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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Help with Single buss 1v/oct keyboard
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capicoso



Joined: Nov 19, 2012
Posts: 126
Location: Argentina

PostPosted: Thu Jan 10, 2013 11:59 am    Post subject: Help with Single buss 1v/oct keyboard
Subject description: Please
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Hello and thanks for reading
I'm having trouble with this one http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=SINGLEBUSSKEYBOARD2007/SINGLEBUSSKEYBOARD2007.php&VPW=1024&VPH=500 ... The bus works fine i think. The dvm reads 1v every 12 resistors so ok(Its a 4octave keyboard). I'm using a +15/-15 power supply. I still didn't wire the jacks, because i want to make sure it works first... i don't know if they need to be wired? One of the main problems is that trigger or TLA voltage is very low. It's at 0v and when a key is pressed it just goes to 0.02v. Gate or GLA instead, works fine i think it's at around -14v and when key pressed it goes to +13v. The ouput of U6A is at about -13.52v and when key pressed -13.48 or so. Output of U6B is ok i think, its at -13 or so and when pressed +13. CV1 and CV2 are always at 8.21v or so, and when i press a key for example the 49th key(last one) It goes down to 4v or so, if i press in the middle it goes to 2v, and the first one very low v. I don't know if thats ok or what but it doesn't matter if i keep the key held down, the V goes up again to 8v(slowly) for example, my DVM reads when i hit the key 4v, then 5v, then 6v, till 8v. CV3 CV4 are always at 8v no matter what i press(maybe because i didn't wire the GLIDE POT, so that doesn't matter i think). I rechecked the resistors and everything, there's one wrong, R35 it should be 47k but i put 27k, i didn't change it yet, because i kept looking and i found a worse error i think the two electrolytic capacitors of the power input... the -15v (C15) was reversed Sad . So i changed it... nothing new.

I think that until the ouput of U2A everthing is right. The output of U2A goes to U4A. The output of U4A goes to U4D and goes out inverted. In other words, ouput Pin 1 of U4A should be 10.5v or so, and the output of U4D pin 14 should be -10.5v or so. Thing is i get 0.15v from pin 1 and -0.15v from pin 14. It's inverting it... but so low voltage. All the resistors around this U4 are fine, the only wrong resistor i found was R35, didn't change it yet, could this be the problem?... It doesn't look like it is, but i'm no expert.


I have no vco so i can't listen anything, i'm testing everything with my DVM.

If needed i'll measure all of the ICs ouputs and post it... i did it already but i don't remember every one.
But yeah there's one wrong for sure U4A

thanks!
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capicoso



Joined: Nov 19, 2012
Posts: 126
Location: Argentina

PostPosted: Fri Jan 11, 2013 6:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

update:

I've made some progess. Changed R35 but nothing happened. Then i changed Q3 transistor and it behaved different, not better. Then changed C19 and now CV1 and CV2 are ALMOST working!

They should be at -0.8v right? And when key 1 pressed 0v, and my last 48th key 4v. Well, they're at -1.21v, -0.40v less than what it should be. SO my last note is at 3.60v. Should i try changing Q2 maybe? I tried making an offset with the trimpot so it's lower than it should but it doesn't change the -1.21v of the CV1 and 2. What could it be?....
Trigger problem persists, but to be honest, by just fixing the CVs i'm happy. I was going to make a midi2cv anyways

edit: Changing the trimpot doesn't seem to change the mininum -1.21v. But it does change when i press a key, i can get to 3.84v now. But the first one is at -0.5v or so, weird
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brock



Joined: May 26, 2011
Posts: 93
Location: Canada

PostPosted: Fri Jan 11, 2013 11:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

hi capicoso,

I don't think the output CVs should ever drop below 0V. The only way that can happen is if the sample and hold is not working and the Q3 JFET is on all the time, or at least when it shouldn't be on.

I'd back up a bit and confirm the current source and keyboard scaling by measuring the outputs of U2 before the sample and hold. It seems that you had this working. With no key pressed I'd expect to see approximately -1.2V. The lowest key should give 0V. If it's anything other than 0V you should check your wiring because the lowest key is basically shorting the input to ground. The highest key for a 49 note 4 octave keyboard should give you 4.0V. If you have a 48 key keyboard it will be 83.3 mV less, 3.917V.

The only adjustment is the scaling with trimpot R3 so once you confirm the lowest key gives 0V at the U2 outputs, adjust the trimpot for the appropriate voltage with the highest key pressed. I think you did this and had it working originally, so if this is all working you can move on to the output.

The next thing I'd check is if the CV outputs are the same as the U2 outputs when a key is pressed and held. If they are different, and low as you described, then the trick is to figure out of it's a trigger hardware problem or the sample and hold.

I'm trying to figure out how to trouble-shoot the rest of this, assuming you don't have a scope to check out the trigger signal and the 0.02V measurement you mentioned was from a meter. The trigger is short, about 1ms probably, so you can't measure it well with a meter unless you have a peak hold function. The only thing I can think of that might cause the outputs to be lower than than the U2 values is if the trigger pulse is too short, but it would mean that as you went down the keyboard the values would likey be too high, so I'll ask a bunch of questions first and see if there are any clues to be had.
    Are the CV outputs the same as U2 output when a key is held down?
    Does the CV output change immediately when a key is released?
    Does the CV output start to drop within a couple of seconds of key release?
    Do you have the trigger indicator LED connected?
    Are the JFETs 2N5457s or something else?
    Have you made any component substitutions?
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capicoso



Joined: Nov 19, 2012
Posts: 126
Location: Argentina

PostPosted: Sat Jan 12, 2013 12:21 am    Post subject: Reply with quote  Mark this post and the followings unread

brock wrote:
hi capicoso,

I don't think the output CVs should ever drop below 0V. The only way that can happen is if the sample and hold is not working and the Q3 JFET is on all the time, or at least when it shouldn't be on.

I'd back up a bit and confirm the current source and keyboard scaling by measuring the outputs of U2 before the sample and hold. It seems that you had this working. With no key pressed I'd expect to see approximately -1.2V. The lowest key should give 0V. If it's anything other than 0V you should check your wiring because the lowest key is basically shorting the input to ground. The highest key for a 49 note 4 octave keyboard should give you 4.0V. If you have a 48 key keyboard it will be 83.3 mV less, 3.917V.

The only adjustment is the scaling with trimpot R3 so once you confirm the lowest key gives 0V at the U2 outputs, adjust the trimpot for the appropriate voltage with the highest key pressed. I think you did this and had it working originally, so if this is all working you can move on to the output.

The next thing I'd check is if the CV outputs are the same as the U2 outputs when a key is pressed and held. If they are different, and low as you described, then the trick is to figure out of it's a trigger hardware problem or the sample and hold.

I'm trying to figure out how to trouble-shoot the rest of this, assuming you don't have a scope to check out the trigger signal and the 0.02V measurement you mentioned was from a meter. The trigger is short, about 1ms probably, so you can't measure it well with a meter unless you have a peak hold function. The only thing I can think of that might cause the outputs to be lower than than the U2 values is if the trigger pulse is too short, but it would mean that as you went down the keyboard the values would likey be too high, so I'll ask a bunch of questions first and see if there are any clues to be had.
    Are the CV outputs the same as U2 output when a key is held down?
    Does the CV output change immediately when a key is released?
    Does the CV output start to drop within a couple of seconds of key release?
    Do you have the trigger indicator LED connected?
    Are the JFETs 2N5457s or something else?
    Have you made any component substitutions?

Hi Brock, thanks for your time.

On U2 output with no keys pressed i see -0.54v. When i press the first key i get 0v, and the last one(49, sometimes i get confused) is 4v.
NOW it's behaving a bit different than earlier. The CV outs are giving me the same values as U2 outputs(earlier CV outs were lower and not 1v/ocT). The only thing i changed in this time was a ceramic capacitor before the non inverting input of U4, i don't think that did anything.

So CV outs are the same as U2 outputs, BUT they don't go back to the original value. If i press 49th key, and release it, it stays at 4v... this didn't happen earlier... maybe i should check something around C19 but i rechecked it 99times.
It keeps holding the note, no matter what, if i play legato it changes ok to that note.
I have the trigger led attached, and it's very very weak.
The GATE led is perfect. When i release the key it shuts down, so everything fine.

Are the CV outputs the same as U2 output when a key is held down?
yes.

Does the CV output change immediately when a key is released?
Now it stays on the note. Doesn't change a bit.

Are the JFETs 2N5457s or something else?
They are 2N5457.

Have you made any component substitutions?

Yes. I changed C19(the first one had leaks).
I also changed all the ICs except for the TL071.
I changed Q3
and a while ago i changed C18 just before U4A, i don't know why...
thanks


update:
i measured Q3 S is ok i think , -0.54 released, 0v first key, 4v last key. It releases okey.
Q3 D makes a fast pulse i think. It's at 0 and with key pressed it triggers fast 0.25v.
Q3 G is at about ~ -13.50 and with key pressed it makes a pulse to -13.35 or so. Is that ok??

Q2 may be acting weird or?... all the legs behave like Q3 G. -13.50 key pressed = very little change

U5B 7 output is the same. I'll keep tracking
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brock



Joined: May 26, 2011
Posts: 93
Location: Canada

PostPosted: Sat Jan 12, 2013 10:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

It seems like everything is working correctly. The CV1 & 2 outputs should hold after a key is released, U2 outputs will drop back below 0V. That's how a note can fade out without changing pitch during the release portion of an envelope.

The trigger LED will be very faint because the pulse is very short. It pulses immediately after a key press, turning on the JFETs Q2 and Q3 to charge C19. The fact that you can see it at all is generally enough to tell you it's working.

I think you've got it working.
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capicoso



Joined: Nov 19, 2012
Posts: 126
Location: Argentina

PostPosted: Sun Jan 13, 2013 12:31 am    Post subject: Reply with quote  Mark this post and the followings unread

brock wrote:
It seems like everything is working correctly. The CV1 & 2 outputs should hold after a key is released, U2 outputs will drop back below 0V. That's how a note can fade out without changing pitch during the release portion of an envelope.

The trigger LED will be very faint because the pulse is very short. It pulses immediately after a key press, turning on the JFETs Q2 and Q3 to charge C19. The fact that you can see it at all is generally enough to tell you it's working.

I think you've got it working.


Really? Well now i think about it i may understand a bit more.
So the correct use of this keyboard is, CV to a VCO for example, and GATE to ADSR no matter the CV keeps the voltage high?
Now makes sense... and all the day/night going crazy trying to fix something that wasn't broken!! Well, the capacitor was broken i think, but oh well.
thank you so much.

edit: SO the trigger led is dim because it's too fast to my eyes. And it's also too fast for my cheap DVM to measure the peak voltage right? now that makes sense... i feel stupid Rolling Eyes
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brock



Joined: May 26, 2011
Posts: 93
Location: Canada

PostPosted: Sun Jan 13, 2013 2:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

You're welcome. Glad I could help explain things. You shouldn't feel stupid to not know things, we all start out not knowing anything. You should be feeling happy that you've learned something and built something that works.
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capicoso



Joined: Nov 19, 2012
Posts: 126
Location: Argentina

PostPosted: Sun Jan 13, 2013 3:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

brock wrote:
You're welcome. Glad I could help explain things. You shouldn't feel stupid to not know things, we all start out not knowing anything. You should be feeling happy that you've learned something and built something that works.

There's one more thing i noticed now. Hope you can give me one more hand

When i calibrated the buss it was ok. I put the DVM on DC voltage, and put the negative probe on earth and the red on the last resistor and it gave me 4.02v. I put it on the first resistor and it gave me 83.3mv. Just like Ray says. BUT when i measure CV output the first resistor is at 0,6 not 0,833. But the last one is at 4.01. Then i say ok, i may give it an offset with the trimpot.
I thought it may be something wrong with my resistor chain or the buttons, but if i measure U2 ouput its ok. So something changes after that...
Yesterday i told you that U2 output and CV outputs were the same because i pressed 4 or 5 buttons in the last part... and my DVM is a cheap one so ithought that 4.01v was the same 4.02v 3.92v same as 3.93v , i don't have a higher resolution, just two decimals.... this may be confusing, i explain it better now

at CV ouput The 1st button gives -16mv(this is WRONG right?? On U2 is 0v), the second 69-73mv, the third 152mv, the fourth 235mv... it seems like the jumps are ok at 83mv, but its the first one that is wrong

As expected if i adjust the trimpot the first button won't change, on U2 it'll be always 0v as it should be, and on CV output it's always at -16mv ...
So if i adjust the trimpot to make the first resistor 83mv that means that the jump is higher than 83mv, because it has to go from -16mv to 83mv, so 99mv. And then my last note will be like 4.30v or more i think.
So in a few words... something after U2 is making the first note below ground when it should be grounded, im lost
I used a 5% 10M resistor on R7 instead of 1% i don't know if it matters...
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brock



Joined: May 26, 2011
Posts: 93
Location: Canada

PostPosted: Sun Jan 13, 2013 8:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

The CV voltage offset is larger than I would expect and it may look wrong but in practice it is a small offset that is unlikely to cause any problem in use. The important thing is that the step size be correct. This circuit is designed so you don't have an offset adjust because it's usually unnecessary as every module it will hook up to will have a tuning control of some kind. The offset may just be an artifact of the way the circuit is terminated and how you are measuring it. I'd add a 100K resistor between the CV1 output and ground, then measure across the resistor. You don't need the resistor in the circuit normally as most modules have a 100K input impedance but this will give you a good idea of how it's going to work when hooked up to something. If things don't look better across the resistor you might want to swap out U3 and see if that makes a difference.
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capicoso



Joined: Nov 19, 2012
Posts: 126
Location: Argentina

PostPosted: Sun Jan 13, 2013 9:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

brock wrote:
The CV voltage offset is larger than I would expect and it may look wrong but in practice it is a small offset that is unlikely to cause any problem in use. The important thing is that the step size be correct. This circuit is designed so you don't have an offset adjust because it's usually unnecessary as every module it will hook up to will have a tuning control of some kind. The offset may just be an artifact of the way the circuit is terminated and how you are measuring it. I'd add a 100K resistor between the CV1 output and ground, then measure across the resistor. You don't need the resistor in the circuit normally as most modules have a 100K input impedance but this will give you a good idea of how it's going to work when hooked up to something. If things don't look better across the resistor you might want to swap out U3 and see if that makes a difference.


Yes, the step size is correct. So i should calibrate it so each resistor has a voltage drop of 83.3mv. And i stay with that. Doesn't matter the offset then? . I get your point, when tuning the oscillators then i'll tune them for THIS keyboard. If for example i want to use another keyboard, with a midi>cv, then the oscillator i calibrated with the single buss keyboard will not be tuned for that midi keyboard?
I'll try adding that resistor and measure
I should have done the midi>cv converter, it seems to be easier, and more precise. I'm going to do it anyways, but i found the schematic AFter this one. I'm happy this thing works though!
thanks for all your help again
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