Hi folks,

I just finished up building the midiDAC and when testing it I ran into some issues regarding Midi In.

When I play a note and measure the voltage on pin 1 of U1 I get -2.556 V regardless of note/octave. As a matter of fact I seem to get the same voltage even if I don't play any note.

I have checked the wiring from the DIN socket to PCB and it is ok. Pin 5 on the DIN goes to pin 1 on the PCB and pin 4 on the DIN goes to pin 2 on the PCB.

Adjusting the INIT trimmer changes the voltage as should but not drastically.
I don't know what sort of values I should get from INIT.

I have also checked my soldering and all seems ok. I will go through those once again.
Synthbuilder
Is the gate LED coming on when you press a note down? And does the glide LED come on when you play legato?

If it is the midi part of the circuit is probably working.

If not, then the usual suspects are wrong midi channel (easily done) or the midi wiring is suspect (it's worth reversing it just in case). A quick check for the midi wiring is to see if the midi through is working OK. If that isn't working then the midi wiring is probably the wrong way around.

Tony
catatemycaps
Thanks again Tony.

It was the midi channel. I checked that last night at least three times and I was 100% sure that I had set it to one.
I just checked it once again as you suggested and after couple of failed tests I noticed something...I was reading the dip switches backwards. very frustrating

So, I had actually set it to 16 instead of 1. very frustrating

midiDAC calibrates just nicely now. applause
-10 V for the highest C and playing a C an octave lower gives me -9 V and so on.

Btw, is the legato led supposed to stay on after releasing the note? It will switch off once I start playing again, though.
Synthbuilder
catatemycaps wrote:
is the legato led supposed to stay on after releasing the note?

Once on it'll stay on until the next un-legato'd note is played. Which sort of makes sense because the legato'd note CV is also still in the buffer.

Tony
catatemycaps
Excellent. Thanks!
Schlumpfhut
I'm sorry to dig up this old thread but i have a similar problem. I hooked up my Keyboard to my newly build MIDI-DAC and got the same problem. I tried to switch arround the cables with no effect. And i do not get any response from the leds when hitting a key.
Power is applied correctly and all components check out fine as far as i can tell as well as the soldering.
I will go through it again but i run out of ideas. Any suggestions where to look?

Thanks very much in advance
Synthbuilder
The first thing to check is the the midi through output. Check to see if midi is being passed from the midi input to the midi output without errors. If it does then the next thing to check is the midi channel selection switch and that the correct midi channel is selected. The easiest way is normally to try all 16 midi channels on your controller or DAW and see if you can find one that the midiDAC responds to.

If that fails - then the problem may be tricky to find. Do you have a scope?

Tony
Schlumpfhut
Thank you for the quick reply.

I just measured the actual voltage at the pitch socket and it checked out fine. I guess i just put the LEDs into the board the wrong way. Tired brain didn't think about this yesterday (and Friday) d'oh!

Gonna check that out in the afternoon and have another cup of coffee

EDIT: switching LEDs didn't change anything and i am back at the start with no change
I do have a scope and when i use my Multimeter on the Midi through i get the same Value (about 5V) as on the input
Synthbuilder
I'm not sure I understand. Is the CV out responding to the midi input but not the LEDs? Is the gate output working? If there is no gate or any LED operation then check U6 is correctly fitted. If U6 is completely dead then the midi through won't be working either.

Unfortunately, measuring the voltage of a midi input or output doesn't tell you a great deal. Midi is a fast moving digital signal and your voltmeter can't respond to it quickly enough. A scope is better but even that won't tell you if you have a midi error. Midi will be seen a brief set of downward pulses on a scope as data is sent.

You'll need to connect another midi instrument to the midi output of the midiDAC and put midi into the midiDAC. If the connected instrument responds properly you'll know that the midiDAC's midi wiring is correct and the midi input circuit is working correctly.

If you only have a DAW based set up simply send the midi out of your computer interface and back in again via the midiDAC. Check to see if data is being received correctly by controlling another synth in your DAW.

Tony
Schlumpfhut
Synthbuilder wrote:
I'm not sure I understand. Is the CV out responding to the midi input but not the LEDs? Is the gate output working? If there is no gate or any LED operation then check U6 is correctly fitted. If U6 is completely dead then the midi through won't be working either.
Tony


Neither the CV nor the Gate output is working nor the LEDs. I swaped U6 to a spare one without a change. Going to test MIDI through next
Schlumpfhut
About 2 minutes ago i found out that the MIDI through is actually not working. I ran through all MIDI channels without change. I also noticed that the MIDI-in socket is much easier to plug in than the MIDI-Through socket. May it be worthwhile to swap the socket?
Synthbuilder
Pop your scope probe on pin 6 of U7. Send some midi data into the midiDAC's input socket. Mod wheel or pitch bend will generate lots of data and you should be able to see the voltage at pin 6 regularly briefly travel to 0V from 5V with the data. If not, the opto is not working or the midi input wiring is wrong.

Tony
Schlumpfhut
Yesterday i replaced the MIDI-in socket and all of its wiring as dicribed in the builders guide. I also replaced the opto without any effect.
When i probe pin 6 U7 i can monitor a somewhat distorted sinewave which i also see on pin 2 of the opto.
I have already ordered some more optos but i don't think this is the problem anymore.
May some other component be the problem?
Synthbuilder
Schlumpfhut wrote:
When i probe pin 6 U7 i can monitor a somewhat distorted sinewave...


What frequency is that sinewave? If it's 50Hz I think you have a 0V connection (grounding) problem somewhere. Do you get a steady 5V at pin 8 of U7. If not you have a problem either with the 0V or with the components around and including U11.

Tony
Schlumpfhut
Synthbuilder wrote:

Do you get a steady 5V at pin 8 of U7.

I get a steady 4.96 V on pin 8 of U7 - and oddly also on pins 6 and 7. It kinda looks like there is some sort of a short between those pins.
Synthbuilder
Schlumpfhut wrote:
I get a steady 4.96 V on pin 8 of U7 - and oddly also on pins 6 and 7.


That sounds good. Pin 6 is your output and should be at around 5V until valid midi data is sent. Once midi data is sent the output voltage at pin 6 will briefly flicker to 0V. There should be no sinewave observable with the scope set to 1V or 2V per vertical division.

The key to checking the opto is working is getting valid midi data on the midi thru output. You'll need to get this bit working before you can test the rest of the midiDAC's circuitry. If it doesn't check that R43 is 1K and that D5 is a 1N4148 diode and around the correct way. Pin 4 of U6 should copy exactly what is going on at pin 6 of U7.

Tony
Schlumpfhut
Synthbuilder wrote:

Pin 4 of U6 should copy exactly what is going on at pin 6 of U7.


I replaced both R43 and D5. Both are correct values/type of diodes and are oriented correctly.
After that i again hooked up my scope and probed pins 4 and 6 on the opto. Pin 6 shows the expected sinewave of roughly 5V. Pin 4 actually shows nothing at all. After that i disconnected both MIDI-in and MIDI-through and probed again. As expected nothing.
After hooking up MIDI-in: Sinewave on pin 6 and still nothing on pin 4
Then hooking up MIDI-through: again both sides dead.
So am i correct in assuming that the opto is working fine but no data is reaching it?
Synthbuilder
There definitely shouldn't be a sine wave on there. The output of the opto-coupler at pin 6, with no midi data being sent, should be a steady voltage of around 5V. If there is a 50Hz sine wave of 5V then you may have a problem with the midiDAC's power supply connections or the scope probe's ground clip is not connected to the midiDAC's or the power supply's 0V.

Pin 4 of U6 should also be a constant 5V.

Once midi data is sent the signals on pin 4 of U6 and pin 6 of U7 should follow each other. Mostly sitting at around 5V with very brief bursts of 0V.

Tony
Schlumpfhut
I found some poor soldering on one of my Dizzy boards and after resoldering them the grounding issue seems to be gone. I now get a steady almost 5 V on pins 6 and 7 of the opto as required. The input however is still a problem. On the scope i can see some low intensity sinewave wich i was able to trace back to the MIDI-in socket. Next step for me schould be (?) to take another MIDI source and check this aout
Synthbuilder
Schlumpfhut wrote:
On the scope i can see some low intensity sinewave wich i was able to trace back to the MIDI-in socket. Next step for me schould be (?) to take another MIDI source and check this aout


If you have a midi keyboard with a wallwart or battery power source then you may well see a 50Hz sine wave signal superimposed on the midi input. This is because the midi input is effectively floating with respect to the 0V of the midiDAC. It may not indicate a fault on its own. Pins 2 and 3 of the opto would have the same sine wave signal in this case.

That said, there's clearly a problem if the midi out doesn't work. I'm not sure what to suggest at this stage. What are you using to drive the midi input?

Tony
Schlumpfhut
So far i am using a Behringer UMX490 to send Data into the midiDAC. And as far i can tell the Data does not reach the actual input
Synthbuilder
It will be worth checking that the controller is sending valid midi data via its midi output. Maybe try to control something else with it and see if that works. Do you have another midi module or a PC with a midi interface to try it with?

Tony
Schlumpfhut
The Keyboard has a USB out witch works fine and also the MIDI-out works fine through a MIDI-to-USB interface on the PC
Yestarday just for fun (or desperation Mr. Green ) i probed the midiDAC without a controller hooked up and the low sinewave is still there. May this be helpful in our quest? hmmm.....
Synthbuilder
I'm not sure what else to suggest. The midi input and output is a simple circuit so there's not a lot to go wrong. The 50Hz sine wave thing may just be the scope picking up stray hum fields from the open wiring on the midi input and as such may not indicate an issue.

But if midi data is not getting through from the input to the output on the midiDAC the only thing it can be is the circuitry around U6 and U7 and the wiring to and from those midi sockets.

It might be worth removing the PIC from the midiDAC board, powering up and checking midi output again. Take care about static discharges when handling the PIC and other ICs on the board.

Tony
Schlumpfhut
I think i found the real problem: After running out of other ideas i replaced the SOCKET of the opto and checked again. Now i still get the constant 5V line on the output pins with short glimpses of a second line arround 0V when pushing a key. Similar can be seen on the input. Also now the Gate LED blinks once when powering up but stays dark afterwards. When i remove the PIC the Gate LED is on permanently so the little nerd is wired correctly.

EDIT: still no response from MIDI-trough or the GATE-LED. Gonna get my hands on a replacement for U6 as soon as i am completely healthy again and check that part of the board
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