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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Single Buss 1V/Oct Troubleshooting
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Hubaswift



Joined: Jul 06, 2016
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PostPosted: Sat Sep 02, 2017 9:22 pm    Post subject: Single Buss 1V/Oct Troubleshooting Reply with quote  Mark this post and the followings unread

The trigger output and sample/hold aren't working on it. U5 pin 7 is just outputting a constant negative voltage so I presume the reason the S&H isn't working is because the JFETS aren't receiving a pulse, which would also explain why the trigger isn't working as it runs off of U5 pin 7 as well. I am getting a gate signal from U5 pin 1 so the gate is working. So as far as I know, it may be an issue with U5 B, C, D or the surrounding circuitry. I can't seem to figure that out though. There isn't any sort of pulse coming from any of them, just dc voltages.
Two thing that need mentioning are that I'm trying to power this with +/- 9v, and that I would like to use a ribbon controller instead of a keyboard.

Schematic: http://musicfromouterspace.com/analogsynth_new/SINGLEBUSSKEYBOARD2007/schematicpage1.gif
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Hubaswift



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PostPosted: Sun Sep 03, 2017 11:41 am    Post subject: Reply with quote  Mark this post and the followings unread

I just realized that the notes mention the op amps saturation voltage which is 10.5v, so is it possible the circuit isn't working because the 9v batteries are not able to reach that voltage? If that is the case is there any way to still have the circuit run off of 9v?
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wackelpeter



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PostPosted: Sun Sep 03, 2017 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm not sure if i'm totally wrong but doesn't effect the supply voltage of an comparator also it's threshold?

From looking at the schematics i would say playing around with the combination of R32 & R33 would alter the threshold value. As it looks it's set at 12V to 6V with increasing R33 or reducing R32 it should be set to a lower threshold...

Well i'm not really sure if this would work but try for example a ratio of 5:7 there and set the reference voltage for the comparator to 5V or with an 4:8 ratio to 4V... Perhaps it wouldn't affect the behaviour of the rest of the circuit... I'm really no expert but maybe this would work out somehow...

Edit: be sure to check if pin 5 of U5 goes higher that the reference voltage at pin 6 of U5 otherwise the comparator wouldn't switch, if it goes below then it should switch back to -Ub.

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gdavis



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PostPosted: Sun Sep 03, 2017 3:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

The saturation voltage is the maximum voltage the op amp can put out. With a 12V supply it will max out at about 10.5V. With a 9V supply it will be less, about 7.5V.

The comparator threshold in the trigger circuit shouldn't be an issue. It's just being set to the midpoint between gnd and V+ and being compared against the full swings of the op amps (saturation voltages).

You should look at the U4 circuitry also. That actually generates a trigger from the changing notes without gate toggling. U5 apparently just prevents a trigger when you release the key (end of gate).

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Hubaswift



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PostPosted: Sun Sep 03, 2017 3:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

U5:
pin 12 checks out at around 4.5v
pin 13 seems to always be high
pin 14 is always low

since the voltages aren't changing there isn't a pulse at in 10
pin 9 is around the same as pin 12
pin 8 is stuck low with no observable pulse.

would this be a problem with the differentiator and trigger generator then?
just in case it is, here are a few notes around there,

U2-A is outputting the corresponding voltages from the ribbon
U4 pin 3 has a very short and low voltage spike
on U4 pin 1 it is around -3.3v and is outputting higher voltage spikes when the ribbon is pressed
U4 pin 14 is at around 3.3v and has negative voltage spikes
U4 pin 8 seems to just hold about 7v, and the same for pin 7, so maybe the problem is there. I can't see any wrong component values or pcb errors there. I'll give the circuit description another read-through. I can't understand all of it though.

Also, this is just a thought that hopefully won't be an issue. By my (possibly wrong) understanding, the differentiator and trigger generator works by putting the controllers voltage through C18, which creates a small spike whenever the pitch changes, which goes through U4-A to make the spike larger. but since the ribbon controller will be gliding from voltage to voltage, is it possible that it won't trigger the sample/hold because the capacitor has no pulse from gliding voltages?
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gdavis



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PostPosted: Sun Sep 03, 2017 3:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks like you caught my post before I edited it. I got a couple wires crossed so it wasn't totally accurate. Embarassed

Anyway, ya, I think we need to be looking at the U4 area.

You're essentially correct, it's taking changes in the CV and make pulses. U4A output will be positive or negative pulses depending on whether the CV changes up or down. U4D and the diodes make the positive and negative pulses into all positive pulse.

Your U4 pin 1 output sounds like a problem. What are pins 2 and 3 when you're not pressing anything? I think pin 1 should normally be 0V with just short +/- pulses when you press the ribbon.

I think you're right, sliding up and down the ribbon won't cause triggers, just if you tap points far enough apart. You may need to modify the circuit if you want a smooth glide up and down the ribbon to keep the sample open as long as the gate is open.

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Hubaswift



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PostPosted: Sun Sep 03, 2017 5:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

U4 pin 2 is at 0v, pin 3 has tiny 0.4v blips when I press the ribbon, pin one now seems to be sitting at -.3v and outputting positive spikes when I press the ribbon, but the spikes look a bit dirty and are slightly inconsistent, not sure if that matters. U4 pin 14 is now at 0.3v as instead of 3.3v. I'm not sure why pin 1 and pin 14 have changed now.

as far as the ribbon tracking goes, I think I'd just need to exchange the S/H with a Track/Hold. But I'll worry about that once it's running properly.
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gdavis



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PostPosted: Sun Sep 03, 2017 6:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well that sounds better. The small blips at pin 3 are expected. They should be much larger out of pin 1. Might be a bad connection around pins 2 and 3 or the components connected to them if pin 1 is behaving inconsistently.

I think the dirty spikes should get cleaned up down the line.

So with the new behavior on pin 1 and 14, do you see the trigger further down the circuit?

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Hubaswift



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PostPosted: Sun Sep 03, 2017 8:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

The signal seems ok until U4 pin 8 where I am just getting 7v. pin 10 is getting little blips and pin 9 is at 0v.
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gdavis



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PostPosted: Mon Sep 04, 2017 1:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sounds like a problem with the feedback resistor R16.

Pin 8 should be amplifying the voltage at pin 10 by a factor of 3.

Pins 9 and 10 should be the same voltage as long as they are within about 2V from gnd. Much more than that and the output will start to saturate and it won't be able to keep them the same. But 9 should only be 0V when 8 and 10 are 0V.

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Hubaswift



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PostPosted: Mon Sep 04, 2017 4:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

R16 is the correct value and there aren't any errors on the pcb in that area, I also put a different 20k resistor in just in case, but the behavior didn't change. pin 10 normally sits at 0.12v and pin 9 normally sits at around 0.2v, and I'm still getting 7v on the output, which seems quite off. pin 10 has a .4v peak to peak rough saw wave, but I'm unsure if that's an issue.

I just noticed that R22 is a 160k resistor instead of a 150k Rolling Eyes That may be the problem.
Edit: It's not the problem Sad
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gdavis



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PostPosted: Mon Sep 04, 2017 5:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

There's a problem in there somewhere, don't assume anything. Short, cut in a trace, bad component, something.

Check that pin 8 and the end of R16 connected to it are the same voltage. Check that pin 9 and the other end of R16 are the same voltage. Check that pin 9 isn't shorted to ground.

You could also try jumping pin 8 to pin 9, and/or jumping across R16. You'd lose the gain but should see the signal on pin 10 come through.

edit:
Actually, if pin 9 is a higher voltage than pin 10 and pin 8 is positive, the ic is probably bad.

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Hubaswift



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PostPosted: Mon Sep 04, 2017 7:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Replacing U4 wasn't the issue. I mentioned that pin 10 was slightly higher than pin 9. not 9 slightly higher than 10. No matter.

The connections between pin 8 9 and R16 are all good. shorting over R16 just gives my constant 0v. Maybe an issue with R21? I'll check that out. This circuit turning out to be a bit of a pain!
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Hubaswift



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PostPosted: Mon Sep 04, 2017 7:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

And, that was it! R21 was the problem. It was the correct value but was faulty.

as suspected, gliding across the ribbon does not work. Do you have any ideas on what changes would need to be made?
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gdavis



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PostPosted: Mon Sep 04, 2017 7:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hubaswift wrote:
And, that was it! R21 was the problem. It was the correct value but was faulty.

Well there you go! See, you just gotta through the circuit systematically, follow the clues and narrow it down till you find the culprit.

Quote:
as suspected, gliding across the ribbon does not work. Do you have any ideas on what changes would need to be made?


Like you said, track/hold instead of sample/hold. I *think* cutting the connection from U5 pin 7 (trigger) to R17 and connecting U5 pin 1 (raw gate source) to R17 would work.

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Hubaswift



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PostPosted: Mon Sep 04, 2017 8:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

The ribbon now glides properly, but it isn't holding the last played note. It just goes to the lowest note.
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gdavis



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PostPosted: Mon Sep 04, 2017 8:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well that's a bummer, but not surprising since the gate is closed by the CV going to the lowest value. Conceptually, an easy solution might be to add a small delay between U2B and Q3 to allow the hold to engage before the CV to it drops, but I couldn't say to how to actually implement that. Sorry but I don't know any easy way to fix it other than adding another button to control the gate.
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Hubaswift



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PostPosted: Mon Sep 04, 2017 8:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Okay, thanks for your help. I'll see if I can figure something out.
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