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 Forum index » DIY Hardware and Software » The Repair Shop
MFOS adsr problem
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the19thbear



Joined: Apr 19, 2007
Posts: 113
Location: denmark

PostPosted: Tue Apr 29, 2014 12:19 pm    Post subject: MFOS adsr problem Reply with quote  Mark this post and the followings unread

Hi. I build the MFOS adsr and it is working perfectly when i use the front manual gate activation, but it never releases when i use gate from a keyboard.
I know it is a sound +5 gate that is put into the circuit from the keyboard and it is off when i release a key (i measured it).
Any suggestions or ways to get it working?
As you can see i have marked the manual gate input that is working and the "real" gate input. Since the manual gate is working I'm assuming that the problem is somewhere before the two signals meet (the manual and the external gate). I think the problem lies somewhere within the left red square - am i right?
And C13 located in the lower left section, reads 100pfarad on the scheme. In real life it reads n10 (it says that on the actual component) - is that correct?

Thanks.

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elmegil



Joined: Mar 20, 2012
Posts: 1504
Location: Chicago
Audio files: 14

PostPosted: Tue Apr 29, 2014 9:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

How does it behave with a trigger instead of a gate? A trigger in addition to a gate?

I think it's clear that something is going wrong with the section you have in the box on the left. I would measure to be sure my input was behaving as I expected, then that pin 1 of U1A was accurately reflecting that. Your gate is going to have to drop below 2V to turn off the TL082 wired as a comparator. That seems like it ought not be a problem, but never assume....
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the19thbear



Joined: Apr 19, 2007
Posts: 113
Location: denmark

PostPosted: Tue Apr 29, 2014 9:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great. Thanks. Its nice to know that I'm not completely dumb.
I don't have anything with a trigger out to test. I guess I could just use a quick gate signal actually - right?
Thanks a lot for the help. I'll test as soon as I have the time.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Apr 30, 2014 5:04 am    Post subject: Reply with quote  Mark this post and the followings unread

the19thbear wrote:
Great. Thanks. Its nice to know that I'm not completely dumb.
I don't have anything with a trigger out to test. I guess I could just use a quick gate signal actually - right?


I usually take a VCO PWM output with a narrow pulse at a low-ish frequency when I don't have a trigger quickly available.
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the19thbear



Joined: Apr 19, 2007
Posts: 113
Location: denmark

PostPosted: Thu May 01, 2014 5:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Hmmm... out of pin 1 on U1A i get +/- 14 volts when i turn gate on/off!?!
On the input side (pin 3) i get 0/+4.something volts when when i turn gate on/off
... Is the +/- 15 volts right? or should it be less?

The IC is getting +/- 15 volts on its power inputs so that is right (pin 8/4)
THanks
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu May 01, 2014 5:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, if you have +/- 15V supplies, pin 1 should swing from +/- 14V (not quite the rails).

Next check the other side of the diode. But that's really the last step of the way that's unique to the gate input.

Did you see if the trigger input works?
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the19thbear



Joined: Apr 19, 2007
Posts: 113
Location: denmark

PostPosted: Thu May 01, 2014 6:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Wait. Just double checked.
When measuring on pin 1 on the IC i get -14 when i turn on the synth, and the i press a key and i get +14 - and it doesn't go back to -14 again.
Just checked with trigger. (haven't measured anything, just used my ears) It never reaches the release point and is just constant on.
Could it be something with pin 2/6 ?
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu May 01, 2014 7:43 am    Post subject: Reply with quote  Mark this post and the followings unread

What is the voltage at pins 2/6 ? Is your input ever falling below that value? Measure that one at pin 3 and at the jack to see. Is C7 shorted?
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the19thbear



Joined: Apr 19, 2007
Posts: 113
Location: denmark

PostPosted: Thu May 01, 2014 8:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Pin 2/6 voltage: 0.28

Input on pin 3 at its low position is -0.24 (weird?).
It's possible that I messed up with resistor values around r6/r7.

C7 is not shorted. It say 104 on the side.
You deserve a medal by now! Thank you sooo much for your help!
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu May 01, 2014 9:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Something is kinda squirrely. It sounds like you're using 15V supplies, right?

15V x (20K / (100K + 20K) ) = 2.5V

So the value is off by a factor of 10. Make sure that R7 is 20K not 2K. Smile

The op amp will try to make Pin 3 match Pin 2, that's what drives the output. What's your voltage at the jack (other side of R10)?
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the19thbear



Joined: Apr 19, 2007
Posts: 113
Location: denmark

PostPosted: Thu May 01, 2014 2:20 pm    Post subject:   Reply with quote  Mark this post and the followings unread

Your are a genius:) Turned out it wasn't R 7 but R6 that was wrong!
Its working now with external gate Laughing
Thank you SOOOO much!
Now i just have to figure out a way to convert a gate signal to a trigger ( to have the 2 different modes available, since i don't have trigger out from the keyboard), but thats a completey different thread.
Thanks again!!

WOHIOOOOOOOO!!!! you rock!
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elmegil



Joined: Mar 20, 2012
Posts: 1504
Location: Chicago
Audio files: 14

PostPosted: Thu May 01, 2014 2:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you for the kind words, but it's not genius, just troubleshooting and a little bit of experience making my own mistakes. Glad I could help.

Here's a bit of help for gate to trigger, this is a commonly used circuit:

http://www.cgs.synth.net/modules/cgs24_gatetotrigger.html
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