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numbernone
Joined: Aug 16, 2006
Posts: 477
Location: new york city
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 Posted: Sat Jan 11, 2014 10:34 am Post subject:
Single Bus Keyboard Query |
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I cannot get my circuit calibrated to anywhere close to the prescribed voltage and its driving me nuts!
All components are right values, with changes for 15v.
At the the KBTP/R5 junction I am getting around 2.7v with a swing of .3v or so from the trimmer.
Nowhere near the .3958v stated!
The keyboard gets quite close to scaling even with this voltage, but not really close enough.
Also seems to be tremendous droop in the held voltage almost immediately, even with key held. Polystyrene cap in place.
Any help appreciated! |
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Broadwave
Joined: Feb 16, 2007
Posts: 347
Location: Manchester UK
Audio files: 6
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| Posted: Wed Feb 12, 2014 1:37 am Post subject:
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I wish I could help, but although I can scale the output correctly, I'm also suffering from the voltage drop problem 
I've checked that the FET is getting the gate signal to hold the voltage (which it is) but the problem persists… been scratching my head over this for too long.
I've ended up getting a cheap MIDI controller and building a MIDI to CV converter - not what I wanted, but it works.
BTW… Try changing the value of R2 (3.9K)… it should help the trimmer come into line. |
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brock
Joined: May 26, 2011
Posts: 112
Location: Canada
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| Posted: Thu Feb 13, 2014 12:24 am Post subject:
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| Quote: | | BTW… Try changing the value of R2 (3.9K)… it should help the trimmer come into line. |
I agree with this suggestion, the resistor needs to be bigger when operating from 15V, 4k3 or 4k7 should do it.
The CV hold needs a bunch of things to work, it's really the heart of this circuit. The sample and hold operates from the trigger generated as soon as the key is pressed. Holding the key has no effect on CV, only gate duration. If you see a gate like pulse on the JFET, the trigger generator is not working.
It probably makes sense to rule out the unlikely stuff first, before looking at the sample and hold just because it's easy to do. I'd make sure the voltage at the source of Q3 holds while a key is pressed, and make sure it shows up on U2 pin 1 also. This indicates the constant current source and buffers are working.
Then I'd check U3 and confirm it is an LF444.
Then look at the sample and hold.
If you have the trgger indicator LED connected you should be able to see it blink briefly when you press a key. It's a very short pulse so the light will be kind of dim; high brightness LEDs are a good idea for this. If you can see the LED blink the signal is probably getting to the JFET too so one or both JFETs are likely toast, unless R17 happens to be bad.
There could be a problem with the trigger generator circuit. The most likely things here are the caps C18 or C21, or backwards diodes. You can use a brute force method to see if the trigger generator circuit is doing anything by forcing U4 pin 3 to V+ and confirm U4 pin 7 goes high.
That's a start. I'd suspect the JFETs, but look at what I could before replacing them.
Good luck. |
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